With regards to temperature, cities contribute constantly to warming; this, however, is not necessarily a negative characteristic of the urban climate. On average, cities are annually 1 to 2 degrees Celsius warmer than their surrounding landscapes. Particularly large temperature differentials arise on clear nights during the daily temperature minimum.

Figure 2/3 shows this relationship for European cities dependent upon on the size of the city. Cities with millions of inhabitants can witness a temperature differential above 10 degrees. One can also discern, however, a recognizable heat island effect throughout the smaller cities.

Studies in Munich (BRÜNDL et al, 1986) have shown that temperature levels in city districts depend largely upon the degree of soil capping. An increase of 10% in the proportion of soil capping thus produces a rise in average annual temperatures by 0.2 degrees.

This generally higher temperature level exerts a perceptible positive effect on the inner-city vegetation. The effect can be noted by the presence of numerous warmth-loving plant types in front yards and parks as well as in the lengthened vegetation period. Opportunities for outdoor activities are also more frequent in cities. Similarly, the need for heating energy is reduced.

Various types of ground cover warm themselves at highly different rates on cloudless summer days with little wind. This depends on the absorption ability, the heat capacity, the heat conductivity, and the evaporation ability of the underlying ground.

For example, asphalt absorbs 80% to 90% of incoming radiation, whereas a white wall absorbs only 20% to 35%. Temperature measurements vary between less than 30 degrees to almost 50 degrees Celsius (LORENZ, 1973).

The diurnal variations in the temperatures of various materials and surfaces on a hot summer day are shown in Figure 2/4 from FEZER (1975).

In addition to the material properties of surfaces, the height and arrangement of buildings is relevant to the temperature conditions in a city. Narrow streets and alleys produce shadowing effects, which leads to a delay in the warming of the urban realm. The artificial narrowing of the horizon also decreases the heat dissipation of the building surfaces, however, which reduces nightly cooling in the streets.

The interaction of these factors inside the various structures and built densities of the city leads to a mosaic of varied thermal microclimates, which join with each other to produce a clearly-defined heat island (or heat archipelago) when compared with the surrounding land.

These heat island conditions are recognizable in infrared heat images (e.g. thermal map of the neighborhood federation of Stuttgart, Figure 5/1) with their large spatial differentials in surface temperatures.

The development of heat islands in Stuttgart is shown also in the following representation (Figure 2/5a) and (Figure 2/5b) of temperature distributions on 21 August 1965 at 6:00 AM (HAMM, 1969). The figure shows the heat island effect in summer; according to the same study, however, the heat island effect is of the same order of magnitude in winter. The large temperature differentials in Stuttgart amount to roughly 6 degrees between the central city and the edge zones of the city basin.