30
Special Features
millibar (mb) — a unit for expressing atmo-
spheric pressure. Sea level pressure is nor-
mally close to 1013 mb.
northeaster — a name given to a strong,
steady northeast wind that is accompanied
by rain and inclement weather. It often devel-
ops when a storm system moves northwest-
erly along the coast of North America.
overrunning — a condition that occurs when
air moves up and over another layer of air.
pressure tendency — the rate of change of
atmospheric pressure within a specified peri-
od 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 pres-
sure at mean sea level.
severe thunderstorms — intense thunder-
storms capable of producing heavy showers,
flash floods, hail, strong and gusty surface
winds, and tornadoes.
shower — intermittent precipitation from a
cumuli form cloud, usually of short duration
but often heavy.
sleet — a type of precipitation consisting of
transparent pellets of ice .20 inches (5 mm)
or less in diameter. Also known as ice pel-
lets.
smog — air that has restricted visibility due
to pollution, or pollution formed in the pres-
ence of sunlight — photochemical smog
(originally smog meant a mixture of smoke
and fog).
snow — a solid form of precipitation com-
posed 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 thunderstorms.
standard atmospheric pressure — pres-
sure of 1013.25 millibars (mb), 29.92 inches
of mercury (Hg), 760 millimeters of mercury
(mm), 14.7 pounds per square inch (lb/in),
101,325 pascals (Pa).
station pressure — the actual air pressure
computed at the observing station.
supercell storm — an enormous severe
thunderstorm whose updrafts and down-
drafts are nearly in balance, allowing it to
maintain itself for several hours. It can pro-
duce 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
