The urban atmosphere contains lots of solid particles. Together with the strongly modified, mostly artificial urban surface, these significantly change the radiation balance in a city compared to non-urban areas. The radiation balance (net radiation) of a given urban surface is given by the following formula:
Q = (1-A) (I · sin h + i) + (Ez - Ea)
Q - total net radiation of all wave lengths (also called the radiation balance)
A - albedo (expressed in tenths e.g. 0.7, not 70%);
(1-A) - amount of short-wave radiation absorbed by the surface
(I · sin h) - intensity of the direct solar radiation reaching the horizontal surface;
h - solar altitude; i - intensity of diffuse solar radiation
Ez - long-wave radiation of the Earth (the heat emitted by the surface into the atmosphere); the atmosphere absorbs about 96% of Ez, so only a small fraction goes to space, but it depends on the water vapour and greenhouse gas content of the air,
Ea - long-wave radiation of the atmosphere, also called back radiation (the heat emitted by the atmosphere to the surface);
(Ez-Ea) - the so-called effective radiation; the amount of heat lost by the Earth.
The value of the balance Q may be positive (i.e. more energy reaches the surface than is lost) or negative (i.e. more energy is lost from the surface than is gained).