Geography Reference
In-Depth Information
As long as no condensation of water vapour occurs, a rising air
parcel will cool at the dry adiabatic lapse rate of 9.8°C per kilome-
tre, while the surrounding air changes temperature at the lower
rate of the environmental lapse rate. However, condensation of
water vapour may occur. Warmer air can hold more water vapour
than cooler air. When an air mass holding water vapour has cooled
sufficiently so that it is saturated (the
dew point
), condensation of
water occurs. (Think of a cold drink you take out of the fridge and
place in a warm room; water droplets form on the outside of the
drink container as the air around it has cooled to the dew point
and becomes fully saturated leading to condensation of water
vapour onto the container.) The condensation process releases heat
which warms the air and counteracts the cooling that results from
expansion of the rising air. This warming may force the air even
higher to form large clouds (the ones we see in the sky) of con-
densed water. So it is the difference between the lapse rates that
determine whether there will be a continual rise of the air mass and
cloud formation, or whether there will be very stable conditions
(i.e. when the environmental lapse rate is less than the dry and sat-
urated adiabatic lapse rates).
The above processes highlight the important role of water
vapour in atmospheric motion. On average the atmosphere holds
about a fortieth of global annual rainfall (about 25 millimetres
depth of water if it were all deposited evenly across the whole
Earth). Regular evaporation from land and oceans maintains rainfall
throughout the year in different parts of the world. This is not
evenly spread across the planet and some areas provide a lot more
evaporation than they receive as rainfall, and vice versa, indicating
that water moves significant distances in the atmosphere. As we
will see later on, evaporation is also important for controlling major
ocean circulations which have a big impact on the Earth's climate.
The above description of vertical atmospheric processes does not
explain why winds and water vapour move horizontally across the
Earth or why the same location can experience periods of calm and
periods of storminess. The Earth's wind circulation patterns help
form the climatic zones. There are two main processes that drive
global wind circulation. The first is the uneven distribution of the
Sun's radiation over the Earth's surface due to its spherical shape.
Figure 1.1 shows how the same amount of solar radiation is spread
