28
Special Features
wise sense (north to northwest to west, for
example).
barometer — an instrument that measures
atmospheric pressure. The two most com-
mon barometers are the mercury barometer
and the aneroid barometer.
blizzard — a severe weather condition char-
acterized by low temperatures 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° and boils at
100°.
cold front — a transition zone where a cold
air mass advances and replaces a warm air
mass.
cold wave — a rapid fall in temperature with-
in 24 hours that often requires increased pro-
tection for agriculture, industry, commerce,
and human activities.
convection — atmospheric motions that are
predominantly vertical, such as rising air cur-
rents 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 differ-
ence between the maximum and minimum
temperatures for any given day.
dew — water that has condensed onto
objects near the ground when their tempera-
tures 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 activ-
ities in the affected area.
dry line — a boundary that separates warm,
dry air from warm, moist air. It usually repre-
sents a zone of instability along which thun-
derstorms 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 lati-
tude 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° and
boils at 212°.
fog — a cloud with its base at the earth’s
surface. It reduces visibility to less than 1
mile (1.6 km).
freeze — the condition that exists when the
surface temperature over a widespread area
remains below freezing (32° F or 0° C) for a
sufficient time to constitute the characteristic
feature of the weather. If the freeze cuts
