Geoscience Reference
In-Depth Information
Plate 4.3
Jetstream cirrus and cirrostratus clouds. The clouds are aligned with the strong upper winds. The semi-arid vegetation
in the foreground is typical of south-east Spain, with an annual average precipitation below 200 mm.
Photo: Peter Smithson
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
sufficiently for rain to fall.
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 Newfoundland in
Canada 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.
and vertical overturning in the upper troposphere.
Photo: Peter Smithson
