ABSOLUTE HUMIDITY
A type of humidity
that considers the mass of water
vapor present per unit volume of space. Also considered as the
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 degrees K (Kelvin) and a boiling
point of +373 degrees K. Related term: Kelvin
Temperature Scale
ABSOLUTE ZERO
Considered to be
the point at which theoretically no molecular activity exists or the
at which the volume
of a perfect gas vanishes. The value is 0 degrees Kelvin,
-273.15 degrees Celsius and -459.67 degrees
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
. Visit WALTER
Severe Weather web-page
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.
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.
Virgaoften falls from these clouds.
AMERICAN METEOROLOGICAL SOCIETY
An organization whose membership promotes the education and
professional advancement of the atmospheric,
hydrologic, and oceanographic
sciences.
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.
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 inchesand 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 degrees South to the Antarctic Circle at approximately 66 1/2 degrees 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 ofpressure 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 degrees North
to the Arctic Circle at approximately 66 1/2 degrees 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 degrees
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
