Content

Surface Temperatures Day and Night 1991

Statistical Base

Earth resource information satellites have been operated by space agencies since the beginning of the 70’s. The US Landsat-5 remote-sensing satellite orbits the earth in an almost-true polar orbit at an altitude of 705 km. Landsat 5 carries a multi-spectral scanning system called the “Thematic Mapper” ™. The satellite images a 185 km-wide strip along the earth during each orbital cycle of 1-1/2 hours. The entire surface of the earth is thus surveyed in 16 days. The satellite passes over Berlin and environs in about 20 seconds. Data is digitalized and radio-transmitted to ground stations, which record it on magnetic tape. The satellite’s thermal resolution quality about 1,500 points, or pixels, per line. Each point equals a surface area of 120 m x 120 m. Flight patterns during the day and night are not identical, resulting in different image sectors in the resulting maps. Night scans of this area must be specially ordered by way of the ground station of the European Space Agency (ESA) in Italy.

The seven total spectral ranges of the Landsat-TM are in wavelengths from the 0.45 mm of blue-green light, to the 12.5 mm of heat-infrared.

The longwave spectrum between 10.4 and 12.5 mm was selected for the imaging of surface temperatures. This portion of longwave radiation emanating from the earth itself can pass relatively undisturbed through the atmospheric layers. It is called the “infrared window”.

Choices for the two imaging scans were coupled to the time periods that the satellite was over the Berlin area (the time periods could not be changed) during the early evening and the morning of the following day. The selection was also coupled to certain meteorlogical requirements. A recording of the behavior of surface structures as precisely as possible requires that there will be no influence on the area to be examined by clouds, previous precipitation, or too high wind velocity. Consideration of these requirements during the summer half-year of 1991 allowed usable images only during the time periods of the evening of 14 September 1991, 21:45 CET (Image 49-222), and the following day of 15 September 1991, 10:30 CET (193-23). The meteorlogical conditions measured at the Dahlem Station of the Free University of Berlin were:

  • 14 Sept., 22:00 CET: Cloudiness, 0/8; wind velocity, 1.0 m/s; air temperature at 2 m altitude, 13°C
  • 15 Sept., 10:00 CET: Cloudiness, 1/8; wind velocity, 2.5 m/s; air temperature at 2 m altitude, 23°C

A high pressure weather condition with weak winds prevailed before and during the time of overflights. The weather was characterized by extreme, continuing dryness, and hours of sunshine approaching those theoretically possible. This time period was very suitable for the survey.

A climate survey was conducted at the same time by the TU (Technical University Berlin) (Maps 04.02 – 04.05 and 04.07). This survey conducted mobile field surveys along selected routes. Measurements were taken of air temperature, wind, and vapor pressure at an altitude of 2 m. The surface temperatures of homogenous structures, such as surface waters and large-area parking lots, were determined by analog technology. It was thus possible to compare calculated with actually measured surface temperatures. This parallel measurement of climate parameters at an altitude of 2 m also facilitated the inclusion of surface temperature maps in preparing Climate Function Map 04.07.