Solar radiation is the main energy source for climate system. The amount of energy received from the Sun at the outer edge of the atmosphere, on a surface perpendicular to the Sun's rays, when the Earth is at a mean distance from the Sun, is named solar constant. Small changes of the solar constant in a yearly cycle depend on the revolution of the Earth around the Sun, which causes the differences in the distance between the Sun and the Earth. Its value varies between 1365 and 1372 W/m2, as the Sun's radiant output varies, too. However, the earth's surface receives only a part of the solar radiation, as it gets weakened by the atmosphere due to scattering and absorption.
Urban atmosphere contains lots of pollution consisting of solid particles. Together with strongly modified, mostly artificial urban surface, they significantly change the radiation balance in a city, in comparison with non-urban areas. The radiation balance (net radiation) of a given urban surface is expressed with the following formula:
Q =(1-A) (I · sin h + i) + (Ez - Ea)
Q - net all-wave radiation (also named radiation balance)
A - albedo (expressed in tenths e.g. 0.7, not 70%); (1-A) - short-wave radiation absorbed by the surface
(I · sin h) - intensity of direct solar radiation reaching the horizontal surface; h - solar altitude; i - intensity of diffuse solar radiation
Ez - long-wave radiation of the earth (heat emitted by the surface to the atmosphere); the atmosphere absorbs about 96% of Ez, only a small fraction goes to the space, but it depends on the content of water vapour and greenhouse gases in the air, Ea - long-wave radiation of the atmosphere, also called back radiation (heat emitted by the atmosphere to the surface); (Ez-Ea) - so-called effective radiation; the heat lost by the earth
The value of the balance Q may be positive (i.e. more energy is coming to the surface than is being lost) or negative (i.e. more energy is lost by the surface than gained).
Albedo (A) is the percent of radiation returning from a surface compared to that which strikes it. It is modified in a city mainly by the very differentiated character of the urban surface, and depends on the kind, colour and humidity of a surface, but also on duration of snow-cover. For building materials, low albedo is typical in comparison with some natural materials and surfaces, e.g. 5-20% for asphalt, 10-35% for concrete, 20-35% for stones, 10-35% for roofing-tile; for fresh snow it icreases up to 75-95%. However, some natural surfaces may have also low albedo, e.g. chernozem (known also as black soil) 5-10%, deciduous forest 15-20%. Water's albedo varies from a few to 90% depending on an angle of incoming solar beams (see table). Therefore, the amount of absorbed radiation is about 15-30% larger in a city than in a non-urban area. Moreover, different kinds of artificial surfaces form a sort of mosaic and cause large spatial variability of albedo, which in turn effects air temperature in a city.
1. Simplified model of the reflection and absorbtion of solar radiation by different surfaces, at the same Sun inclination. Author: Sebastian Wypych, Mateusz Kaminski
2. Dependence between albedo and Sun inclination, for the same surface. Water is given as an example. Attention: for various surfaces the dependence may be different. Author: Sebastian Wypych, Mateusz Kaminski
water albedo (%)
3. Water albedo values in connection with Sun inclination
The amount of global solar radiation (i.e. both direct and diffuse solar radiation) may be reduced in a city by 10-20% due to air pollution and increased cloudiness. The direct radiation, however, may be reduced by as much as 50%. It means that also the amount of incoming UV radiation is reduced; it is biologically active radiation which improves aerosanitary conditions, e.g. kills bacteria that cause various diseases. The air pollutants form aerosols which absorb the long-wave radiation of the earth (Ez), and then radiate it back (Ea). All the mentioned factors cause air temperature increase in urban areas.
4. Changes of short-wave radiation's streams (received from the Sun) in a city compared with non-urban areas; e.g. "direct radiation -15%" means that in a city the direct radiation is lower by 15% than in non-urban areas Author: Sebastian Wypych
5. Changes of long-wave radiation's streams (i.e. infrared radiation) in a city compared with non-urban areas; e.g. "back radiation of the atmosphere +10%" means that in a city that stream of radiation is higher by 10% than in non-urban areas Author: Sebastian Wypych
Recently, radiation balance of urban areas in Central Europe is modified by the changes of air pollution due to economic and political changes. In the 1990s, most post-communistic countries experienced economic crisis and decrease of industrial production followed by lower emission of air pollution. Additionally, new, low-emission technologies were introduced in many factories, further improving the air quality. In the years 1996-1999, the back scattering capability of clouds over mid-Europe (due to the indirect aerosol effect) decreased by 2.8% (in comparison to the years 1985-1989) leading to an increase of solar flux of roughly 1.5 W/m2.
Authors: Sebastian Wypych, Anita Bokwa - Jagiellonian University - Cracow / Poland Supporter: Anna Gorol 1. Scientific reviewer: Prof. Barbara Obrebska-Starkel - Jagiellonian University - Cracow / Poland - 2003-06-20 2. Scientific reviewer: Dr. Marek Nowosad - Maria Curie-Sklodowska University - Lublin / Poland - 2003-06-16 educational reviewing: last update: 2004-12-17