|
 What
is a cloud? Although they may look fairly solid and may be
beloved of poets, they are in fact merely a collection of
water droplets, ice crystals or a complex mixture of both.
Clouds are formed by climatic forces acting on moisture in
the air. This moisture is constantly renewed by water molecules
escaping from land surfaces and water into the air and evaporating
as a gas or vapour. The amount of the water vapour that can
be held in the air depends on the temperature. The cooler
the air, the less water it can hold (which is why areas near
the equator of often cloudy
Once the air has become
cold enough some of the water vapour condenses to form a visible
mass of tiny droplets. This condensation depends on the presence
in the atmosphere of minute particles. Often these particles
(known as condensation nuclei) can be salt from ocean spray
or particles from fires or volcanic eruptions. When such condensation
from vapour to droplets occurs on the ground, it is called
dew. Near the ground it is called fog or mist. Up in the sky
it is called clouds.
A classification of
clouds was first introduced by Luke Howard (1772-1864) who
used Latin words to describe their characteristics.
- Cirrus - a tuft or filament
(e.g. of hair)
- Cumulus - a heap or pile
- Stratus - a layer
- Nimbus - rain bearing
There are now ten
basic cloud types with names based on combinations of these
words (the word 'alto' meaning high but it is also used nowadays
to denote medium-level cloud).
Clouds form when moist
air is cooled to such an extent that it becomes saturated.
The main mechanism for cooling air is to force it to rise.
As air rises it expands, because the pressure decreases through
the atmosphere, and therefore cools. Eventually it may become
saturated and the water vapour then condenses into the tiny
water droplets mentioned above. If the temperature reaches
below about -20C many of the cloud droplets will have frozen
so the cloud mainly composes of ice crystals.
There are four main
ways in which air rises to form cloud:
- Rapid local ascent when
heated air at the earth's surface rises in the form of thermal
currents (convection).
- Slow widespread mass ascent
where warm moist air is undercut by cold air (the barrier
between the warm and cold air is called a 'front').
- Upward motion associated
with turbulent eddies resulting from the frictional effect
of the earth's surface.
- Air forced to rise over
a barrier of mountains or hills.
The
first of these tends to produce cumulus-type clouds, whereas
the next two usually produce layered clouds. The last can
produce with cumulus-type cloud or layered cloud depending
upon the state of the atmosphere. The range of ways in which
clouds can be formed and the variable nature of the atmosphere
give rise to the enormous variety of shapes, sizes and textures
of clouds.
Related links
Cumulus
Cumulonimbus
Stratus
Cirriform
Stratocumulus
and Altocumulus
Types of Clouds
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