2008 REED’s East Coast Edition - Weather
changing declination are termed “seasonal • The heap cloud, or cumulus, formed
winds.” The well-known monsoon of the In- from strong vertical air motion (convec-
dian Ocean and the China Sea is an example tion).
of a seasonal wind. • The layer cloud, or stratus, formed from
You can estimate the speed of surface winds more gradual horizontal and vertical as-
using the Beaufort Wind Scale (page 297) cent of air.
that relates sea state to wind speed. A calm • The streak cloud, or cirrus, composed of
sea indicates winds of less than one knot ice crystals.
(one knot = 1.15 mph), while Force 12 (the The three cloud forms are subdivided into
highest number on the scale) indicate winds ten important modifications or principal
of hurricane strength (63-plus knots). cloud types based on cloud base height:
Effect of the Earth’s Rotation: Air moving High clouds (10,000-60,000 ft): Cirrus (ci),
over the earth’s surface is deflected to the Cirrostratus (cs), Cirrocumulus (cc)
right in the Northern Hemisphere and to the Middle clouds (6,500-25,000 ft): Altostra-
left in the Southern Hemisphere due to the tus (as), Altocumulus (ac)
earth’s rotation. This phenomenon is known Low clouds (up to 6,500 ft.): Stratus (st),
as the Coriolis effect. Absent the Coriolis ef- Stratocumulus (sc), Nimbostratus (ns)
fect, winds would flow directly from high- to Variable clouds—low to high: Cumulus
low-pressure centers. Because of the deflec- (cu), Cumulonimbus (cb)
tion caused by the Coriolis effect, winds Following are some cloud descriptions (see
circulate clockwise around areas of high page 301 for visual reference).
pressure and counterclockwise around lows Cirrus clouds are detached and have a
in the Northern Hemisphere. The reverse is delicate, fibrous, often silky appearance. They
true for the Southern Hemisphere. In addi- are usually white in color, not shaded. Cirrus
tion, close to the earth, friction caused by clouds that look like separate brush strokes
surface topography affects wind direction. are popularly called “mares’ tails.”
Surface winds angle somewhat more toward Cirrocumulus clouds form small white
and away from pressure centers than do flakes or very small globular masses without
winds at 3,000 feet and higher. A useful rule shadows. They are arranged in groups or
of thumb states that if an observer in the lines with their configuration often resem-
Northern Hemisphere faces into the wind, bling ripples or wave-created ridges of sand
low barometric pressure will lie to his or her on the seashore.
right and high pressure to his or her left. Cirrostratus clouds create a transparent,
Jet Streams: In middle and high latitudes, whitish veil of fibrous or smooth appearance
upper-level winds called jet streams circle that totally or partially cover the sky and
the globe from west to east at approximately usually produce halo phenomena.
18,000 feet and higher. With no surface fric- Altocumulus clouds are composed of fair-
tion to inhibit their movement, these undu- ly small elements, with or without shading.
lating ribbons of air often travel at speeds These clouds form rather flattened globular
in excess of 200 knots, although 50- to masses that appear in a layer or in patches.
150-knot speeds are more prevalent. Altostratus clouds form a striated or fi-
Jet stream winds are caused by the atmo- brous veil, and are more or less gray or bluish
sphere constantly seeking equilibrium by in color. They form a uniform layer covering
transferring hot air from equatorial regions all or part of the sky.
to polar regions, and the deflection of airflow Nimbostratus clouds form a low, amor-
caused by the earth’s rotation. The strength phous, and rainy layer. The color is dark gray
and path of jet streams is important to sur- and nearly uniform.
face mariners because jet streams act as steer- Stratocumulus clouds form globular mass-
ing currents for surface weather. es or rolls that appear in a layer or patches.
Clouds
The smallest of the regularly arranged ele-
ments are fairly large; they are soft and gray
Clouds are gatherings of minute water
with darker parts.
drops, ice crystals, or both, held in suspen-
Stratus clouds form a low, gray layer with
sion in the atmosphere. There are three pri-
tops and bases at uniform heights, not unlike
mary forms:
fog at the surface.
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