Geoscience Reference
In-Depth Information
CHAPTER FOUR
atmosphere
On a hot day the picture of clouds building up often
signifies that a storm is imminent. We do not always
appreciate what is happening, but these growing clouds
represent one of the most vital processes in the atmos-
phere - the condensation of water as it is raised to higher
levels and cooled within strong updraughts of air.
The water, of course, was derived from the surface -
evaporated from the oceans, from the soil, or transpired
by the vegetation. But within the atmosphere a variety of
events combine to convert the water vapour, which is
produced by evaporation, to water droplets. The air must
rise and cool for condensation to occur. In this chapter
we shall be looking at the nature and consequences of
these processes. Precipitation at the ground surface and
the evaporation process will be covered in
Chapter 5.
Gases expand on heating more than liquids or solids, so
this effect is quite marked. Moreover, as the air expands
its density falls; in simple terms the same mass of air now
occupies a larger volume. As its density falls so it becomes
lighter than the surrounding air and it tends to rise.
Reverse the process, cool the parcel of air, and the opposite
occurs. It contracts, its density increases and it sinks.
One effect of the heating and cooling of the surface
atmosphere is therefore to cause vertical movements of air.
But there are other effects. As the air becomes cooler its
ability to hold moisture in the form of water vapour is
reduced. If it cools to the point where it can no longer hold
the water vapour as a gas, condensation occurs and water
droplets appear. If the air is heated, these droplets tend to
evaporate and become water vapour once more. Thus
heating and cooling are intimately linked with the
processes of evaporation, condensation and precipitation
formation. Let us consider these main effects in turn.
EFFECTS OF HEATING AND
COOLING IN THE ATMOSPHERE
Vertical movements
The rising of warm air is a process we can see on a hot
day by the shimmering effect of air near the surface; we
can see it too if we watch the beautiful and immense
movements usually develop if local heating of the surface
takes place, so that individual parcels of air become
warmer and lighter than the air around them.
Local heating occurs for a number of reasons. Variation
in the colour or wetness of a surface may cause differences
in atmospheric heating; the air above dark-coloured or
dry surfaces heats up more rapidly than that above light
General effects
The atmosphere is a highly complex system, and the
effects of changes in any single property tend to be
transmitted to many other properties. Thus heating and
cooling of the air cause adjustments in relative humidity
and buoyancy; they may cause condensation and evapora-
tion, cloud formation and the development of storms.
What happens, then, when air is heated? To simplify
the problem we will consider a parcel of air in contact with
the ground. As the ground is heated, the air in contact
with it will warm also; its temperature rises and it expands.
