preventing solar radiation from reaching the ground, (2) absorbing warmth that is re-emitted
from the ground, and (3) as the source of precipitation. When there are no clouds, there is
less insulation. As a result, cloudless days can be extremely hot, and cloudless nights can
be very cold. For this reason, cloudy days tend to have a lower range of temperatures than
clear days.
Clouds form when air reaches itsdew point, the temperature when the air is saturated with
water vapor. This can happen in two ways. First, the air temperature can stay the same
whilethehumidityincreases. Thisiscommoninlocationsthatarewarmandhumid. Second,
the humidity can remain the same, but the temperature decreases. When this happens, the
air will eventually cool enough so that it reaches 100% humidity, and water droplets form.
Air cools when it comes into contact with a cold surface or when it rises. There are three
ways that rising air can create clouds: (1) It can be warmed at or near the ground level, (2)
It can be pushed up over a mountain or mountain range, or (3) It can be thrust over a mass
of cold, dense air.
Water vapor in the atmosphere is not visible, unless it condenses to become a cloud. Water
vapor condenses around a nucleus, such as dust, smoke, or a salt crystal. This forms a tiny
liquid droplet. Billions of these water droplets together make a cloud. If the atmosphere is
very cold, the droplets freeze into ice. Most clouds appear white because sunlight reflects off
the water droplets. If the clouds are thick, the droplets scatter or absorb the light and less
solar radiation can travel through them. This is why storm clouds are dark black or gray.
Clouds have been classified in several ways. The most common classification used today di-
vides clouds into four separate cloud groups, which are determined by their altitude (Figure
16.2). High clouds, which have the prefix ’cirro-,’ are found above 6,000 m (20,000 feet) in
altitude. Middle clouds, which have the prefix ’alto-,’ are between 2,000 to 7,000 m (6,500
to 23,000 feet). Low clouds, which have the word ’stratus’ in their names, occur beneath
2,000 m (6,500 feet). Each of the clouds that occur in these groups is layered, and they grow
horizontally.
Another group of clouds, which have the prefix ’cumulo-,’ describes clouds that grow verti-
cally instead of horizontally. These impressive clouds have their bases at low altitude and
their tops at high or middle altitude.
Table 16.2:High clouds: Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus (Cc)
Middle clouds: Altostratus (As) Altocumulus (Ac)
Low clouds: Stratus (St) Stratocumulus (Sc) Nimbostratus (Ns)
Vertical clouds: Cumulus (Cu) Cumulonimbus (Cb)(Source:CK-12 Foundation,License: CC-BY-SA)
High clouds form where the air is extremely cold and can hold little water vapor. The