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Weather Glossary. A.

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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A: Absolute humidity...,
B: Backing...,
C: Calm...,
D: Daily mean...,
E: Earthlight...,
F: Fahrenheit temperature scale...,
G: Gale...,
H: Haboob...,
I: Ice...,
J: Jet streak...,
K: Katabatic wind...,
L: Lake effect snow...,
M: Mackerel sky...,
N: Nadir...,
O: Obscuration...,
P: Palmer drought index...,
Q: Quantitative precipitation forecast...,
R: Radar...,
S: Saffir...,
T: Teleconnections...,
U: Ultraviolet...,
V: Valley breeze...,
W: Walker circulation...,
X: X-rays...,
Y: Year...,
Z: Zodiac....

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