5. Thermal maps
Physical principlesAtmospheric transmission is not uniform for radiation of different wavelengths. Gases in the atmosphere rather bring about different "absorption bands".
Larger areas with a high transmittance can be found in the visible spectral range and in the "infrared" range, namely between a wavelength of 2 and 5.5 µm and between 7 and 15 µm.
In this context, we speak of atmospheric "windows" as the earth is visible from space in these spectral ranges and vice versa.
Infrared thermography benefits from the physical property of bodies to release heat according to their surface temperatures. Energy is transported through electromagnetic waves here with radiation signifying the flow of electromagnetic waves per time and area. The physical relations are described by the Stefan-Boltzmann law and by Planck's radiation law.
As the heat release of the ground is low compared with solar radiation, radiometric measurements used to be impracticable for determining ground temperatures in the field of meteorology. It was only in recent years that semiconductor detectors were developed which are sensitive enough for perceiving these low intensities in the infrared wavelength range.
Surface consistency
|
Reflection coefficient |
| black earth, dry |
14 |
| fair sand |
8 |
| snow, clean |
30 - 40 |
| water surfaces
|
5 - 15 |
| green grass |
26 |
wheat
|
10 - 25 |
| concrete |
14 - 22 |
| wall, white |
65 - 80 |
| wall, yellow |
35 - 50 |
| wall, grey |
20 - 45 |
| asphalt |
12 - 25 |
| gravel
|
5 - 10 |
| average value of the earth's surface |
35 |
Table 5.1: Albedo (reflectivity) of different surfaces in %
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Fig. 5.1: Development of different surface temperatures in the course of the day (summer day)
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What plays a role for the warming of the ground is the subsoil's reflectivity and the thermal conductivity and effective heat capacity (table 5.1).
Significant temperature differences can occur on the same irradiation conditions depending on the type and consistency of the surface. This is proved by measurements on different surfaces, for example, which were carried out on a bright summer day (fig. 5.1).
In the mentioned example, asphalt reaches a temperature of more than 131 °F (55 °C) during the day, while a sunny lawn stays relatively cool with a temperature of less than 97 °F (36 °C). During the night, asphalt has a temperature of 77 °F (25 °C) and is still warmer by 5-7 K than the lawn or farmland.
In the late afternoon, that means before sunset, the lawn temperature sinks already below the air temperature and therefore starts to produce cold air near the ground.
Surface temperatures are not exclusively determined by radiation influences, however, but they are the result of an interaction between different energy conversions through radiation, evaporation and condensation of water and water vapour, man-made influences (combustion processes) as well as the energy conversion in the ground and through advection (atmospheric heat transport).
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| © City of Stuttgart, Office for Environmental Protection, Section of Urban Climatology |
