Environmental Engineering Reference
In-Depth Information
SENSIBLE HEAT
How do these processes which use the net radiational energy operate? One way is by
sensible heat transfer.
Sensible heat is the exchange of warm air down the temperature gradient. By day, this
will normally be upwards, but at night there may be a weak transfer of sensible heat
down to the cooler ground surface. It takes place because the air in contact with the
surface becomes warmer through conduction. Being warmer, the air will be less dense
than its surroundings and, like a cork in water, will tend to rise until it has the same
density (temperature) as its surroundings. Occasionally this process can be seen
operating. If the ground is being warmed intensely, the rate of sensible heat transfer is
high. The rising air can be seen as a 'shimmering' of the air layer near the ground due to
the variable refractive indices of light through the air of
Figure 3.8
Global distribution of the vertical transfer of
sensible heat. Units are W m
-2
.
Source: After Budyko et al. (1962).
different temperatures. Replacing the rising warm air are pockets of cooler air descending
towards the ground.
The significance of sensible heat in the local heat budget depends upon the frequency
and intensity of surface heating. Where the surface is usually hotter than the air, values
may be high, but where there is little temperature difference, as over most ocean surfaces,
sensible heat transfer will be low (Figure 3.8).
