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- ABSOLUTE HUMIDITY
- A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.
- ABSOLUTE INSTABILITY
- 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
- Considered to be the point at which theoretically 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
- Statements that are 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
- This is considered the mixture of gases that make up the earth's atmosphere. The principal 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 that is produced by convection within an unstable
air mass through an
instability mechanism. Such thunderstorms 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, that does not preclude them from having brief heavy downpours.
- AIR POLLUTION
- The soiling of the atmosphere
by
contaminants to the 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 with respect 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 in respect to a constant pressure surface or above mean sea level.
- ALTOCUMULUS
- Composed of flattened, thick, gray, globular masses, this middle cloud genus is 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
- This middle cloud genus is 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
- A 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.
- Related term: Dave's Dictionary
- 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
- This 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 or air 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.
- For further information, 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
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