The atmosphere always contains, in greater or smaller amounts, tiny particles,
such as dust from roads, desert sand, plant pollen, salt particles from oceans,
and factory smoke. These fragments are hygroscopic nuclei, the term means
particles that readily condense moisture. A cloud is merely a mass of
hygroscopic nuclei that has soaked up moisture from the air.
The heat generated by the Suns energy causes earthbound moisture to
evaporate (turn into water vapor). Water vapor is lighter than air; thus, it rises.
If the air it passes into is cold enough, the vapor condenses; that is, it turns
back into moisture. The water droplets that result from this process cling to
the hygroscopic nuclei. Many of these water-soaked nuclei bunched together
form a cloud. Fog is the same in principle, but its a cloud on the ground.
Changes in atmospheric conditions account for the many different shapes of
clouds and for their presence at various altitudes. Formations of clouds give
clues concerning the existing forces at play in the atomsphere. Thats why you
must keep an accurate record of cloud genera (types).
With respect to clouds, the atmosphere is broken down into three layers
or etages. In the middle latitudes or temperate region, the low etage is
from the surface to 6,500 feet; the mid etage, from 6,500 feet to 18.500
feet; and the high etage, from 18,500 feet on up to near 45,000 feet (fig.
10-2). The limits of the etages are generally lower in the polar regions
(mid etage, from 6,500 to 10,000 feet and high etage from 10,000 to
25,000 feet) and higher in the tropics (mid etage from 6,500 to 20,000
feet and high etage from 20,000 to 60,000 feet).
The low-etage cloud may be cumuliform, such as the cumulus genera or
cumulonimbus (identified by their size and extent of development);
stratiform, such as the stratus; or have mixed characteristics, such as the
stratocumulus. The mid-etage cloud genera are mostly identified with
the prefix alto. The mid etage contains the cumuliform clouds, such as
altocumulus, and the stratiform clouds, such as altostratus and
nimbostratus. The high-etage cloud genera contain the prefix cirro.
Cumuliform clouds in this etage are called cirrocumulus, while
stratiform clouds are called cirrostratus. Another form of cloud found
only in the high etage is the cirriform clouds that are the normally thin,
wispy or hairlike ice-crystal clouds that can be defined as neither
cumuliform nor stratiform, but are simply called cirrus clouds.
Take a moment to study figure 10-2, which shows many of the cloud genera
and their associated heights above ground.