Page 258 - NS-2 Textbook

Clouds and Fog

Water is always present in the au; in greater or smaller weather reports. Such information is inlpOl·tant in fore-
amounts. It can be present in three states: solid, liquid, casting.
and vapor. In chapter 1 we discussed water vapor in the
air, called humidity. Relative humidity was defuled as
the percentage of the amount of vapor the air can hold at CLOUD CLASSIFICATIONS
a given temperature. In this chapter we will discuss how
TIlere are three basic cloud types: cirl'lls (wispy), CUl111ilus
water vapor is formed into clouds, and what kind of
(heaped-up), and stmtus (layered). In addition to the
weather the various kinds of clouds may foretell. This in-
three basic types, there are other types having names
formation is vital to meteorologists, but it can also be that are combinations of these with the word nimbus
both helpful and interesting to the average person.
(meaning "rain") or the prefix a/to- (meaning "high"),
identifying clouds in the middle altitudes. Another
DEFINITION OF A CLOUD prefix, jracto-, is often used to describe fragmented or
windblown clouds.
Tiny particles of dust, sand, pollen from plants, factory
Clouds are often classified in accordance with the al-
smoke, and salt particles from oceans are always present
titudes at which they most frequently occur. The altitude
in the air. These fragments of matter are called hygro-
classes are high, middle, or low. Sometimes a fourth class,
scopic nuclei, a term meaning "particles that readily ab-
towering, is used to identify an exceptionally high cloud
sorb moisture." A cloud is a mass of hygroscopic nuclei
with its base Ul the low-altitude area. Altitudes associ-
that have soaked up moisture from the water vapor in
ated with each of these classes are (1) low: surface to
the air. 7,000 feet; (2) middle: 7,000 to 20,000 feet; and (3) high:
The heat generated by the Sun's energy causes earth-
above 20,000 feet.
bound moisture to evaporate into the sky in the form of
Middle clouds seldom attain heights greater than
water vapor. TIlis water vapor rises, since it is lighter
13,000 feet Ul the polar regions, though they may reach
than air. If the au' it passes Ulto is cold enough, the vapor
23,000-45,000 feet Ul the temperate and tropical zones.
condenses-in other ,vords, it turns back into moisture.
Clouds are usually named accOl'dillg to their appear-
The water droplets that result from this process c1ulg to
ance. Appearance, though, is largely dependent upon the
the hygroscopic nuclei. Bunched togethe1; these water-
altitude Ul which they are fotmd. Grouped by appear-
soaked nuclei form a cloud. Fog is formed the same way;
ance and altitude, there are ten general cloud types.
it is a cloud very close to the ground. Low Clouds. Low clouds are of five main types:
As these droplets ride air currents, one of three
things can happen, depending upon the temperature and 1. Stratus clouds, the lowest cloud type, are often
wind. They may reevaporate and rise farther into the at- like a gray layer with a uoiform base. TIley may
mosphere; they may rise and freeze into ice crystals, cause drizzle, but never rain. Fog becomes strahlS
sometimes in sufficient amounts to form ice crystallille when it lifts.
clouds; or they may collide with other nuclei and form 2. Nimbostratus are dark, shapeless, rain-laden
larger drops that become heavy enough to fall as rain, clouds, often blanketing the sky. They are true
snow, or sleet. rain clouds and "look wet" because they often
Changes in atmospheric conditions account for the have streaks of raUl extending to the grotmd be-
many different shapes of clouds, and for theu' presence neath them. They are often seen in the summer at
at various altitudes. Cloud formations give a clue con- the base of thunderheads. In the winter they
cerning the forces at work in the atmosphere. Navy and bring steady, heavy snow.
civilian meteorologists must keep accurate records of 3. Stratocllmlilus are irregulm; rounded masses of
clouds and must account for cloud cover in their periodic clouds spread out in puffy or rollulg layers. These

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