WX 200 Weather Station
(630-1015) Weather Reference Faxback Doc. # 38350
GLOSSARY OF WEATHER TERMS
air mass - a large body of air that has similar horizontal temperature
and moisture characteristics.
air (atmospheric) pressure - the pressure exerted by the weight of air
above a given point. Usually expressed
in millibars (mb) or inches (in) of mercury
(Hg).
altimeter - an instrument that indicates the altitudes of an object
above a fixed level. Pressure altimeters use an aneroid
barometer with a scale graduated in altitude instead of
pressure.
anemometer - an instrument that measures wind speed.
Atmosphere - the envelope of gases that surrounds a planet and are held
to it by the planet's gravitational attraction. The earth's
atmosphere is mainly nitrogen and oxygen.
autumnal equinox - the equinox at which the sun approaches the Southern
Hemisphere and passes directly over the equator.
Occurs around September 23.
backing wind - a wind that signifies cooling and changes direction
in a counterclockwise sense (north to northwest to
west, for example).
barometer - an instrument that measures atmospheric pressure. The two
most common barometers are the mercury barometer and the
aneroid barometer.
blizzard - a severe weather condition characterized by low temperature
and strong winds (greater than 32 mph) bearing a great amount
of snow.
Celsius scale - a temperature scale where (at sea level) water freezes at
0 degrees and boils at 100 degrees.
cold front - a transition zone where a cold air mass advances and replaces
a warm air mass.
cold wave - a rapid fall in temperature within 24 hours that often
requires increased protection for agriculture, industry,
commerce, and human activities.
convection - motions in a fluid that result in the transport and mixing
of the fluid's properties. In meteorology, convection
usually refers to atmospheric motions that are predominantly
vertical, such as rising air currents due to surface heating.
The rising of heated surface air and the sinking of cooler
air aloft is often called free convection.
cyclone - an area of low pressure around which the winds blow
counterclockwise in the Northern Hemisphere and clockwise in the
Southern Hemisphere.
daily range of temperature - the difference between the maximum and
minimum temperatures for any given day.
dew - water that has condensed onto objects near the ground when their
temperatures have fallen below the dew point of the surface air.
dew point (dew-point temperature) - the temperature to which air must
be cooled (at constant pressure and
constant water vapor content) for
saturation to occur. When the dew
point falls below freezing. It is
called the frost point.
downburst - a severe localized downdraft that can be experienced beneath
a severe thunderstorm.
drizzle - small drops between 0.2 and 0.5 mm in diameter that fall slowly
and reduce visibility more than light rain.
drought - a period of abnormally dry weather sufficiently long enough to
cause serious effects on agriculture and other activities in the
affected area.
dry line - a boundary that separates warm, dry air from warm, moist air.
It usually represents a zone of instability along which
thunderstorms form.
evaporation - the process by which a liquid changes into a gas.
extratropical cyclone - a cyclonic storm that most often forms along
a front in middle and high latitudes. Also
called a middle latitudes storm, a depression,
and a low. It is not a tropical storm or
hurricane.
eye - a region in the center of a hurricane (tropical storm) where
the winds are light and skies are clear to partly cloudy.
eye wall - a wall of dense thunderstorms that surrounds the eye of a
hurricane.
Fahrenheit scale - a temperature scale where (at sea level) water freezes
at 32 degrees and boils at 212 degrees.
fog - a cloud with its base at the earth's surface. It reduces visibility
to less than 1 km.
freeze - a condition occurring over a widespread area when the surface air
temperature remains below freezing for a sufficient time to
damage certain agricultural crops. A freeze most often occurs
as cold air moves into a region, causing freezing conditions to
exist in a deep layer of surface air.
freezing rain/drizzle - rain or drizzle that fails in liquid form and then
freezes upon striking a cold object or ground.
front - the transition zone between two distinct air masses.
frost (hoarfrost) - a covering of ice produced by deposition (sublimation)
on exposed surfaces when the air temperature falls
below the frost point (the dew point is below
freezing).
frozen dew - the transformation of liquid dew into tiny beads of ice
when the air temperature drops below freezing.
funnel cloud - a rotating cone-like cloud that extends downward from
the base of a thunderstorm. When it reaches the
surface, it is called a tornado.
graupel - ice particles between 2 and 5 mm in diameter that form in a
cloud. Snowflakes that become rounded pellets due to
riming (being frosted over) are called graupel or snow
pellets.
gust front - a boundary that separates a cold downdraft of a thunderstorm
from warm, humid surfaces air. On surface its passage
resembles that of a cold front.
hailstones - transparent or partially opaque particles of ice that range
in size from that of a pea to that of golf balls and larger.
haze - fine dry or wet dust or salt particles dispersed through a portion
of the atmosphere. Individually these are not visible, but
cumulatively they will diminish visibility. Dry haze particles
are very small, on the order of 0.1 fm. Wet haze particles are
larger.
heat index (HI) - an index that combines air temperature and relative
humidity to determine an apparent temperature - how
hot it actually feels.
humidity - a general term that refers to the air's water vapor content.
hurricane - a severe tropical cyclone having winds in excess of 64
knots (74 mph).
hurricane warning - a warning given when it is likely that a hurricane
will strike an area within 24 hours.
hurricane watch - a hurricane watch indicates that a hurricane poses a
threat to an area (often within several days) and
residents of the watch area should be prepared.
jet stream - relatively strong winds concentrated within a narrow band
in the atmosphere.
knot - a unit of speed equal to 1 nautical mile per hour. 1 knot equals
1.15 mph.
lake-affect snows - localized snowstorms that form on the downwind
side of the lake. Such storms are common in late
fall and early winter near the Great Lakes as cold,
dry air picks up moisture and warmth from the unfrozen
bodies of water.
lightning - a visible electrical discharge produced by thunderstorms.
mean annual temperature - the average temperature at any given location
for the entire year.
mean daily temperature - the average of the highest and lowest temperature
for a 24-hour period.
meteorology - the study of the atmosphere and atmospheric phenomena
as well as the atmosphere's interaction with the earth's
surface, oceans, and life in general.
millibar (mb) - a unit for expressing atmospheric pressure. Sea level
pressure is normally close to 1013 mb.
northeaster - a name given to a strong, steady northeast wind that is
accompanied by rain and inclement weather. It often
develops when a storm system moves northeastward along
the coast of North America.
overrunning - a condition that occur when air moves up and over another
layer of air.
pressure tendency - the rate of change of atmospheric pressure within
a specified period of time, most often three hours.
Also known as barometric tendency.
rain - precipitation in the form of liquid water drops that have diameters
greater than that of drizzle.
rainbow - an arc of concentric colored bands that spans a section of the
sky when rain is present and the sun is behind the observer's
back.
rain gauge - an instrument designed to measure the amount of rain that
falls during a given time interval.
relative humidity - the ratio of the amount of water vapor actually
in the air compared to the amount of water vapor
the air can hold at that particular temperature
and pressure. The ratio of the air's actual vapor
pressure to its saturation vapor pressure.
sea breeze - a coastal local wind that blows from the ocean onto the
land. The leading edge of the breeze is called a sea
breeze front.
sea level pressure - the atmospheric pressure at mean sea level.
severe thunderstorms - intense thunderstorms capable of producing heavy
showers, flash floods, hail, strong and gusty
surface winds, and tornadoes.
shower - intermittent precipitation from a cumuliform cloud, usually
of short duration but often heavy.
sleet - a type of precipitation consisting of transparent pellets
of ice 5 mm or less in diameter. Also known as ice pellets.
smog - originally smog meant a mixture of smoke and fog. Today, smog
means air that has restricted visibility due to pollution
formed in the presence of sunlight - photochemical smog.
snow - a solid form of precipitation composed of ice crystals in
complex hexagonal forms.
snowflake - an aggregate of ice crystals that falls from a cloud.
snow flurries - light showers of snow that fall intermittently.
snow squall (shower) - an intermittent heavy shower of snow that
greatly reduces visibility.
squall line - any non-frontal line or band of active thunderstorm.
standard atmospheric pressure - pressure of 1013.25 millibars (mb.)
29.92 inches of mercury (Hq.),
760 millimeters of mercury (mm),
14.7 pounds per square inch (lb/10),
101,325 pascals (Pa).
station pressure - the actual air pressure computed at the observing
station.
supercell storm - an enormous severe thunderstorm whose updrafts and
downdrafts are nearly in balance, allowing it to
maintain itself for several hours. It can produce
large hail and tornadoes.
temperature - the degree of hotness or coldness of a substance as measured
by a thermometer. It is also a measure of the average speed
or kinetic energy of the atoms and molecules in a
substance.
thermograph - an instrument that measures and records air temperature.
thermometer - an instrument for measuring temperature.
thunder - the sound created by rapidly expanding gases along the channel
of a lightning discharge.
thunderstorm - a local storm produced by cumulonimbus clouds. Always
accompanied by lightning and thunder.
tornado - an intense, rotating column of air that protrudes from a
cumulonimbus cloud in the shape of a funnel or a rope
and touches the ground.
tornado outbreak - a series of tornadoes that form within a particular
region - a region that may include several states.
Often associated with wide-spread damage and
destruction.
tornado warning - a warning issued when a tornado has actually been
observed either visually or on a radar screen.
tornado watch - a forecast issued to alert the public that tornadoes
may develop within a specified area.
trace (of precipitation) - an amount of precipitation less than 0.01
inch (0.025 cm).
visibility - the distance an observer can see and identify prominent
objects.
warm front - a front that moves in such a way that warm air replaces cold
air.
wet-bulb temperature - the lowest temperature that can be obtained by
evaporating water into the air.
wind - air in motion relative to the earth's surface.
wind-chill factor - the cooling effect of any combination of temperature
and wind, expressed as the loss of body heat. Also
called wind-chill index.
wind direction - the direction from which the wind is blowing.
wind vane - an instrument used to indicate wind direction.
winter solstice - approximately December 22 in the Northern Hemisphere
where the sun is lowest in the sky and directly overhead
at latitude 23.5 degrees S, the Tropic of Capricorn.
CLOUD CLASSIFICATION CHART
Clouds can float from a few feet above the earth (fog) all the way up
to 40,000 feet and higher. There are 10 different classes of clouds.
As the following chart illustrates, you can tell clouds apart based on
their altitude and sharp.
(LB/km-03/12/1997)