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Pixel Thermometer

Many remote sensing sensors can detect the thermal radiation emitted by objects upon the surface of the earth via their thermal bands. This thermal radiation is invisible to the human eye. In most cases, the spatial resolution of the thermal band is worse than that of bands of the visible light. This is due to the fact that in the case of long-wavelength thermal radiation less energy on the surface of the earth is emitted per area than in the case of short-wavelength radiation. Nevertheless, different surfaces can be distinguished well.
For instance, a city emits more warmth than forests do. Bodies of water, however, have a considerably lower temperature and can be spotted quite easily on thermal images as they stick out as black spots and lines.



Berlin as seen in the thermal band, day and night. (Processed, original images by courtesy of USGS/NASA Landsat Program)


The swipe above shows the city of Berlin during daytime and nighttime in the thermal band of a satellite sensor. Due to the colouring of the image, very cold areas are blue and very warm areas are red.
The Tiergarten (park) and the Spree (river) are clearly visible during daytime as they are cold areas contrasting with their warm surroundings. At night, however, the temperatures have cooled off – the colour scale stayed the same – and adapted to the situation within the city. Now, the Spree, for example, is hardly visible. Only the area surrounding Berlin-Tempelhof Airport sticks out as a cold air reservoir. In addition, the road network of the German capital is clearly visible. This is due to the fact that asphalt emits warmth much more slowly than, for example, the surrounding houses made of bricks and concrete.




How warm was it Sunday in Berlin or in Bödefeld (Sauerland) a year ago? Satellites constantly record the temperatures of every place on the surface of the earth.