Small areas of vegetation, for example green inner-city areas, do not have any effect on a town or city’s climate, though they do affect its microclimate. Small green zones are oases in the urbanised area, where the shade provided by trees and the fewer hot, paved surfaces mean that air and surface temperatures are lower. Many possibilities for introducing green areas into towns and cities on a small scale have not yet been utilised, for example between the rails of tramlines and in places of unused paved surfaces.
Parks of 2.5 hectares or more in size have a measurable cooling effect on the surrounding areas. Ideally, the parks should be designed to have grass and shrubs, and the trees should be spread far enough apart to allow the wind to blow through them freely. The cooling effect is caused by the evapotranspiration and the shade. At night the cooling effect is greatest, when air flows between the park and the surrounding areas. The effect reaches half of the diamter more or less proportionate to its diameter.
Grids of connected green areas function in a similar manner. Ministerium für Klimaschutz NRW, 2011
See also the measure: Reducing pavements and improving the ground: patios out, green in.