Heat at the Surface

Heat balance at the surface
Chart (Stull 1988)
Scale 1:100

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The heat balance terms at the surface, by day (a) and by night (b) (Stull 1988)

Heat exchanges between the soil and the atmosphere modify the temperature, humidity and stability of the atmospheric boundary layer, thus ultimately influencing the microclimatic conditions of our living environments.
As a first approximation, and in particular neglecting the role of urban areas, we can consider the heat balance to be valid at the surface (Stull 1988)
– Q*s = QH + QE – QG
where we assume that upward flows have a positive sign, and
Q*s = net radiation
QH = sensible heat flux
QE = latent heat flux
QG = molecular heat flux from the underlying ground

Heat flows have different values ​​between day and night, and also vary according to the season, weather conditions, soil water content, soil type and land cover (type of vegetation, etc.). 

At noon on a sunny day (above the ground), -Q*s is positive because more radiation from the sun (directly or indirectly) reaches the surface than the surface reflects and emits back into the atmosphere. QH and QE are positive because sensible heat and moisture are lost from the ground to the atmosphere. -QG is positive because the surface loses heat to the ground below by molecular conduction.

At night (above the ground), however, -Q*s is often negative (the ground continues to emit infrared radiation, even when it no longer receives radiation from the sun). QH is negative because the air releases heat to the ground (which cools down more quickly). QE is negative because daytime evaporation has given way to the formation of dew and frost. -QG also changes sign, becoming negative: now it is the ground below that releases heat to the surface.

Source: Urban microclimate: impact of urbanization on local climate conditions and mitigation factors
Edited by Giovanni Bonafè
 

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