front—engender the rising air necessary for cloudiness. Topographic barriers are
places for greater cloudiness on the upwind side because of rising air and decreased
cloudiness on the downwind side because of the sinking of air. Continental interiors
generally, but not universally, are less cloudy then their oceanic and coastal coun-
terparts because of the relative lack of water vapor from oceanic sources.
Vertically, clouds are common only in the troposphere. The water vapor that
makes clouds is most plentiful in the lowest portions of the troposphere because
the water source is evaporation from oceans. Few clouds exist above the tropo-
pause and these involve so little mass that they do not provide precipitation to the
surface. It has been suggested, however, that the presence of these thin, ice crystal
clouds can impact the amount of solar energy reaching the surface and, so, changes
in their distribution and mass might be related to subtle changes in climate.
Clouds are classified like other features of physical geography and the classifi-
cation of clouds used by the World Meteorological Organization today is directly
descended from the work Luke Howard published in 1802. Although there are
hundreds of variant cloud types in the modern classification, the most common
cloud types can be readily given. Clouds are arranged into four families: (1) low
clouds, (2) middle clouds, (3) high clouds, and (4) clouds of vertical development.
While middle latitude altitudes will be given here, these families occur at higher
altitudes in the tropics and lower altitudes in the colder air over the poles.
Low clouds occur from the surface to 2,000 m. They tend to have a lot of water
in them and are ice-free in many places even during the winter season (not all
cloud droplets freeze until the air is at− 40 °C). The primary types are stratus (from
the Latin for “layer”) and stratocumulus, which is an intermediate form between
stratus and cumulus clouds. Low clouds form dense overcasts. They differ because
the rate of rising air is greater in stratocumulus clouds. An important subtype is
nimbostratus, which is a stratus cloud that is precipitating. Fog is a stratus cloud
with its base at Earth’s surface.
Middle clouds range from 2,000 through 7,000 m in the middle latitudes. They
are a mixture of ice particles and liquid water droplets. They tend to be less mas-
sive than low clouds. The two principal types are altostratus and altocumulus.
Altostratus clouds appear layered while the faster rise of air in altocumulus clouds
makes the individual clouds look puffy and organize them into bands and waves.
Middle cloud decks sometimes extend over hundreds of thousands of square kilo-
meters. Even though these clouds can precipitate small amounts, this moisture
rarely reaches the surface.
High clouds range from 5,000 through 13,000 m (note the overlap of altitudes
with middle clouds). These clouds are composed entirely of ice crystals and have lit-
tle mass, rendering them incapable of precipitating to the surface except
minimally in some high mountain locations. The three principal types are cirrus,66 Clouds