Fig. 4a: Thermal image of Freiburg i.Br. 
The dark area near the middle left margin is the airfield, 
the city centre is at the same height farther to the right 
(1. April 1976, 5 h, altitude 4000 m) 
Same image as 4a, equidensity representation, 19 C - grey 
tones ranging from < 0° (dark) to > 5° (light)