Environmental Engineering Reference
In-Depth Information
water occurs over warm-water surfaces such as the Great Lakes in the autumn, or over
the Arctic Ocean, where water will evaporate from the relatively warm sea surface and
rapidly condense into the cold air. Almost calm conditions are needed to avoid the
saturated air being mixed with drier air above.
Radiational or contact cooling at a cold ground surface may also be sufficient to
produce saturation, but as these are ground-based processes the resulting condensation is
known as fog. It is like cloud in being composed of myriads of water droplets but the
detailed mechanisms of formation are different. As there is very little upward movement
or mixing, the droplets do not increase in size and rain does not fall.
THE EFFECTS OF CONDENSATION
FOG
Fogs are a common feature of the climate of some parts of the world. For example, they
are frequent on the North Sea coast of Britain in summer, off the Grand Banks of
Newfoundland and in coastal Peru. There are two weather situations which can form fog.
First, when the ground loses heat at night by long-wave radiational cooling, usually with
the clear skies of an anticyclone; second, when warm air flows from a warm region to
cover a cold surface, particularly a melting snow surface with lots of moisture about. The
first type of fog is called
radiation
fog and the second
advection
fog.
Fog consists of microscopic droplets of water between 1 µm and 20 µm in diameter.
Visibility in a fog will depend upon the size and concentration of droplets in it. When the
droplets are small and numerous, visibility is poor, perhaps as little as 5 m. If pollution
adds suitable nuclei, condensation of water vapour is favoured. The actual formation of
radiation fog represents a delicate balance between
CLOUD TYPES
systems
Clouds are a vital element of Earth's energy budget. They reflect and absorb some of the
incoming solar radiation and trap much of the outgoing long-wave radiation. Such is their
importance in climate control that when models of the atmosphere are used to predict
future climate change the results can vary widely according to the assumptions about the
nature of the cloud systems. Cloud top height, thickness, density and spatial distribution
are of vital significance in affecting where energy can be absorbed and where it is lost.
Despite this importance, cloud features are one of the least well observed climate
variables. Most climate stations will observe the amount of cloud only once a day; it is
noted as the proportion of the sky covered by cloud expressed in oktas (eighths) in
Europe or in tenths in the United States. Few stations record the type of cloud or its
height; observations at night are not easy.
Because clouds develop in an infinite variety of forms and shapes attempts have been
made to classify them. The easiest and most widely used way is on the basis of their
appearance, a system largely devised by Luke Howard in 1803. Genetic systems based
upon the origin of the cloud have been suggested. As it is not always clear exactly how a
cloud formed they have been less successful.
