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Absolute Humidity |
A measure of humidity that considers the mass of
water vapor present per unit volume of space. Also the density
of the water vapor. It is usually expressed in grams per cubic
meter. |
Absolute Instability |
Condition occurring when the lapse rate of a
column of air is greater than the dry adiabatic lapse rate.
The term absolute is used because this applies whether or not
the air is dry or saturated. Related term: instability |
Absolute Temperature Scale |
A temperature scale with a freezing point of
+273°K (Kelvin) and a boiling point of +373°K. Related term:
Kelvin Temperature Scale |
Absolute Zero |
The point at which no molecular activity exists
or the temperature at which the volume of a perfect gas
vanishes. The value is 0° Kelvin, -273.15° Celsius and
-459.67° Fahrenheit. |
Absorption |
The process in which incident radiant energy is
retained by a substance. The absorbed radiation is then
transformed into molecular energy. |
Abyssal Plain |
The flat, gently sloping or nearly level region
of the sea floor. |
Adiabatic Process |
A thermodynamic change of state in a system in
which there is no transfer of heat or mass across the
boundaries of the system. In this process, compression will
result in warming and expansion will result in cooling. |
Advection |
The horizontal transfer of any property in the
atmosphere by the movement of air (wind). Examples include
heat and moisture advection. |
Advection Fog |
Fog that develops when warm moist air moves over
a colder surface, cooling that air to below its dew
point.
Related terms: Arctic Sea Smoke and sea fog |
Advisory |
Statement issued by the National Weather Service
for probable weather situations of inconvenience that do not
carry the danger of warning criteria, but, if not observed,
could lead to hazardous situations. Some examples include snow
advisories stating possible slick streets, or fog advisories
for patchy fog condition causing temporary restrictions to
visibility. |
AFOS |
Acronym for Automation of Field Operations and
Services. It is the computer system that links National
Weather Service offices together for weather data
transmission. |
Air |
The mixture of gases in the earth's atmosphere.
The gases that compose dry air are Nitrogen (N2) at 78.09%,
Oxygen (O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide
(CO2) at 0.033%. One of the most important constituents of air
and most important gases in meteorology is water vapor (H2O). |
Air Mass |
An extensive body of air throughout which the
horizontal temperature and moisture characteristics are
similar. |
Air Mass Thunderstorm |
A thunderstorm produced by convection within an
unstable air mass through an instability mechanism. They
normally occur within a tropical or warm, moist air mass
during the summer afternoon as the result of afternoon heating
and dissipate soon after sunset. Such thunderstorms are not
generally associated with fronts and are less likely to become
severe than other types of thunderstorms. However, they do
produce brief heavy downpours. |
Air Pollution |
The contamination of the atmosphere to a point
that may cause injury to health, property, plant, or animal
life, or prevent the use and enjoyment of the outdoors. |
Air Quality Standards |
The maximum level which will be permitted for a
given pollutant. Primary standards are to be sufficiently
stringent to protect the public health. Secondary standards
must protect the public welfare, including property and
aesthetics. |
Alaskan Winds |
The downslope air flow that blows through the Alaskan valleys.
It is usually given local names, such as Knik, Matanuska,
Pruga, Stikine, Taku, Take, Turnagain, or Williwaw. |
Albedo |
The ratio of the amount of radiation reflected
from an object's surface compared to the amount that strikes
it. This varies according to the texture, color, and expanse
of the object's surface and is reported in percentage.
Surfaces with high albedo include sand and snow, while low
albedo rates include forests and freshly turned earth.
Related term: Dave's Dictionary |
Alberta Clipper |
A fast moving, snow-producing weather system
that originates in the lee of the Canadian Rockies. It moves
quickly across the northern United States, often bring gusty
winds and cold Arctic air. |
Aleutian Low |
A semi-permanent, subpolar area of low pressure
located in the Gulf of Alaska near the Aleutian Islands. It is
a generating area for storms and migratory lows often reach
maximum intensity in this area. It is most active during the
late fall to late spring. During the summer, it is weaker,
retreating towards the North Pole and becoming almost
nonexistent. During this time, the North Pacific High pressure
system dominates.
Related term: Icelandic Low |
Altimeter |
An instrument used to determine the altitude of
an object relative to a fixed level. The type normally used by
meteorologists measures the altitude with respect to sea level
pressure. |
Altimeter Setting |
The pressure value to which an aircraft
altimeter scale is set so that it will indicate the altitude
above mean sea level of an aircraft on the ground at the
location for which the value was determined. |
Altitude |
In meteorology, the measure of a height of an
airborne object relative to a constant pressure surface or
above mean sea level. |
Altocumulus |
Clouds composed of flattened, thick, gray,
globular masses primarily made of water droplets. In the
mid-latitudes, cloud bases are usually found between 8,000 and
18,000 feet. A defining characteristic is that it often
appears as a wavy billowy layer of cloud, giving it the
nickname of "sheep" or "woolpack" clouds.
Sometimes confused with cirrocumulus clouds, its elements
(individual clouds) have a larger mass and cast a shadow on
other elements. It may form several sub-types, such as
altocumulus castellanus or altocumulus lenticularis. Virga may
also fall from these clouds. |
Altocumulus Castellanus |
A middle cloud with vertical development that forms from
altocumulus clouds. It is composed primarily of ice crystals
in its higher portions and characterized by its turrets,
protuberances, or crenelated tops. Its formation indicates
instability and turbulence at the altitudes of occurrence. |
Altostratus |
Middle cloud genus composed of water droplets,
and sometimes ice crystals, In the mid-latitudes, cloud bases
are generally found between 15,000 and 20,000 feet. White to
gray in color, it can create a fibrous veil or sheet,
sometimes obscuring the sun or moon. It is a good indicator of
precipitation, as it often precedes a storm system. Virga
often falls from these clouds. |
American Meteorological Society |
An organization whose membership promotes the
education and professional advancement of the atmospheric,
hydrologic, and oceanographic sciences.
For further information, contact the AMS.
Related term: National Weather Association |
Anabatic Wind |
Wind that is created by air flowing uphill.
Valley breezes, produced by local daytime heating, are an
example of these winds. The opposite of a katabatic wind. |
Anemometer |
An instrument that measures the speed or force
of the wind. |
Aneroid Barometer |
An instrument for measuring the atmospheric
pressure. It registers the change in the shape of an evacuated
metal cell to measure variations on the atmospheric pressure.
The aneroid is a thin-walled metal capsule or cell, usually
made of phosphor bronze or beryllium copper. The scales on the
glass cover measure pressure in both inches and millibars.
Related term: mercurial barometer |
Anomalous Propagation |
Refers to the non-standard propagation of a beam
of energy, radio or radar, under certain atmospheric
conditions, appearing as false (non-precipitation) echoes. May
be referred to as A.P. |
Antarctic |
Of or relating to the area around the geographic
South Pole, from 90° South to the Antarctic Circle at
approximately 66 1/2°South latitude, including the continent
of Antarctica. Along the Antarctic Circle, the sun does not
set on the day of the summer solstice (approximately December
21st) and does not rise on the day of the winter solstice
(approximately June 21st). |
Antarctic Ocean |
Although not officially recognized as a separate
ocean body, it is commonly applied to those portions of the
Atlantic, Pacific, and Indian Oceans that reach the Antarctic
continent on their southern extremes. |
Anticyclone |
A relative pressure maximum. An area of pressure
that has diverging winds and a rotation opposite to the
earth's rotation. This is clockwise the Northern Hemisphere
and counter-clockwise in the Southern Hemisphere. It is the
opposite of an area of low pressure, or a cyclone.
Related term: high pressure |
Anvil |
The upper portion of a cumulonimbus cloud that
becomes flat and spread-out, sometimes for hundreds of miles
downstream from the parent cloud. It may look smooth or
fibrous, but in shape, it resembles a blacksmith's anvil. It
indicates the mature or decaying stage of a thunderstorm. |
Aphelion |
The point on the earth's orbit that is farthest
from the sun. Although the position is part of a 21,000 year
cycle, currently it occurs around July, when the earth is
about 3 million miles farther from the sun than at perihelion.
This term can be applied to any other celestial body in orbit
around the sun. It is the opposite of perihelion. |
Apogee |
The point farthest from the earth on the moon's
orbit. This term can be applied to any other body orbiting the
earth, such as satellites. It is the opposite of perigee. |
Arctic |
Of or relating to the area around the geographic
North Pole, from 90° North to the Arctic Circle at
approximately 66 1/2 North latitude. |
Arctic Air Mass |
An air mass that develops around the Arctic, it
is characterized by being cold from surface to great heights.
The boundary of this air mass is often defined by the Arctic
front, a semi-permanent, semi-continuous feature. When this
air mass moves from its source region, it may become more
shallow in height as it spreads southward. |
Arctic Jet |
The jet stream that is situated high in the
stratosphere in and around the Arctic or Antarctic Circles. It
marks the boundary of polar and arctic air masses. |
Arctic Sea Smoke |
A type of advection fog that forms primarily
over water when cold air passes across warmer waters. Related
term: steam fog |
Argon (A) |
A colorless, odorless inert gas that is the
third most abundant constituent of dry air, comprising 0.93%
of the total. |
Arid |
A term used for an extremely dry climate. The
degree to which a climate lacks effective, life-promoting
moisture. It is considered the opposite of humid when speaking
of climates. |
ASOS |
Acronym for Automated Surface Observing System.
This system is a collection of automated weather instruments
that collect data. It performs surface based observations from
places that do not have a human observer, or that do not have
an observer 24 hours a day. |
Astronomical Twilight |
The time after nautical twilight has commenced
and when the sky is dark enough, away from the sun's location,
to allow astronomical work to proceed. It ends when the center
of the sun is 18° below the horizon.
Related term: twilight |
Atmosphere |
The gaseous portion of the physical environment
that encircles a planet. In the case of the earth, it is held
more or less near the surface by the earth's gravitational
attraction. The divisions of the atmosphere include the
troposphere, the stratosphere, the mesosphere, the ionosphere,
and the exosphere. |
Atmospheric Pressure |
The pressure exerted by the atmosphere at a
given point. Its measurement can be expressed in several ways.
One is in millibars. Another is in inches or millimeters of
mercury (Hg). Related term: barometric pressure |
Aurora |
It is created by the radiant energy emission
from the sun and its interaction with the earth's upper
atmosphere over the middle and high latitudes. It is seen as a
bright display of constantly changing light near the magnetic
poles of each hemisphere. In the Northern Hemisphere, it is
known as the aurora borealis or Northern Lights, and in the
Southern Hemisphere, this phenomena is called the aurora
australis. |
Autumn |
The season of the year which occurs as the sun
approaches the winter solstice, and characterized by
decreasing temperatures in the mid-latitudes. Customarily,
this refers to the months of September, October, and November
in the North Hemisphere and the months of March, April, and
May in the Southern Hemisphere. Astronomically, this is the
period between the autumnal equinox and the winter solstice. |
AVHRR |
Acronym for Advanced Very High Resolution
Radiometer. It is the main sensor on the U.S. polar orbiting
satellites. |
Aviation Weather Center |
As one of the National Centers for Environmental
Prediction, it is the national center for weather information
that is used daily by the Federal Aviation Administration,
commercial airlines, and private pilots. It is entering a new
phase of service, growing to accept global forecasting
responsibilities. contact the AWC, located in Kansas City,
Missouri. |
AWIPS |
Acronym for Advanced Weather Interactive
Processing System. It is the computerized system that
processes NEXRAD and ASOS data received at National Weather
Service Forecast Offices. |
Azores High |
A semi-permanent, subtropical area of high
pressure in the North Atlantic Ocean that migrates east and
west with varying central pressure. Depending on the season,
it has different names. In the Northern Hemispheric winter and
early spring, when the Icelandic Low dominates the North
Atlantic, it is primarily centered near the Azores Islands.
When it is displaced westward, during the summer and fall, the
center is located in the western North Atlantic, near Bermuda,
and is known as the Bermuda High. Related term: North Pacific
High |
