Environmental Engineering Reference
In-Depth Information
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 towers of a
convectional cloud (Plate 4.1). Such vertical 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
Plate 4.1
Cumulus clouds developing in an unstable air flow
in Slovakia. Individual turrets of rising saturated air can be
seen. Parts of the cloud tops can be seen evaporating into the
drier air above.
Photo: Peter Smithson.
surfaces heats up more rapidly than that above light or wet surfaces. Differences in slope
angle may have the same effect. But there is another factor that plays an important role in
these vertical air movements. It is the vertical change in air temperature away from the
ground surface. It is known as the environmental lapse rate.
ENVIRONMENTAL LAPSE RATE
If we measured air temperatures in the troposphere at different heights under cloudless
conditions, we would find that temperature usually falls with height. The reason is quite
