basis of the greenhouse effect. Moreover, clouds are an integral part of the atmosphere
so wind moves them over long distances. Clouds are measured by eye and by human
observation. Usually measured are cloud type, the height of the cloud bases, and por-
tion of the sky covered. The international standard is to report cloud coverage in
eighths known as octas. Coverage, therefore, is reported on a 0 to 8 scale.
Clouds are made because there are impurities in the air. Even before the advent
of human-caused air pollution the air was never pure. There are a number of solid
and liquid substances that serve as condensation and freezing nuclei around which
pieces of water can gather. These nuclei are so light that they float on air and are
composed of many substances, including dust from weathered soils and rocks,
pollen, sulfate particles from oceanic phytoplankton, and sea salt. The nuclei best
suited to gather water have radii of 0.1 millionths of a meter and larger. Most pla-
ces in the lower troposphere have on the order of 100 to 1,000 of these nuclei in a
cubic centimeter, although their density decreases away from the surface sources.
Some of the nuclei are hygroscopic and present surfaces on which water can
gather and start to make pieces of clouds at relative humidities much lower than
the 100 percent defining saturation. Indeed, on summer days with plentiful water
vapor it is common to experience haze, which is the beginning of water agglom-
erating onto condensation nuclei.
Clouds are made as substantial amounts of water are gathered around the nuclei.
The atmosphere needs to be saturated for clouds to form. Increases in relative
humidity to saturation can occur in two ways. The first is by the addition of water
vapor into the air. This goes on via evaporation or sublimation and is most important
where there is plentiful water vapor. This takes place on the order of hours and can
produce some substantial low clouds but rarely significant precipitation. The second
method is associated with the rise of air. Rising air dramatically cools via the
decompression of air molecules. Here, air cools until it becomes saturated and this
takes place on time scales of only a few minutes and the air does not initially need
to be close to saturation to have this take place. Four basic mechanisms make air
rise. The first is the orographic effect, which makes air rise because of topography
in the path of air flow; note that mountain peaks are cloudier than their surround-
ings. The second is the lifting of air alongfronts. The third is the rise of air because
of heating of the surface—convection. The fourth is the rise of air in the centers
of low pressure as air streams meet and are forced upward. At individual locations
they can occur singly, in combination, or not at all. Any one of them can cause
cloudiness.
In that the presence of significant cloudiness is dependent on the rise of air,
topography, and presence of water vapor, there is a definite geography to cloudiness
on Earth. Cloud geography is well related to global winds and pressure belts.
Latitudes with average low pressure—the Intertropical Convergence and the polar
Clouds 65