Urban Climate

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Radiation balance in a city

The Sun delivers enormous amounts of energy to the Earth.  In this section we look at how this energy is altered in the urban environment and how air pollution changes the radiation balance.

Solar radiation is the main energy source for the climate system. The amount of energy from the Sun at the outer edge of the atmosphere on a surface perpendicular to the Sun's rays, when the Earth is at its mean distance from the Sun, is called the solar constant.  Changes in the distance between the Sun and the Earth resulting from changes in the orbit of the Earth around the Sun and changes in the radiation output of the Sun lead to small changes in the solar constant.  Over a yearly cycle, the value varies between 1365 and 1372 W m^-2.   However, the Earth only receives a fraction of this solar radiation as it is scattered and absorbed in the atmosphere.

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)

where:

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).

The Albedo (A) is the percent radiation reflected from a surface compared to the total amount which strikes it.  The albedo depends on the kind, colour and humidity of a surface and also on the amount of snow.  Building materials tend to have low albedos (they absorb solar radiation efficiently) compared to natural surfaces.  For example, 5 - 20% for asphalt, 10 - 35% for concrete, 20 - 35% for stone and 10 - 35% for roofing-tiles.  In comparison, the albedo for fresh snow is between 75 -95%, so most of the solar radiation which hits snow is reflected back into space.  Some natural surfaces do have low albedos, for example, chernozem (which is also known as black soil) has an albedo of 5 - 10% and deciduous forest is between 15 - 20%.  The albedo for water varies from a few percent to 90% depending on the angle of the incoming solar radiation (see the table below). 

The amount of radiation absorbed by a city is about 15 to 30% greater than a non-urban area.  In addition, all the different artificial surfaces form a sort of mosaic and cause large spatial variations in the albedo.  This, in turn, affects the air temperature of the city.

A

B

Sun inclination	1°	5°	10°	20°	30°	40°	50°
water albedo (%)	89.6	58.6	35.0	13.6	6.2	3.5	2.5

3.  Variation of the albedo of water as the angle of the Sun changes.

The amount of global solar radiation (i.e. both direct and diffuse solar radiation) may be between 10 and 20% less in a city due to air pollution and increased cloudiness.   The direct radiation may be reduced by as much as 50%.  This means that the amount of incoming ultra-violet radiation is less.  Reductions in this biologically active radiation lowers our risk of skin damage but also decreases the number of harmful bacteria which are killed.  The air pollutants form aerosols which absorb the long-wave radiation from the Earth (Ez), and then radiate it back into space (Ea). All these factors cause an increase in air temperature in the cities compared to the surrounding non-urban areas.

4. Changes in the amount of short-wave radiation from the Sun in a city compared with non-urban areas.  "Direct radiation -15%" means that the amount of direct radiation from the Sun is 15% lower in the city than in the non-urban area.  Author: Sebastian Wypych.

5. Changes in the amount of long-wave (infra-red) radiation in a city compared with non-urban areas.  "Back radiation of the atmosphere +10%" means that the amount of radiation in the city is 10% higher than in the non-urban area.  Author: Sebastian Wypych.

Political and economic changes in Central Europe have led to reductions in the amount of air pollution and changed the urban radiation balance in this area. In the 1990's, most post-communist countries experienced economic crisis and a decline in industrial production.  This resulted in lower emissions of air pollution.  In addition, new low emission technologies were introduced into many factories, further improving the air quality.  This reduced the number of aerosols in the air and changed the amount of cloud (through the indirect aerosol effect).  Between 1996 and 1999, the amount of radiation scattered back into space by clouds over mid-Europe decreased by 2.8% (compared to the years 1985-1989) leading to an increase in the solar flux of roughly 1.5 W m^-2. 

Related pages:

Read more about Earth's radiation balance in:
Lower Atmosphere - More - Unit 2 - Radiation

About this page:

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: 2003-08-21