Green in Urban Climate

Summary

Two negative aspects of urban climate are the urban heat island (= the overheating of the city) and air pollution, both of which can be diminished by areas of green vegetation. Only the former will be discussed here.

It is shown that the effect of diminishing the difference of temperature in its range and its reach increases with the size of a green area. But this effect on the wider environment should not be overrated. Even in the environment of very large green areas the effect can be noticed only within a relatively small distance. Convection reduces and suppresses wider effects.

The special climates of green areas, their effect on air temperature resp., were proved in measurements with different scales in Hannover <4,Beckröge,1980; 45,Wilmers,1972; 48,Wilmers,1978b; 49,Wilmers,1982> and as well in Bonn <37,Sperber, 1974>. These show that even in small spaces between buildings green areas have favourable bioclimatic effects.

The ranges of temperature, that occured in the environment were examined by measuring the surface temperatures with handheld scanner. Some examples of measurements of infrared temperature in garden courts in Hannover make clear that vegetattion surfaces show considerably different temperatures than other surfaces, above all dry and inanimate ones <45,Wilmers,1972; 48,Wilmers,1978b; 51, Wilmers,1986>. In this treatise we mention some of the values of infrared radiation temperatures, which may not be absolutely precise, because the grades of emissivity of some surfaces are not exactly known. Those values are near 1 for all natural surfaces, and small deviations from the mean extreme temperature differences. However we have measured great dissimilarities in temperature. There were ranges of surface temperature, during periods of intense solar insolation, of more than 20 Kelvin, dependent on the types of surface.

At night the differences are rather small, but the parts, which got less direct insolation, remain cooler, except for those parts which are protected against effective outgoing radiation.

The demonstrated measurements show important dissimilarities in the thermal behaviour of our surroundings. They were done in small areas but nevertheless they have a good indication of the different thermal zones of the urban environment.