Geography Reference
In-Depth Information
river flows. Where there are glaciers these can modify the local
climate. Glaciers cool air in contact with the surface and depending
on the moisture content of the air they can act as either a local
moisture source or sink. For example, if the air is warm then water
vapour can undergo
sublimation
(change directly from the solid
to gaseous state) into the air above the glacier and this uses energy
and thereby cools the air.
Box 1.3 Rain and snow
Clouds contain tiny droplets of condensed water. The droplets are very
light and remain suspended in the air. As these droplets move around
some of them collide and join together. As they grow bigger they
become heavier and can start to fall as the gravitational pull drawing
them down will be greater than the force of the rising air keeping them
buoyant. This process is typical in warm clouds and the deeper the
cloud the bigger the drops will grow and the faster the rainfall.
In the middle and high latitudes, however, many clouds form where
it is well below 0°C. Here, clouds contain ice crystals as well as water
droplets. The latter exist because the very small size of the droplets
means they do not freeze straight away and they become 'supercooled'.
A peculiar property related to the lower saturation point of ice com-
pared to that of water means that some of the water droplets evapo-
rate and then freeze onto the ice crystals causing them to grow. As they
become heavier and fall they also collide and stick to each other,
forming snow. As the flakes fall they may warm and melt and then
produce rain. Clearly, if the air is cold near the surface then this melting
does not occur and snow will reach the ground.
Land and sea breezes
Water is slow to heat up and cool down. Therefore, deep water
bodies witness little daily change in surface temperature. The air
over these water bodies does not experience large daily fluctuations
in temperature. Over the land there are greater daily changes in air
temperature, particularly in the summer when the Sun's energy is
stronger and the land heats up during day and cools down at night.
In the middle latitude summer half-years, sea surface temperatures
(and the air in the layers close to the surface) are cooler than land
