Advection
The horizontal transfer of any property in the
atmosphere by the movement of air. Examples include heat and moisture advection.
Air
This is considered the
mixture of gases that make up
the earth's atmosphere. The principle gases that compose dry air are Nitrogen at
78.084%, Oxygen at 20.946%, Argon at .93%, and Carbon Dioxide .03%
Airmass
An extensive body of air throughout which the
horizontal temperature and moisture characteristics are similar.
ASOS
Acronym for Automated
Surface Observing System. This
system is a collection of automated surface weather instruments that collect
data. It performs surface observations in places that either do not have a human
observer, or an observer 24-hours a day.
Barometer
An instrument used to measure atmospheric
pressure. Examples include the aneroid barometer and the mercurial barometer.
Barometric Pressure
The pressure exerted by the
atmosphere at a given point. The measurement can be expressed in millibars (mb) or
in inches of mercury (Hg).
Blizzard
A severe weather condition characterized by low
temperatures, winds 35mph of greater, blowing snow that can reduce visibilities
to 1/4 mile or less for more than 3 hours. A severe blizzard is characterized by
temperatures at or below 10 degrees Fahrenheit, winds exceeding 45mph, and
visibility reduced by snow to near zero.
Blue Norther
Refers to a fast-moving cold front in the
southern Great Plains , marked by a dark, blue-black sky, strong north winds and
temperatures that may drop 20-30 degrees Fahrenheit in a matter of minutes
Calm
Atmospheric conditions devoid of wind or any other
air motion.
CAPE
Acronym for Convective Available
Potential Energy. The amount of energy available to create convection. Higher
values indicate an increasing possibility of severe weather.
Ceiling
The lowest cloud layer that is reported as
broken or overcast. If the sky is totally obscured for example by fog, then the
ceiling is defined by vertical visibility.
Chinook
A type of foehn wind. Refers to the warm
downslope wind in the Rocky Mountains that may occur after an intense cold spell
when the temperatures may rise 20-40 degrees in a matter of minutes.
Cirrus
High clouds usually above 18,000 feet, composed of
ice crystals
Climate
The historical record of average daily and
seasonal weather events. Statistics are generally drawn over several decades.
The word is derived from the Greek "klima" meaning inclination, and reflects the
importance early scholars attributed to the sun's influence.
Climate Prediction Center
A branch of the National Centers for Environmental
Prediction. The center maintains a continuous watch on short-term climate
fluctuations and diagnoses and predicts them.
Climatology
The study of climate. Includes climatic
data, the analysis of the causes of the differences in climate, and the
application of climatic data to the solution of specific design or operational
problems.
Cloudburst
A sudden, heavy rainfall of a showery nature.
Related
Coalescence
The merging of two water drops into a single
larger drop.
Cold Air Funnel
Funnel clouds, usually short-lived, that
develop from relatively small showers or thunderstorms when the air aloft is
very cold. Cold air funnels tend to touch down briefly, but in general are less
violent than most other types of tornadoes.
Cold Front
The leading edge of an advancing cold air
mass that is under running and displacing the warmer air in its path. Generally,
when a cold front passes the temperature and humidity decrease, the pressure
rises, and the wind shifts from southwest to northwest. Precipitation is usually
along or ahead of the front in the form of thunderstorms.
Condensation
The process by which water vapor undergoes
a change in state from a gas to a liquid. It's opposite is evaporation.
Convection
Motions in a fluid that transport and mix the
properties of the fluid. These properties could be heat and/or moisture. Often
the term convection is used to describe upward motion of water vapor (moisture)
forced to rise by surface heating in turn creating rain or thunderstorms
Convergence
Wind movement that results in a horizontal
net inflow of air into a particular region. Convergent winds at lower levels are
associated with upward motion.
Coriolis Force
A force per unit mass that arises solely
from the earth's rotation, acting as a deflecting force. It is dependent on the
latitude and speed of the moving object. In the Northern Hemisphere the air is
deflected to the right, and in the Southern Hemisphere to the left. The coriolis
effect is almost non-existent at the equator.
Cumulonimbus Cloud
A vertically developed cloud, often
capped by an anvil shaped cloud. This cloud is otherwise known as a
thundercloud. A cumulonimbus cloud can produce tornadoes, hail, lightning,
strong winds and heavy rain.
Cyclone
An area of closed pressure circulation with
rotating and converging winds. The circulation is counterclockwise in the
Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low
pressure system and the term used for tropical cyclone in the Indian Ocean .
Other phenomena with cyclonic flow may be referred to as dust devils, tornadoes,
tropical and extratropical systems.
Data Buoys
Buoys placed throughout the Gulf of
Mexico
and along the Atlantic and Pacific coasts of the U.S. that relay information on
air and water temperature, air pressure, wind speed and wave conditions via
radio signals.
Depression
In meteorology it is another name for an area
of low pressure, a low or trough. It also applies to the initial stage of a
developing tropical cyclone.
Dew
Condensation in the form of small water drops that
form on grass and other object near the ground when the temperature has fallen to
the dewpoint. Dew generally forms during the nighttime hours and evaporates by
mid to late morning.
Dewpoint
The temperature to which air must be cooled at
a constant pressure to become saturated. Example: If the air temperature is 70
degrees and the dewpoint temperature is 70 degrees the air is saturated and dew
will form i.e., the relative humidity is 100%
Difluence
A rate at which wind flow spreads apart along
an axis oriented normal to the flow in question.
Divergence
Wind movement that results in a horizontal
net outflow of air from a particular region. Divergence at lower levels is
associated with a downward movement of air from aloft.
Doppler Radar
Weather radar that measures the direction
and speed of a moving object, such as drops of precipitation, by determining
whether atmospheric motion is horizontally toward or away from the radar.
Downburst
A severe localized downdraft from a
thunderstorm or shower. This outward burst of cool air creates damaging winds at
or near the surface.
Drought
Abnormal, dry weather for a specific area that
is prolonged and causes serious hydrological imbalance.
Dry Line
The boundary between the dry desert air mass of
the southwestern U.S. and moist air mass from the Gulf of Mexico . It usually
lies north-south across the central and southern High Plains states during the
spring and summer months. When a dry line passes it results in a decrease in
humidity, clearing skies, and wind shift from east/southeasterly to
west/southwesterly. Its presence influences severe weather development in the
Great Plains .
Easterlies
Usually applied to the broad patterns of
persistent winds with an easterly component, such as the easterly trade winds.
Easterly Wave
An inverted, migratory wave-like
disturbance or trough in the tropical region that moves from east to west,
generally creating only a shift in winds and rain. The low level convergence and
associated convective weather occur on the eastern side of the wave axis. It is
often associated with possible tropical cyclone development and is also known as
a tropical wave.
Echo
The energy return of a radar signal after it has
hit the target. Related
radar echo.
El Nino
The cyclical warming of East Pacific Ocean sea
water temperatures off the western coast of South America that can result in
significant changes in weather patterns in the United States and elsewhere. This
occurs when warm equatorial Pacific waters move in and displace the colder
waters, cutting off the upwelling process.
Environment
The sum total of all the external conditions
that effect an organism, community, material, or energy.
Equator
The geographic circle at 0 degrees latitude on
the earth's surface. It is equal distance from the North and South Poles and
divides the Northern Hemisphere from the Southern.
Equinox
The point at which the ecliptic intersects the
celestial equator. Days and nights are most nearly equal in duration. In the
Northern Hemisphere, the vernal equinox falls on or about March 20 and the
autumnal equinox on or about September 22.
Evaporation
The physical process by which a liquid, such
as water is transformed into a gaseous state, such as water vapor. It is the
opposite physical process of condensation.
Evapotranspiration
The total amount of water that is
transferred from the earth's surface to the atmosphere. It is made up of the
evaporation of liquid or solid water plus the transpiration from plants.
Extratropical Cyclone
Any cyclone not of tropical
origin. Generally considered to be a migratory frontal cyclone found in the
middle and high latitudes.
Eye
The center of a tropical storm or hurricane,
characterized by a roughly circular area of light winds and rain-free skies. An
eye will usually develop when the maximum sustained wind speeds exceed 78 mph.
It can range in size from as small as 5 miles up to 60 miles, but the average
size is 20 miles. In general, when the eye begins to shrink in size, the storm
is intensifying.
Eyewall
An organized band of convection surrounding the
eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense
rainfall and very strong winds.
Fahrenheit Temperature Scale
A temperature scale where
water at sea level has a freezing point of +32 degrees F and a boiling point of
+212 degrees F. More commonly used in areas that observe the English system of
measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German
physicist, who also invented the alcohol and mercury thermometers.
Fair
This is a subjective description. Considered as
pleasant weather conditions with regard to the time of year and the physical
conditions.
Feeder Bands
In tropical parlance, the lines or bands of
thunderstorms that spiral into and around the center of a tropical system. Also
known as outer convective bands, a typical hurricane may have three or more of
these bands. They occur in advance of the main rain shield and are usually 40 to
80 miles apart. In thunderstorm development, they are the lines or bands of low
level clouds that move or feed into the updraft region of a thunderstorm.
Flanking Line
A line of attached cumulus or towering
cumulus clouds of descending height, appearing as stair steps (usually on the
southwest side) of the most active part of a supercell.
Flash Flood
A flood that rises and falls quite rapidly
with little or no advance warning, usually as the result of intense rainfall
over a relatively small area. Flash floods can be caused by situations such as a
sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.
Flood
High water flow or an overflow of rivers or
streams from their natural or artificial banks, inundating adjacent low lying
areas.
Foehn
A warm dry wind on the lee side of a mountain
range, whose temperature is increased as the wind descends down the slope. It is
created when air flows downhill from a high elevation, raising the temperature
by adiabatic compression. Classified as a katabatic wind.
Fog
A visible aggregate of minute water droplets
suspended in the atmosphere at or near the surface of the earth, reducing
horizontal visibility to less than 5/8 statute miles. It is created when the
temperature and the dew point of the air have become the same, or nearly the
same, and sufficient condensation nuclei are present. It is reported as "FG" in
an observation and on the METAR.
Forecast
A statement of expected future occurrences.
Weather forecasting includes the use of objective models based on certain
atmospheric parameters, along with the skill and experience of a meteorologist.
Freezing Drizzle
Drizzle, falling as a liquid, but
freezing on impact with the colder ground or other exposed surfaces. It is
reported as "FZDZ" in an observation and on the METAR.
Freezing Rain
Rain that falls as liquid and freezes upon
impact to form a coating of glaze on the colder ground or other exposed
surfaces. It is reported as "FZRA" in an observation and on the METAR.
Front
The transition zone or interface between two air
masses of different densities, which usually means different temperatures. For
example, the area of convergence between warm, moist air and cool, dry air.
Frost
The covering of ice crystals that forms by direct
sublimation on exposed surfaces whose temperature is below freezing.
Fujita-Pearson Scale
A scale that classifies the
severity of wind damage intensity based on the degree of destruction as it
relates to the wind speed as well as path length and path width of the event. It
is normally used to identify the most intense damage exhibited by a tornado.
Developed by T. Theodore Fujita and Allen Pearson.
Funnel Cloud
A violent, rotating column of air visibly
extending from the base of a towering cumulus or cumulonimbus toward the ground,
but not in contact with it.
Geostationary Satellite
An orbiting weather satellite
that maintains the same position over the equator during the earth's rotation.
Also known as GOES, an acronym for Geostationary Operational Environmental
Satellite.
Geostrophic Wind
A steady horizontal motion of air along
straight, parallel isobars or contours in an unchanging pressure or contour
field. It is assumed that there is no friction, that the flow is straight with
no curvature and there is no divergence or convergence with no vertical
acceleration.
Graupel
A form of frozen precipitation consisting of
snowflakes or ice crystals and supercooled water droplets frozen together.
Gravity
The force of attraction of the earth on an
object. The direction is downward relative to the earth, and it decreases with
elevation or altitude away from the earth's surface.
Green Flash
A brilliant green coloration of the upper
edge of the sun, occasionally seen as the sun's apparent disk is about to set
below a clear horizon.
Greenhouse Effect
The overall warming of the earth's
lower atmosphere primarily due to carbon dioxide and water vapor which permit
the sun's rays to heat the earth, but then restrict some heat-energy from
escaping back into space.
Greenwich Mean Time (GMT)
The name of the twenty-four
hour time scale which is used throughout the scientific and military
communities. This is the Prime Meridian of Longitude. The globe is divided into
twenty-four time zones of 15 degrees of arc, or one hour in time apart. To the
east of this meridian, time zones are numbered from 1 to 12 and prefixed with a
minus (-), indicting the number of hours to be subtracted to obtain Greenwich
Time (GMT). To the west, the time zones are also numbered 1 through 12, but are
prefixed with a plus (+), indicating the number of hours to be added to obtain
GMT.
Ground Clutter
A pattern of radar echoes reflecting off
fixed ground targets such as buildings or hills near the radar. This may hide or
confuse the proper return echo signifying actual precipitation.
Gulf Stream
The warm, well-defined, swift, relatively
narrow ocean current which exists off the east coast of the United States ,
beginning near Cape Hatteras . The term also applies to the oceanic system of
currents that dominate the western and northern Atlantic Ocean: the Florida
current, which flows through the Florida Straits between the Florida Keys and
Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and
flows northeasterly off the continental slope into the North Atlantic; and the
North Atlantic current, which begins around the Grand Banks off Newfoundland and
continues east-northeastwards towards the British Isles.
Gust
A sudden significant increase in or rapid
fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per
hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles
per hour). The duration is usually less than twenty seconds.
Gust Front
The leading edge of the cool, gusty surface
winds produced by thunderstorm downdrafts. Sometimes confused with an outflow
boundary.
Gustnado
A weak, and usually short-lived, tornado that
forms along the gust front of a thunderstorm, appearing as a temporary dust
whirl or debris cloud.
Hail
Precipitation that originates in convective clouds,
such as cumulonimbus, in the form of balls or irregular pieces of ice, which
comes in different shapes and sizes. Hail is considered to have a diameter of 5
millimeter or more; smaller bits of ice are classified as ice pellets, snow
pellets, or graupel. Individual lumps are called hailstones.
Haze
A suspension of fine dust and/or smoke particles in
the air. Invisible to the naked eye, the particles reduce visibility by being
sufficiently numerous to give the air an opalescent appearance.
Heat
A form of energy transferred between two systems by
virtue of a difference in temperature. The first law of thermodynamics
demonstrated that the heat absorbed by a system may be used by the system to do
work or to raise its internal energy.
Heat Exhaustion
The effect of excessive heat,
particularly when combined with high humidity on a human being. Signs of heat
exhaustion include a general weakness, heavy sweating and clammy skin, dizziness
and/or fainting, and muscle cramps.
heat index
The combination of air temperature and
humidity that gives a description of how the temperature feels. This is not the
actual air temperature.
Heat Lightning
Lightning that appears as a glowing flash
on the horizon. It is actually lightning occurring in distant thunderstorms,
just over the horizon and too far away for thunder to be heard.
Heat Stroke
Introduced to the body by overexposure to
high temperatures, particularly when accompanied by high humidity. The signs of
heat stroke include when an individual's body temperature is greater than 105
degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular
pulse, perspiration has stopped, and one has lost consciousness. Seek immediate
medical aid. May be called a sun-stroke when caused by direct exposure to the
sun.
Heat Wave
A period of abnormally and uncomfortably hot
weather. It could last from several days to several weeks.
heating degree day
One heating degree day is given for
each degree that the daily mean temperature is below 65 degrees Fahrenheit. It
is used as an indication of fuel consumption.
High Clouds
A term used to signify cirriform clouds that
are composed of ice crystals and generally have bases above 20,000 feet. The
main types of high clouds are cirrus, cirrocumulus, and cirrostratus. This
altitude applies to the temperate zone. In the polar regions, these clouds may
be found at lower altitudes. In the tropics, the defining altitudes for cloud
types are generally higher.
High Pressure System
An area of relative pressure
maximum that has diverging winds and a rotation opposite to the earth's
rotation. This is clockwise in the Northern Hemisphere and counterclockwise in
the Southern Hemisphere. It is the opposite of an area of low pressure or a
cyclone.
Hook Echo
A radar reflectivity pattern observed in a
thunderstorm, appearing like a fish hook and indicating favorable conditions for
tornadic development. However, hook echoes and tornadoes do not always accompany
each other.
Humidity
The amount of water vapor in the air. It is
often confused with relative humidity or dew point.
Hurricane
The name for a tropical cyclone with sustained
winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean,
Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same
tropical cyclone is known as a typhoon in the western Pacific and a cyclone in
the Indian Ocean .
Hurricane Warning
A formal advisory issued by
forecasters at the National Hurricane Center when they have determined that
hurricane conditions are expected in a coastal area or group of islands within a
24 hour period. A warning is used to inform the public and marine interests of
the storm's location, intensity, and movement.
Hurricane Watch
A formal advisory issued by forecasters
at the National Hurricane Center when they have determined that hurricane
conditions are a potential threat to a coastal area or group of islands within a
24 to 36 hour period. A watch is used to inform the public and marine interests
of the storm's location, intensity, and movement.
Hydrologic Cycle
Often called the water cycle, it is the
vertical and horizontal transport of water in all its states between the earth,
the atmosphere, and the seas.
Hydrometeor
Any form of atmospheric water vapor,
including those blown by the wind off the earth's surface. Liquid or solid water
formation that is suspended in the air includes clouds, fog, ice fog, and mist.
Drizzle and rain are examples of liquid precipitation, while freezing drizzle
and freezing rain are examples of freezing precipitation. Solid or frozen
precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and
ice crystals. Water vapor that evaporates before reaching the ground is virga.
Examples of liquid or solid water particles that are lifted off the earth's
surface by the wind includes drifting and blowing snow and blowing spray. Dew,
frost, rime, and glaze are examples of liquid or solid water deposits on exposed
objects.
Hygrometer
An instrument that measures the water vapor
content of the atmosphere.
Hypothermia
This situation occurs when the core
temperature of one's body falls below normal. It is the failure of the body to
maintain adequate production of heat under conditions of extreme cold.
Ice
Water in a solid state. It can be found in the
atmosphere in the form of ice crystals, snow, ice pellets, and hail.
Inches of Mercury
The name comes from the use of
mercurial barometers which equate the height of a column of mercury with air
pressure. One inch of mercury is equivalent to 33.86 millibars. First devised in
1644 by Evangelista Torricelli, an Italian physicist and mathematician, to
explain the fundamental principles of hydromechanics.
Infrared
The long wave, electromagnetic radiation of
radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can
be found between microwave radiation and visible light. Water vapor, ozone, and
carbon dioxide are efficient at absorbing or transmitting infrared radiation.
Insolation
Solar radiation or heating received at the
earth's surface. The name is derived from INcoming SOLar radiATION.
Instability
It is the condition of the atmosphere when
spontaneous convection and severe weather can occur. Air parcels, when displaced
vertically, will accelerate upward.
Instrument Flight Rules (IFR)
Refers to the general
weather conditions a pilot can expect at the surface and applies to the weather
situations at an airport during which a pilot must use instruments to assist in
take off and landing.
Instrument Shelter
A boxlike structure designed to
protect temperature measuring instruments from exposure to direct sunshine,
precipitation, and condensation, while at the same time providing adequate
ventilation.
Intertropical Convergence Zone (ITCZ)
An area where the
Northern and Southern Hemispheric trade winds converge. It is a broad area of
low pressure where both the coriolis force and the low-level pressure gradient
are weak, occasionally allowing tropical disturbances to form. The ITCZ
fluctuates, moving northward over the south Atlantic during the Northern
Hemisphere summer.
Inversion
It refers to an increase in an atmospheric
property with height. For example...A temperature inversion is when the
temperature increases with altitude, which is a departure from the usual
decrease of temperature with height.
Ionosphere
An atmospheric zone of ionized gases that
extends between 50 and 400 miles above the surface of the Earth. It is located
between the mesosphere and the exosphere.
Isallobar
The line of equal change in atmospheric
pressure during a certain time period. It marks the change in pressure
tendency.
Isobar
The line drawn on a weather map connecting points
of equal barometric pressure.
Isodrosotherm
The line drawn on a weather map connecting
points of equal dew point.
Isohyet
The line drawn through geographic points
recording equal amounts of rainfall during a given time or for a give storm.
Isopleth
A line connecting equal points of value.
Isotach
A line connecting equal wind speeds.
Isotherm
The line of equal or constant temperature.
Jetstreak
A region of accelerated wind speed along the
axis of a jet stream.
Jetstream
A area of strong winds that are concentrated
in a relatively narrow band in the upper troposphere of the Northern and
Southern Hemispheres. Flowing in a semi-continuous band around the globe from
west to east, it is caused by the changes in air temperature where the cold
polar air moving towards the equator meets the warmer equatorial air moving
northward toward the poles.
K Index
The measure of thunderstorm potential based on
the vertical temperature lapse rate, the moisture content of the lower atmosphere
and the vertical extent of the moist layer.
Katabatic Wind
A wind that is created by air flowing
downhill. When the air is warm, it may be called a foehn wind, and regionally
may be known as a Chinook, or Santa Ana . When this air is cool, it is called a
drainage wind, mountain breeze or glacier wind.
Knot
A nautical unit of wind speed equal to the velocity
at which one nautical mile is traveled in one hour. Used primarily by marine
interests and in weather observations. 1 knot = 1.151 statute miles per hour.
Land Breeze
A diurnal coastal breeze that blows
offshore, from the land to the sea. It is caused by the temperature difference
when the sea surface is warmer than the adjacent land. Predominate during the
night, it reaches its maximum around dawn.
Lapse Rate
The change of an atmospheric variable,
usually temperature, with height. A steep lapse rate implies a rapid decrease in
temperature with height. This is a sign of instability.
Latitude
The location north or south in reference to the
equator, which is designated at zero degrees. Parallel lines that circle the
globe both north and south of the equator. The poles are at 90 degrees North and
South latitude.
Lee/Leeside
The side of an object, such as a ship's
sail, a mountain, or a hill, furthest away from the wind, and therefore
protected from the direct force of the wind.
Lifted Index
A measure of atmospheric instability that
is obtained by computing the temperature that the air near the ground would have
if it were lifted to a higher level and compared to the actual temperature at
that altitude. Positive values indicate more stable air and negative values
indicate instability.
Lightning
A rapid, visible discharge of electricity
hotter than the surface of the sun. Lightning is caused by the build up of
electrical potential between cloud and ground, between clouds, or between clouds
and the surrounding air.
Longitude
The location east or west in reference to the
Prime Meridian, which is designated at 0 degrees longitude. The distance between
lines of longitude are greater at the equator and smaller at the higher
latitudes. Time zones are correlated to longitude.
Low Clouds
A term used to describe clouds with bases
below 6,000 feet. Types of low clouds include stratus, stratocumulus, cumulus
and cumulonimbus.
Low Level Jet
Strong winds that are
concentrated in
relatively narrow bands in the lower part of the atmosphere. It is often
amplified at night.
Low Pressure System
An area of a relative pressure
minimum that has converging winds and rotates in the same direction as the
Earth...counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere. Also known as a cyclone.
Lunar Eclipse
A eclipse of the moon occurs when the
Earth is in a direct line between the sun and the moon. The moon does not have
any light of its own, instead it reflects the sun's light. During a lunar
eclipse the moon is in the earth's shadow.
Macroburst
A large downburst
with an outflow diameter of
2.5miles or larger and damaging winds.
Macroscale
The meteorological scale covering an area
ranging from the size of a continent to the entire globe.
Mare's Tail
The name given to thin, wispy cirrus clouds
composed of ice crystals that appear as veil patches of strands, often
resembling a horse's tail.
Mean Sea Level
The
average height of the sea surface water level. For the United States , it is
computed by averaging the levels of all tide stages over a nineteen year period,
determined from hourly height readings measured from a fixed predetermined
reference level. It is used as a basis for determining elevations, as the
reference for all altitudes in upper air measurements, and as the level above
which altitude is measure by a pressure altimeter for aviation. Often referred
to as MSL
Mean Sea Level
The
average height of the sea surface water level. For the United States , it is
computed by averaging the levels of all tide stages over a nineteen year period,
determined from hourly height readings measured from a fix, predetermined
reference level. It is used as a basis for determining elevations, as the
reference for all altitudes in upper air measurements, and as the level above
which altitude is measured by a pressure altimeter for aviation. Often referred
to as MSL.
Mercurial Barometer
An instrument used for measuring the
change in atmospheric pressure. It uses a long glass tube, open at one end and
closed at the other. After first filling the open end with mercury, it is then
temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum
is established at the closed end after the mercury descends. The height of the
column of mercury in the tube is a measurement of air pressure. As atmospheric
pressure increases, the mercury is forced from the cistern up the tube; when the
atmospheric pressure decreases, the mercury flows back into the cistern.
Measurement is taken in inches of mercury. First used by Evangelista Torricelli
(1608-1647), an Italian physicist and mathematician, to explain the fundamental
principles of hydromechanics.
Mesocyclone
An area of rotation of storm size that may
often be found on the southwest part of a supercell. Its circulation can be
larger than the tornado that may develop within it, but not necessarily.
Originally a radar term for a rotation signature that met certain criteria, it
is best seen on Doppler radar.
Mesoscale
The scale of meteorological phenomena that
range in size from several kilometers to around 100 kilometers. This includes
MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale
while larger are classified as synoptic-scale.
Mesoscale Convective Complex (MCC)
A large mesoscale
convective system (MCS) which is about the size of the state of Ohio or Iowa and
lasts at least 6 hours. Generally forming during the afternoon and evening, the
complex normally reaches its peak intensity at night when heavy rainfall and
flooding become the primary threat. Severe weather may occur at anytime.
Mesoscale Convective System (MCS)
A large organized
convective weather system comprised of a number of individual thunderstorms. It
normally persists of several hours and may be rounded or linear in shape. This
term is often used to describe a cluster of thunderstorms that does not meet the
criteria of a mesoscale convective complex (MCC).
Metar
Acronym for METeorological Aerodrome Report. It is
the primary observation code used in the United States to satisfy requirements
for reporting surface meteorological data. Minimum reporting requirements
includes wind, visibility, runway visual range, present weather, sky condition,
temperature, dew point, and altimeter setting.
Meteorology/Meteorologist
The science and study of the
atmosphere and atmospheric phenomena. Various areas of meteorology include
agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology,
operational, and synoptic, to name a few. A scientist who studies the atmosphere
and atmospheric phenomena.
Microburst
A severe localized wind blasting down from a
thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter
and is of short duration, usually less than 5 minutes.
Microscale
The smallest scale of meteorological
phenomena that range in size from a few centimeters to a few kilometers. Larger
phenomena are classified as mesoscale. It also refers to small scale
meteorological phenomena with life spans of less than a few minutes that affect
very small areas and are strongly influenced by local conditions of temperature
and terrain.
Middle Clouds
A term used to signify clouds with bases
between 6,000 and 18,000 feet. At the higher altitudes, they may also have some
ice crystals, but they are composed mainly of water droplets. Altocumulus,
altostratus, and nimbostratus are the main types of middle clouds. This altitude
applies to the temperate zone. In the polar regions, these clouds may be found
at lower altitudes. In the tropics, the defining altitudes for cloud types are
generally higher.
Millibar (MB)
The standard unit of measurement for
atmospheric pressure used by the National Weather Service. One millibar is
equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2
millibars.
Mixed Precipitation
Any of the following combinations of
freezing and frozen precipitation: snow and sleet, snow and freezing rain, or
sleet alone. Rain may also be present.
Moisture
Refers to the water vapor content in the
atmosphere, or the total water, liquid, solid or vapor, in a given volume of
air.
Monsoon
The seasonal shift of winds created by the great
annual temperature variation that occurs over large areas in contrast with
associated ocean surfaces. The monsoon is associated primarily with the moisture
and copious rains that arrive with the southwest flow across southern India .
The name is derived from the word mausim, Arabic for season. This pattern is
most evident on the southern and eastern sides of Asia, although it does occur
elsewhere, such as in the southwestern United States .
Multicell Storm
A thunderstorm made up of two or more
single-cell storms.
Multiple Vortex Tornado
A tornado which has two or more
condensation funnels or debris clouds, often rotating around a common center.
National Center for Atmospheric
Research (NCAR)
A division of the University Corporation for
Atmospheric Research, the Center plans, organizes, and conducts atmospheric and
related research programs in collaboration with universities. For further
information, contact NCAR, located in Boulder , Colorado .
National Centers for Environmental Prediction (NCEP)
As
part of the National Weather Service, the centers provide timely, accurate, and
continually improving worldwide forecast guidance products. Some of the centers
include the Aviation Weather Center , the Climate Prediction Center , the Storm
Prediction Center , and the Tropical Prediction Center . Formerly known as NMC.
For further information, contact the NCEP, with central offices located in
Silver Spring , Maryland .
National Climatic Data Center (NCDC)
The agency that
archives climatic data from the National Oceanic and Atmospheric Administration
as well as other climatological organizations. For further information, contact
the NCDC, located in Asheville , North Carolina .
National Hurricane Center (NHC)
A
branch of the Tropical Prediction Center , it is the office of the National
Weather Service that is responsible for tracking and forecasting tropical
cyclones over the North Atlantic, Caribbean, Gulf of Mexico , and the Eastern
Pacific. For further information, contact the NHC, located in Miami , Florida .
National Meteorological Center (NMC)
Now incorporated into the National Centers for Environmental
Prediction, it was the division of the National Weather Service that produced,
processed, handled, and distributed meteorological and oceanographic information
to users throughout the Northern Hemisphere, specifically U.S. governmental
organizations.
National Oceanic and Atmospheric Administration (NOAA)
A
branch of the U.S. Department of Commerce, it is the parent organization of the
National Weather Service. It promotes global environmental stewardship,
emphasizing atmospheric and marine resources. For further information, contact
NOAA, located in Silver Spring , Maryland .
National Severe Storms Forecast Center (NSSFC)
As of October 1995, the responsibilities of this Center were
divided into two branches, the Storm Prediction Center and the Aviation Weather
Center .
National Severe Storms Laboratory (NSSL)
A branch of the
National Oceanic and Atmospheric Administration, it provides accurate and timely
forecasts and warnings of hazardous weather events, especially flash floods,
hail, lightning, tornadoes, and other severe wind storms. For further
information, contact the NSSL, headquartered in Norman , Oklahoma .
National Weather Association (NWA)
An organization whose
membership promotes excellence in operational meteorology and related
activities, recognizing the professional as well as the volunteer. For further
information, contact the NWA.
National Weather Service (NWS)
A primary branch of the
National Oceanic and Atmospheric Administration, it is responsible for all
aspects of observing and forecasting atmospheric conditions and their
consequences, including severe weather and flood warnings. For further
information, contact the NWS.
Nimbostratus
This cloud exhibits a combination of rain
or snow, and sometimes the base of the cloud cannot be seen because of the
heaviness of precipitation. They are generally associated with fall and winter
conditions, but can occur during any season.
Nitrogen
A colorless, tasteless, odorless gas that is
the most abundant constituent of dry air. It comprises 78.09%.
Nor'easter
A cyclonic storm occurring off the east coast
of North America . These winter weather events are notorious for producing heavy
snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing
beach erosion and structural damage. Wind gusts associated with these storms can
exceed hurricane force in intensity. A nor'easter gets its name from the
continuously strong northeasterly winds blowing in from the ocean ahead of the
storm and over the coastal areas.
Normal
The recognized standard value of a meteorological
element as it has been averaged in a given location over a fixed number of
years. Normals are concerned with the distribution of data within limits of
common occurrence. The parameters may include temperatures (high, low, and
deviation), pressure, precipitation (rain, snow, etc.), winds (speed and
direction), thunderstorms, amount of clouds, percent relative humidity, etc.
Numerical Forecasting
The use of numerical models, such
as the fundamental equations of hydrodynamics subjected to observed initial
conditions, to forecast the weather.
Observation
In meteorology, the evaluation of one or more
meteorological elements, like temperature, pressure or wind that describe the
state of the atmosphere at a given time. A trained observer is one who records
the evaluations of the meteorological records.
Oceanography
The study of the ocean, embracing and
integrating all knowledge pertaining to the ocean's physical boundaries, the
chemistry and physics of sea water, and marine biology.
Omega Block
A warm high aloft which has become displaced
and is on the polarward side of the jetstream. It frequently occurs in the late
winter and early spring in the Northern Hemisphere. The name comes from its
resemblance to the Greek letter, Omega, when analyzed on upper air charts.
Outflow
Also referred to as an outflow boundary. It is
the outward flow of air from a system such as a thunderstorm. It is the result
of cold downdrafts and its passage includes a wind shift and most often a
temperature drop. Outflow boundaries sometimes help produce thunderstorms as
they move into regions of instability.
Overcast
When the sky is completely covered by clouds.
Overrunning
This occurs when a relatively warm air mass
is forced above a cooler air mass of greater density. Weather generally
associated with this event include cloudiness, cool temperatures and steady
precipitation.
Ozone Layer
An atmospheric layer that contains a high
proportion of oxygen that exists as ozone. It acts as a filtering mechanism
against incoming ultraviolet radiation. It is located between the troposphere and
the stratosphere between 9.5 and 12.5 miles above the Earth's surface. Ozone at
the surface is not healthy for humans to breathe.
Palmer Drought Index
A long-term meteorological drought
severity index produced by the NOAA/USDA Joint Agricultural Weather Facility.
The index depicts prolonged times, in months or years, of abnormal dryness or
wetness. It responds slowly. Changing little from week to week, it reflects
long-term moisture runoff, recharge and deep percolation, as well as
evapotranspiration.
Partly Cloudy
The state of the weather
when clouds are
conspicuously present, but do not completely cover the sky at a given time.
Sometimes interchanged with mostly sunny.
Pilot Report
A report of in-flight weather by an
aircraft
pilot or crew member. Often referred to as a PIREP.
Polar Front
A semi-continuous, semi-permanent boundary
between polar airmasses and tropical air masses. An integral part of an early
meteorological theory known as the Polar Front Theory.
Polar-Orbiting Satellite
A satellite whose orbit passes
over both of the Earth's poles gathering cloud and temperature data.
Precipitation
Any and all forms of water, liquid or
solid, that falls from clouds and reaches the ground. This includes, drizzle,
freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow,
snow pellets, and snow grains.
Pressure
The force per unit area exerted by the weight
of the atmosphere above a point on or above the earth's surface.
Pressure Gradient
The amount of pressure change that
occurs over a fixed distance at a fixed altitude.
Prevailing Wind
A wind that blows from one direction
more frequently than any other during a given period, such as a day, month,
season, or year.
Profiler
A type of Doppler radar that typically measures
both wind speed and direction from the surface to 55,000 feet in the atmosphere.
Used to monitor winds with height in determining severe weather threat.
Psychrometer
An instrument used to measure the water
vapor content of the atmosphere. It consists of two thermometers, a wet bulb and
dry bulb. May also be referred to as a sling psychrometer.
Quasi-stationary Front
A front which is nearly
stationary and moves very little since the last synoptic position. Also known as
a stationary front.
Radar
Acronym for RAdio Detection And Ranging. An
electronic instrument used to detect distant objects and measure their range by
how they scatter or reflect radio energy. Precipitation and clouds are detected
by measuring the strength of the electromagnetic signal reflected back. Related
Terms: Doppler Radar and NEXRAD
Radiation
The process by which energy is propagated
through any medium by virtue of the wave motion of that medium. Electromagnetic
radiation, which emits heat and light, is one form. Sound waves are another.
Radiation Fog
Fog that is created when radiational
cooling at the earth's surface lowers the temperature of the air near the ground
to or below its dew point. Formation is best when there is a shallow surface
layer of relatively moist air beneath a drier layer, clear skies, and light
surface winds. This primarily occurs during the night or early morning. Related
ground fog.
Radiosonde
An instrument attached to a weather balloon
used to measure pressure, temperature, humidity, and winds aloft. Observations
are made when the radiosonde is aloft and emits radio signals as it ascends. May
be referred to as a RAOB, an acronym for RAdiosonde OBservation.
Rainbow
A luminous arc featuring all colors of the
visible light spectrum (red, orange, yellow, green, blue, indigo, and violet).
It is created by refraction, total reflection, and the dispersion of light. It
is visible when the sun is shining through air containing water spray or
raindrops, which occurs during or immediately after a rain shower. The bow is
always observed in the opposite side of the sky from the sun.
Reflectivity
A measure of the process by which a surface
can turn back a portion of incident radiation into the medium through which the
radiation approached. It also refers to the degree by which precipitation is
able to reflect a radar beam. Related albedo.
Relative Humidity
A type of humidity that considers the
ratio of the actual vapor pressure of the air to the saturation vapor pressure.
It is usually expressed in percentage.
Retrogression
In meteorology, it is the movement of a
weather system in a direction opposite to the direction of the basic flow in
which it is embedded.
Rime
The rapid freezing of supercooled water droplets as
they touch an exposed object, forming a white opaque granular deposit of ice. It
is one of the results of an ice storm, and when formed on aircraft it is called
rime icing. Related glaze
Roll Cloud
A relatively rare, low-level, horizontal,
tube-shaped cloud. Although they are associated with a thunderstorm, they are
completely detached from the base of the cumulonimbus cloud.
Rossby Waves
The movement of ridges and troughs in the
upper wind patterns, primarily the jet stream, circling the earth. Named for
Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee, who first theorized
about the existence of the jet stream in 1939.
Saffir-Simpson Damage-Potential Scale
Developed in the
early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then
Director of the National Hurricane Center , it is a measure of hurricane
intensity on a scale of 1 to 5. The scale categorizes potential damage based on
barometric pressure, wind speeds, and surge. Related Saffir Simpson Scale
Salinity
A measure of the quantity of dissolved salts in
sea water. The total amount of dissolved solids in sea water in parts per
thousand by weight.
Santa Ana Winds
The hot, dry winds,
generally from the east, that funnel through the Santa Ana river valley south of
the San Gabriel and San Bernadino Mountains in southern California , including
the Los Angeles basin. Classified as katabatic, it occurs most often during the
winter and it is an example of a foehn wind.
Satellite
Any object that orbits a celestial body, such
as a moon. However, the term is often used in reference to the manufactured
objects that orbit the earth, either in a geostationary or a polar manner. Some
of the information that is gathered by weather satellites, such as GOES9,
includes upper air temperatures and humidity, recording the temperatures of
cloud tops, land, and ocean, monitoring the movement of clouds to determine
upper level wind speeds, tracing the movement of water vapor, monitoring the sun
and solar activity, and relaying data from weather instruments around the world.
Saturation Point
The point when the water vapor n the
atmosphere is at its maximum level for the existing temperature.
Scattering
The process by which small particles
suspended in the air diffuse a portion of the incident radiation in all
directions. This is a primary reason for colors, such as blue skies, rainbows,
and orange sunsets.
Scud
Low fragments of clouds, usually stratus fractus,
that are unattached and below a layer of higher clouds, either nimbostratus or
cumulonimbus. They are often along and behind cold fronts and gust fronts, being
associated with cool moist air, such as an outflow from a thunderstorm. When
observed from a distance, they are sometimes mistaken for tornadoes.
Sea Breeze
A diurnal coastal breeze that blows onshore,
from the sea to the land. It is caused by the temperature difference when the
surface of the land is warmer than the adjacent body of water. Predominate
during the day, it reaches its maximum early to mid afternoon. It blows in the
opposite direction of a land breeze.
Sea Breeze Front
A coastal phenomena, it is restricted
to large bodies of water and their immediate coast lines. This is usually the
landward extent of the sea breeze. Due to the imbalance of heating between land
and water, a region of maximum upward motion or convergence occurs by
mid-afternoon in the summer some 10 to 15 miles inland.
Sea Fog
A type of advection fog which forms in warm
moist air cooled to saturation as the air moves across cold water. Related
Arctic Sea Smoke
Sea Level
The height or level of the sea surface at any
time. It is used as a reference for elevations above and below. Related mean sea
level
Sea Level Pressure
The atmospheric pressure at mean sea
level, usually determined from the observed station pressure.
Severe Thunderstorm
A thunderstorm with winds measuring
50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornadoes. Severe
thunderstorms may also produce torrential rain and frequent lightning. Related
supercell
Severe Weather
Generally, any destructive weather event,
but usually applies to localized storms, such as blizzards, intense
thunderstorms, or tornadoes.
Shear
It is the rate of change over a short duration. In
wind shear, it can refer to the frequent change in wind speed within a short
distance. It can occur vertically or horizontally. Directional shear is a
frequent change in direction within a short distance, which can also occur
vertically or horizontally. When used in reference to Doppler radar, it
describes the change in radial velocity over short distances horizontally.
Short Wave
A progressive wave of smaller amplitude, wave
length, and duration than a long wave. It moves in the same direction as the
basic current in which it is embedded and may induce upward vertical motion
ahead of it. They are more numerous than long waves and often disappear with
height in the atmosphere.
Skew T-Log P Diagram
A thermodynamic diagram, using the
temperature and the logarithm of pressure as coordinates. It is used to evaluate
and forecast air parcel properties. Some values that can be determined are the
Convective Condensation Level (CCL), the Lifting Condensation Level (LCL), and
the Level of Free Convection (LFC).
Sky Cover
The amount of the celestial dome that is
hidden by clouds and/or obscurations.
Sleet
Also known as ice pellets, it is winter
precipitation in the form of small bits or pellets of ice that rebound after
striking the ground or any other hard surface.
Small Craft Advisory
An advisory issued for marine
interests, especially for operators of small boats or other vessels. Conditions
include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).
Snow
Frozen precipitation in the form of white or
translucent ice crystals in complex branched hexagonal form. It most often falls
from stratiform clouds, but can fall as snow showers from cumuliform ones. It
usually appears clustered into snowflakes.
snow flakes
An ice crystal or an aggregate of ice
crystals which fall from clouds.
Snow Flurry/Flurries
Light showers of snow, generally
very brief without any measurable accumulation.
Snow Flurry/Flurries
Light showers of snow, generally
very brief without any measurable accumulation.
Snow Grains
Frozen precipitation in the form of very
small, white, opaque grains of ice. The solid equivalent of drizzle.
Snow Grains
Frozen precipitation in the form of very
small, white, opaque grains of ice. The solid equivalent of drizzle.
Snow Pellets
Frozen precipitation in the form of white,
round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2
inch (2 to 5 mm). They are easily crushed and generally break up after
rebounding from a hard surface, unlike hail. Sometimes it is called small or
soft hail.
Snow Pellets
Frozen precipitation in the form of white,
round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2
inch (2 to 5 mm). They are easily crushed and generally break up after
rebounding from a hard surface, unlike hail. Sometimes it is called small or
soft hail.
Snowflakes
An ice crystal or an aggregate of ice
crystals which fall from clouds.
Solar Eclipse
An eclipse of the sun occurs when the moon
is in a direct line between the sun and the earth, casting some of the earth's
surface in its shadow. The moon's disk shaped outline appears to cover the sun's
brighter surface, or photosphere. That part of the earth that is directly in the
moon's shadow will see a total eclipse of the sun, while the areas around it
will see a partial eclipse.
Solar Eclipse
An eclipse of the sun occurs when the moon
is in a direct line between the sun and the earth, casting some of the earth's
surface in its shadow. The moon's disk shaped outline appears to cover the sun's
brighter surface, or photosphere. That part of the earth that is directly in the
moon's shadow will see a total eclipse of the sun, while the areas around it
will see a partial eclipse.
Solstice
The point at which the sun is the furthest on
the ecliptic from the celestial equator. The point at which sun is at maximum
distance from the equator and days and nights are most unequal in duration. The
Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude
which lies directly beneath a solstice. In the Northern Hemisphere, the winter
solstice falls on or about December 21 and the summer solstice on or about June
21.
Southern Oscillation
A periodic reversal of the pressure
pattern across the tropical Pacific Ocean during El Nino events. It represents
the distribution of temperature and pressure over an oceanic area.
Spring
The season of the year which occurs as the sun
approaches the summer solstice, and characterized by increasing temperatures in
the mid-latitudes. Customarily, this refers to the months of March, April, and
May in the Northern Hemisphere, and the months of September, October, and
November in the Southern Hemisphere. Astronomically, this is the period between
the vernal equinox and the summer solstice.
Squall
A sudden onset of strong winds with speeds
increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more
knots (25 miles per hour) for at least one minute. The intensity and duration is
longer than that of a gust.
Squall Line
A narrow band of line of active
thunderstorms that is not associated with a cold front. It may form from an
outflow boundary or the leading edge of a mesohigh.
Station Pressure
The atmospheric pressure with respect
to the station elevation.
Stationary Front
A front which is nearly stationary or
moves very little since the last synoptic position. May be known as
quasi-stationary front.
Storm Prediction Center (SPC)
A branch of the National
Centers for Environmental Prediction, the Center monitors and forecasts severe
and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena
across the United States. Formerly known as the Severe Local Storms (SELS) unit
of the National Severe Storms Forecast Center .
Storm Winds
On the Beaufort Wind Scale, a wind with
speeds from 56 to 63 knots (64 to 72 miles per hour).
Straight-Line Winds
Any surface wind that is not
associated with rotation. An example is the first gust from a thunderstorm, as
opposed to tornadic winds.
Stratiform
Clouds composed of water droplets that
exhibit no or have very little vertical development. The density of the droplets
often blocks sunlight, casting shadows on the earth's surface. Bases of these
clouds are generally no more than 6,000 feet above the ground. They are
classified as low clouds, and include all varieties of stratus and
stratocumulus.
Stratocumulus
A low cloud composed of layers or patches
of cloud elements. It can form from cumulus clouds becoming more stratiformed
and often appears as regularly arranged elements that may be tessellated,
rounded, or roll-shaped with relatively flat tops and bases. It is light or dark
gray in color, depending on the size of the water droplets and the amount of
sunlight that is passing through them.
Stratopause
The boundary zone or transition layer
between the stratosphere and the mesosphere. Characterized by a decrease in
temperature with increasing altitude.
Stratosphere
The layer of the atmosphere located between
the troposphere and the mesosphere, characterized by a slight temperature
increase and absence of clouds. It extends between 11 and 31 miles (17 to 50
kilometers) above the earth's surface. It is the location of the earth's ozone
layer.
Stratus
One of the three basic cloud forms (the others
are cirrus and cumulus). It is also one of the two low cloud types. It is a
sheetlike cloud that does not exhibit individual elements, and is, perhaps, the
most common of all low clouds. Thick and gray, it is seen in low, uniform layers
and rarely extends higher than 5,000 feet above the earth's surface.
Sublimation
The process of a solid (ice) changing
directly into a gas (water vapor), or water vapor changing directly into ice, at
the same temperature, without ever going through the liquid state (water).
Subsidence
A sinking or downward motion of air, often
seen in anticyclones. It is most prevalent when there is colder, denser air
aloft. It is often used to imply the opposite of atmospheric convection.
Subtropical
The region between the tropical and
temperate regions, an area between 35 and 40 degrees North and South latitude.
This is generally an area of semi-permanent high pressure that exists and is
where the Azores and North Pacific Highs may be found.
Subtropical Jet
Marked by a concentration of isotherms
and vertical shear, this jet is the boundary between the subtropical air and the
tropical air. It is found approximately between 25 and 35 degrees North latitude
and usually above an altitude of 40,000 feet. Its position tends to migrate
south in the Northern Hemispheric winter and north in the summer.
Summer
Astronomically, this is the period between the
summer solstice and the autumnal equinox. It is characterized as having the
warmest temperatures of the year, except in some tropical regions. Customarily,
this refers to the months of June, July, and August in the North Hemisphere, and
the months of December, January, and February in the South Hemisphere.
Supercell
A severe thunderstorm characterized by a
rotating, long-lived, intense updraft. Although not very common, they produce a
relatively large amount of severe weather, in particular, extremely large hail,
damaging straight-line winds, and practically all violent tornadoes.
Supercooling
The reduction of the temperature of any
liquid below the melting point of that substance's solid phase. Cooling a
substance beyond its nominal freezing point. Supercooled water is water that
remains in a liquid state when it is at a temperature that is well below
freezing. The smaller and purer the water droplets, the more likely they can
become supercooled.
Surface Boundary Layer
The lowest layer of the earth's
atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's
surface, where the wind is influenced by the friction of the earth's surface and
the objects on it.
Surge
The increase in sea water height from the level
that would normally occur were there no storm. Although the most dramatic surges
are associated with hurricanes, even smaller low pressure systems can cause a
slight increase in the sea level if the wind and fetch is just right. It is
estimated by subtracting the normal astronomic tide from the observed storm
tide.
Swell
Ocean waves that have traveled out of their
generating area. Swell characteristically exhibits a more regular and longer
period and has flatter wave crests than waves within their fetch.
Synoptic Chart
Any map or chart that depicts
meteorological or atmospheric conditions over a large area at any given time.
Synoptic Scale
The size of migratory high and low
pressure systems in the lower troposphere that cover a horizontal area of
several hundred miles or more.
Temperature
The measure of molecular motion or the
degree of heat of a substance.
Terminal Doppler Weather Radar (TDWR)
Doppler radar
installed at major airports throughout the United States to detect microbursts.
Terrestrial Radiation
Long wave radiation that is
emitted by the earth back into the atmosphere. Most of it is absorbed by the
water vapor in the atmosphere, while less than ten percent is radiated directly
into space.
Thermal Low
Also known as heat low, it is an area of low
pressure due to the high temperatures caused by intensive heating at the
surface. It tends to remain stationary over its source area, with weak cyclonic
circulation.
Thermometer
An instrument used for measuring
temperature. The different scales used in meteorology are Celsius, Fahrenheit,
and Kelvin or Absolute.
Thermosphere
A thermal classification, it is the layer
of the atmosphere located between the mesosphere and out space. It is a region of
steadily increasing temperature with altitude.
Thunder
The sound emitted by rapidly expanding gases
along the channel of a lightning discharge. Over three-quarters of lightning's
electrical discharge is used in heating the gases in the atmosphere in and
immediately around the visible channel. Temperatures can rise to over 10,000
degrees Celsius in microseconds, resulting in a violent pressure wave, composed
of compression and rarefaction. The rumble of thunder is created as one's ear
catches other parts of the discharge, the part of the lightning flash nearest
registering first, then the parts further away.
Thunder Snow
A wintertime thunderstorm from which falls
snow instead of rain.
Thunderstorm
Produced by a cumulonimbus cloud, it is a
microscale event of relatively short duration characterized by thunder,
lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate
to extreme up and downdrafts, and under the most severe conditions, tornadoes.
Tide
The periodic rising and falling of the earth's
oceans and atmosphere. It is the result of the tide-producing forces of the moon
and the sun acting on the rotating earth. This propagates a wave through the
atmosphere and along the surface of the earth's waters.
Tornado
A violently rotating column of air in contact
with and extending between a convective cloud and the surface of the earth. It
is the most destructive of all storm-scale atmospheric phenomena. They can occur
anywhere in the world given the right conditions, but are most frequent in the
United States in an area bounded by the Rockies on the west and the Appalachians
in the east.
Tornado Alley
A geographic corridor in the United States
which stretches north from Texas to Nebraska and Iowa . In terms of sheer
numbers, this section of the United States receives more tornadoes than any
other.
Towering Cumulus
Another name for cumulus congestus, it
is a rapidly growing cumulus or an individual dome-shaped clouds whose height
exceeds its width. Its distinctive cauliflower top often mean showers below, but
lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.
Trace
Generally, an
immeasurable or insignificant
quantity. A precipitation amount of less than 0.005 inch.
Trade Winds
Two belts of prevailing winds that blow
easterly from the subtropical high pressure centers towards the equatorial
trough. Primarily lower level winds, they are characterized by their great
consistency of direction. In the Northern Hemisphere, the trades blow from the
northeast, and in the Southern Hemisphere, the trades blow from the southeast.
Transpiration
The process by which water in plants is
transferred as water vapor to the atmosphere.
Triple Point
The point at which any three atmospheric
boundaries meet. It is most often used to refer to the point of occlusion of an
extratropical cyclone where the cold, warm, and occluded fronts meet.
Cyclogenesis may occur at a triple point. It is also the condition of
temperature and pressure under which the gaseous, liquid, and solid forms of a
substance can exist in equilibrium.
Tropic of Cancer
The most northern point on the earth
where the sun is directly overhead, located at approximately 23.5 degrees North
latitude.
Tropic of Capricorn
The most southern point on the earth
where the sun is directly overhead, located at approximately 23.5 degrees South
latitude.
Tropical Air Mass
An air mass that forms in the tropics
or subtropics over the low latitudes. Maritime tropical air is produced over
oceans and is warm and humid, while continental tropical air is formed over arid
regions and is very hot and dry.
Tropical Cyclone
A warm core low pressure system which
develops over tropical, and sometimes subtropical, waters, and has an organized
circulation. Depending on sustained surface winds, the system is classified as a
tropical disturbance, a tropical depression, a tropical storm, or a hurricane or
typhoon.
Tropical Depression
A tropical cyclone in which the
maximum sustained surface winds are 38 miles per hour (33 knots) or less.
Characteristically having one or more closed isobars, it may form slowly from a
tropical disturbance or an easterly wave which has continued to organize.
Tropical Disturbance
An area of organized convection,
originating in the tropics and occasionally the subtropics, that maintains its
identity for 24 hours or more. It is often the first developmental stage of any
subsequent tropical depression, tropical storm, or hurricane.
Tropical Prediction Center (TPC)
A division of the
National Centers for Environmental Prediction, the Center issues watches,
warnings, forecasts, and analyses of hazardous weather conditions in the tropics
for both domestic and international communities. The National Hurricane Center
is a branch. For further information, contact the TCP, located in Miami ,
Florida .
Tropical Storm
A tropical cyclone in which the maximum
sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per
hour (63 knots). At this point, the system is given a name to identify and track
it.
Tropical Wave
Another name for an easterly wave, it is
an area of relatively low pressure moving westward through the trade wind
easterlies. Generally, it is associated with extensive cloudiness and showers,
and may be associated with possible tropical cyclone development.
Tropics/Tropical
The region of the earth located between
the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of
Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region,
an area of high temperatures and considerable precipitation during part of the
year.
Tropopause
The boundary zone or transition layer between
the troposphere and the stratosphere. This is characterized by little or no
increase or decrease in temperature or change in lapse rate with increasing
altitude.
Troposphere
The lowest layer of the atmosphere located
between the earth's surface to approximately 11 miles (17 kilometers) into the
atmosphere. Characterized by clouds and weather, temperature generally decreases
with increasing altitude.
Trough
An elongated area of low atmospheric pressure
that is associated with an area of minimum cyclonic circulation. The opposite of
a ridge.
Tsunami
An ocean wave with a long period that is formed
by an underwater earthquake or landslide, or volcanic eruption. It may travel
unnoticed across the ocean for thousands of miles from its point of origin and
builds up to great heights over shallower water. Also known as a seismic sea
wave, and incorrectly, as a tidal wave.
Turbulence
The irregular and instantaneous motions of
air which is made up of a number of small eddies that travel in the general air
current. Atmospheric turbulence is caused by random fluctuations in the wind
flow. It can be caused by thermal or convective currents, differences in terrain
and wind speed, along a frontal zone, or variation in temperature and pressure.
Twilight
Often called dusk, it is the evening period of
waning light from the time of sunset to dark.
Twister
A slang term used in the United States for a
tornado.
Typhoon
The name for a tropical cyclone with sustained
winds of 74 miles per hour (65 knots) or greater in the western North Pacific
Ocean . This same tropical cyclone is known as a hurricane in the eastern North
Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean .
Ultraviolet
Electromagnetic radiation that has a
wavelength shorter than visible light and longer than x-rays. Although it
accounts for only 4 to 5 percent of the total energy of insolation, it is
responsible for many complex photochemical reactions, such as fluorescence and
the formation of ozone.
United States Weather Bureau
The
official name of the National Weather Service prior to 1970.
Universal Time Coordinate
One of several names for the
twenty-four hour time which is used throughout the scientific and military
communities.
Unstable/Instability
Occurs when a rising air parcel
becomes less dense than the surrounding air. Since its temperature will not cool
as rapidly as the surrounding environment, it will continue to rise on its own.
Updraft
A small scale current of air with vertical
motion. If there is enough moisture, then it may condense, forming a cumulus
cloud, the first step towards thunderstorm development.
Upper Air/Upper Level
The portion of the atmosphere
which is above the lower troposphere. It is generally applied to the levels
above 850 millibars. Therefore, upper level lows and highs, troughs, winds,
observations, and charts all apply to atmospheric phenomena above the surface.
Upslope Effect
The cooling of an air flow as it ascends
a hill or mountain slope. If there is enough moisture and the air is stable,
stratiform clouds and precipitation may form. If the air is unstable, there
might be an increased chance of thunderstorm development.
Vapor Trail
A cloudlike streamer or trail often seen
behind aircraft flying in clear, cold, humid air. A vapor trail is created when
the water vapor from the engine exhaust gases are added to the atmosphere. Also
called a contrail, for condensation trail.
Vernal Equinox
Taking place in the Northern Hemispheric
spring, it is the point at which the ecliptic intersects the celestial equator.
Days and nights are most nearly equal in duration. It falls on or about March 20
and is considered the beginning of spring in the Northern Hemisphere and autumn
in the Southern Hemisphere.
Vertical Temperature Profile
A series of temperature
measurements taken at various levels in the atmosphere that show the thermal
structure of the atmosphere over a specific location. Obtained through a
rawinsonde sounding or comparable method, and exhibited in a skew t-log p
diagram.
Vertical Wind Profile
A series of wind direction and
wind speed measurements taken at various levels in the atmosphere that show the
wind structure of the atmosphere over a specific location. Obtained through a
rawinsonde sounding or comparable method, and exhibited in a skew t-log p
diagram.
Virga
Streaks or wisps of precipitation, such as water
or ice particles, that fall from clouds but evaporate before reaching the
ground.
Visibility
A measure of the opacity of the atmosphere,
and therefore, the greatest distance one can see prominent objects with normal
eyesight.
Visible Light
The portion of the electromagnetic
spectrum that can be detected by the human eye. It travels at the same speed as
all other radiation, that is at 186,000 miles per second. It has a wave length
longer than ultraviolet light and shorter than x-rays.
Visual Flight Rules (VFR)
Refers to the general weather
conditions pilots can expect at the surface. VFR criteria means a ceiling
greater than 3,000 feet and greater than 5 miles.
Vortex
Any circular or rotary flow in the atmosphere
that possesses vorticity.
Vorticity
The measurement of the rotation of a small air
parcel. It has vorticity when the parcel spins as it moves along its path.
Although the axis of the rotation can extend in any direction, meteorologists
are primarily concerned with the rotational motion about an axis that is
perpendicular to the earth's surface. If it does not spin, it is said to have
zero vorticity. In the Northern Hemisphere, the vorticity is positive when the
parcel has a counterclockwise, or cyclonic, rotation. It is negative when the
parcel has clockwise, or anticyclonic, rotation.
Vorticity Maximum
A center of vorticity, or the maximum
of the vorticity field of a fluid.
Wall Cloud
An abrupt lowering of a cloud from its parent
cloud base, a cumulonimbus or supercell, with no visible precipitation
underneath. Forming in the area of a thunderstorm updraft, or inflow area, it
exhibits rapid upward movement and cyclonic rotation. It often develops before
strong or violent tornadoes.
Warm Advection
The horizontal movement of warmer air
into a location.
Warm Front
The leading edge of an advancing warm air
mass that is replacing a retreating relatively colder air mass. Generally, with
the passage of a warm front, the temperature and humidity increase, the pressure
rises, and although the wind shifts (usually from the southwest to the northwest
in the Northern Hemisphere), it is not as pronounced as with a cold frontal
passage.
Warm High
A high pressure system that has its warmest
temperatures at or near the center of circulation.
Warm Low
A low pressure system that has its warmest
temperatures at or near the center of circulation. Also referred to as a warm
core low.
Warning
A forecast issued when severe weather has
developed, is already occurring and reported, or is detected on radar. Warnings
state a particular hazard or imminent danger, such as tornadoes, severe
thunderstorms, flash and river floods, winter storms, heavy snows, etc.
Watch
A forecast issued well in advance of a severe
weather event to alert the public of the possibility of a particular hazard,
such as tornadoes, severe thunderstorms, flash and river floods, winter storms,
or heavy snows.
Waterspout
A small, weak tornado, which is not formed by
a storm-scale rotation. It is generally weaker than a supercell tornado and is
not associated with a wall cloud or mesocyclone. It may be observed beneath
cumulonimbus or towering cumulus clouds and is the water equivalent of a
landspout.
Weather
The state of the atmosphere at a specific time
and with respect to its effect on life and human activities. It is the short
term variations of the atmosphere, as opposed to the long term, or climatic,
changes.
Weather Surveillance Radar (WSR-88D)
The newest
generation of Doppler radars, the 1988 Doppler weather radar. The radar units,
with help from a set of computers, show very detailed images of precipitation
and other phenomena, including air motions within a storm.
Westerlies
Usually applied to the broad patterns of
persistent winds with a westerly component. It is the dominant persistent
atmospheric motion, centered over the midlatitudes of each hemisphere.
Wet Bulb Depression
Dependent on the temperature and the
humidity of the air, it is the difference between the dry bulb and the wet bulb
readings.
Wet Bulb Thermometer
A thermometer used to measure the
lowest temperature in the ambient atmosphere in its natural state by evaporating
water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature
is used to compute dew point and relative humidity. One of the two thermometers
that make up a psychrometer.
Whiteout
When visibility is near zero due to blizzard
conditions or occurs on sunless days when clouds and surface snow seem to blend,
erasing the horizon and creating a completely white vista.
Wind
Air that flows in relation to the earth's surface,
generally horizontally. There are four areas of wind that are measured:
direction, speed, character (gusts and squalls), and shifts. Surface winds are
measured by wind vanes and anemometers, while upper level winds are detected
through pilot balloons, rawin, or aircraft reports.
Wind Chill Index
The calculation of temperature that
takes into consideration the effects of wind and temperature on the human body.
Describes the average loss of body heat and how the temperature feels. This is
not the actual air temperature.
Wind Direction
The direction from which the wind is
blowing.
Wind Shear
The rate of wind speed or direction change
with distance. Vertical wind shear is the rate of change of the wind with
respect to altitude. Horizontal wind shear is the rate of change on a horizontal
plane.
Wind Vane
An instrument that indicates the wind
direction. The end of the vane which offers the greatest resistance to the
motion of the air moves to the downwind position.
Windward
The direction from which the wind is blowing.
Also the upwind side of an object. The opposite of the downwind or leeward side.
Winter
Astronomically, this is the period between the
winter solstice and the vernal equinox. It is characterized as having the
coldest temperatures of the year, when the sun is primarily over the opposite
hemisphere.
World Meteorological Organization (WMO)
From weather
prediction to air pollution research, climate change related activities, ozone
layer depletion studies and tropical storm forecasting, the World Meteorological
Organization coordinates global scientific activity to allow increasingly prompt
and accurate weather information and other services for public, private and
commercial use, including international airline and shipping industries.
Established by the United Nations in 1951, it is composed of 184 members.
X-rays
The portion of the electromagnetic spectrum that
has a very short wave length. It has a wave length longer than gamma rays, yet
shorter than visible light. X-rays can penetrate various thicknesses of all
solids, and when absorbed by a gas, can result in ionization.
Zonal Flow
The flow of air along a latitudinal component
of existing flow, normally from west to east.