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2 to 3°C warmer than the adjacent northwestern suburbs, al- 
though here again the structure is similar. This can be seen 
best in Fig. 4b, an equidensity representation of Fig. 4a. 
After applying a digital low-pass filter [Goßmann & Nübler 1977], 
the 0...59C range of the original data (Fig. 4a) is split into 
5 grey tones representing 1°C each; black is lower than 0°C, 
white is warmer than 5°C. 
The problem of differing ST in regions of the town with com- 
parable structure can be explained by the influezce of a local 
circulation system. Under anticyclonic conditions at night cold 
air flows from the slopes and valleys of the Black Forest to- 
wards the Rhine Valley. 
The wind field in the urban area of Freiburg under the influence 
of a well developed mountain wind system is shown in Fig. 2a. 
Fig. 2b shows that a similar wind field occured when the thermal 
image (Fig. 4a) was recorded on 1st April 1976 between 5 am and 
6 am. Although the general wind direction is West to Southwest 
(Feldberg), in Freiburg southeasterly wind prevails throughout 
the night. 
Due to the topographic situation, with the city center slightly 
north of the opening of the Dreisam valley, this stream of cold 
air reaches only the southern part of the city centre, cools it 
and displaces its heat island westwards, thus warming the 
western suburbs. This explanation is confirmed by extensive 
dir temperature measurements and by registrations of wind speed 
and wind direction [Nübler 1977]. 
This means that when the structure of the surface cover is known 
and corroborative evidence from ground truth measurements is 
available, the cooling of surfaces by cold air-flows and the 
extent of their influence can be deduced from thermal images. 
Example 2 
In the second example it is more evident that wind must have 
shaped the distribution of ST. The area covered in Fig. 5a is 
situated 8 km northwest of Freiburg in the Rhine Valley (see 
Fig. 1). The villages Gottenheim and Umkirch are near the upper 
left and lower right corners respectively. The region is partly 
wooded and partly open field. The thermal image was recorded 
from an altitude of 2000 m on 2nd April 1976, before sunrise. 
Again the forests are warmest with ST between 5 and 10°C. The 
most remarkable feature, however, are narrow zones of lower 
ST on most northern margins of forest areas. It is clear by