The terms of the surface heat balance, by day (a) and by night (b) (Stull 1988)
The exchanges of heat between the soil and the atmosphere modify the temperature, humidity and stability of the atmospheric boundary layer, therefore 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 on the surface (Stull 1988)
- Q * s = QH + QE - QG
where we assume that the upward flows are positive, and
Q * s = net radiation
QH = sensible heat flow
QE = latent heat flow
QG = molecular heat flow from the underlying soil
The heat flows have different values between day and night, moreover they vary according to the season, to the meteorological conditions, to the water content in the soil, to the type of soil and to the ground cover (type of vegetation etc.).
At noon on a sunny day (above the earth), -Q * s is positive because the radiation that comes from the sun (directly or indirectly) and reaches the surface is greater than that which the surface reflects and emits towards the atmosphere. QH and QE are positive, since sensible heat and humidity are transferred from the soil to the atmosphere. -QG is positive because the surface gives heat to the underlying soil by molecular conduction.
At night instead (above the earth) -Q * s is often negative (the soil 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 faster). QE is negative, because daytime evaporation has given way to dew and frost. Even -QG changes sign, becoming negative: now it is the ground below to give heat to the surface.
Source: Urban microclimate: impact of urbanization on local climatic conditions and mitigation factors
By Giovanni Bonafè
Surface heat balance
Graph (Stull 1988)
Scale 1: 100