Environmental Engineering Reference
In-Depth Information
place. This air is heated in turn. Consequently we have convection currents rapidly
transferring heat to the cooler layers of the lower atmosphere. If there is a strong wind
blowing, the mixing of heat is encouraged and the temperature profile in the lower
atmosphere becomes less steep (Figure 8.3).
THE RADIATION BALANCE AT NIGHT
At night the net radiation at the ground surface is negative (Figure 8.2). More long-wave
radiation is lost from the
Figure 8.3
Daytime and night-time temperature profiles on
windy and calm days with clear skies.
ground than is returned as counter-radiation from the atmosphere. This is especially true
with clear skies and dry air, which allow the long-wave radiation to escape to space more
easily. In cloudy or humid conditions heat loss is less effective, for water vapour readily
absorbs long-wave radiation and re-emits it towards the ground. During the night,
therefore, the surface normally gets cooler, although heat may flow up from lower levels
in the soil to reduce the cooling effect of radiation loss. In a sandy soil, with its large air
spaces, this process is limited, so the surface becomes particularly cool, as anyone who
has slept outdoors on beach sand will know. The air in contact with the ground also gets
cooler, making the air denser and preventing any thermals of warmer air rising and
mixing with the air above.
The night-time profile of temperature during calm conditions is shown in Figure 8.3,
which illustrates the major cooling at the surface. If conditions are windy, the cooler air
will be mixed with the warmer air above so there may be little temperature change with
height. Clouds are efficient emitters or radiators of long-wave radiation, so low clouds
encourage counter-radiation to the surface and the net loss of energy from the surface is
reduced. On cloudy and windy nights the cooling at the surface is small and temperatures
decrease away from the surface. The factors most favouring low surface temperatures at
night are consequently clear skies, dry air, no wind, and sandy, dry peat or snow-covered
soil. If such conditions occur at the beginning of the growing season in most temperate
latitudes then damage to frost-sensitive crops is likely.
WIND NEAR THE GROUND
