Surface Temperatures Day and Night 2000
IR thermal imaging does not directly measure surface temperature. Surface temperatures are calculated from the longwave radiation emanating from structures. The radiation temperature, as it is called, is measured. Radiation temperature is a transport of energy by electromagnetic waves. Radiation is then defined as the flow of electromagnetic waves per given area during a given period of time. The radiation and temperature of an object’s immediate surface are in a functional relationship to each other, as expressed in the Stefan-Boltzmann equation. This relationship occurs when the surface approaches its full emissivity (emissive capability) (theoretical emissive value = 1). Values are known for all important surface elements within the imaged wavelength spectrums of 10.4 to 12.5 µm, so that the influence of the atmosphere on measured emissive behavior remains minimal. The difference between the radiation temperature measured by satellite and the calculated surface temperature is then usually negligible. Only metal surfaces, such as used in flat roofs, deviate significantly with emissive values of 0.1. They must be given a special category during interpretation.
Of much greater significance is the degree of spatial resolution of the image elements in pixels of 60 m x 60 m. These are transposed into pixels of 30 m x 30 m before delivery by the ESA ground station. However, the larger initial size often means the registration of mixed signals which impedes the determination of traffic areas, smaller urban squares, or various vegetal structures. Each grid of approximately 3,600 m² can initially only be given an average radiation temperature integrating all the surface elements within the grid.
The initial material was digitally processed by ERDAS, an image processing system. It was necessary to perform a geometric distortion correction first by means of basing point definition referring to the vector data of the landscape structure of the Informationsystem Urban and Development and second by the assignment of clear defined objects in the panchromatic map from the Landsat-day-scene. The panchromatic record of the 14.08. offered the best spatial dissolution of the satellite data with a grid of 15m x 15m. These data were corrected geometrically with help of the passport points distributed over the whole city area.
Die radiation amounts could be derived from the gray values of the satellite data. The gray values must be converted into spectral ray densities at first. In a next treatment step they were converted into the surface temperatures of the observed area. The values gotten in the evaluation in °C were rounded up to 1°C to simplify and reduce the amount of data.
The calculation of the day-/night temperature differences was executed for the two scenes day and night. The results were also rounded on 1 °C. Statements about the level at which the temperature differences range, i.e., whether at relatively high or at lower surface temperatures, are offered by Figure 2.
Remarks for Interpretation
The possibilities and limits of the survey technology and time periods described above should be mentioned again as a basis for the interpretation and comparative analysis of day and night levels:
- Small areas of differentiated horizontal and vertical structures, such as interior courtyards, street areas, and city squares, could be recorded only as mixed pixel images.
- The overflight times during the morning and early evening did not record the time periods of greater heating or greater cooling. Material-dependent heat conduction and heat storage exert a special influence. The dry sand soil, with high air content, of farmlands and dried-off vacant areas has a poor heat conductivity, particularly on sunny weather days with weak winds. This produces a quick morning heating and a quick evening cooling in the map image. Inversely, the high heat storage properties of building materials such as concrete, asphalt, and stone lead to a slower heating and cooling, and thus to a limited representation of “the urban heat-island”.
- Seasonal changes in open areas are of great importance. Critical modifications in temperature behavior sometimes occur during harvest or large-scale dying of surface stocks, particularly in field areas and rough meadows.