Content

Surface Temperatures Day and Night 1991

Introduction

The inclusion of climatological aspects in evaluating environmental situations of urban metropolitan areas and their spatial planning requires a definition of the term urban climate. Urban climate is understood, according to Schirmer et al. 1987, to be “the strongly modified mesoclimate (local climate) of cities and concentrated industrial areas in comparison to their surrounding areas. It encompasses the entire volume of near-surface air layers above and in the direct vicinity of the city and its urban borders. It is caused by the type and density of building, the heat storage capacity of the soil, a lack of vegetation, a changed water balance and increased emissions of waste gasses, aerosols, and heat wastes.”

Definite limit values and guidelines for evaluating climate situations are lacking. The ideal urban climate to be striven for is one largely free of pollutants. It offers its inhabitants as great a diversity of atmospheric conditions as possible, and avoids extremes (cf. Deutsche Meteorologische Gesellschaft 1989 – German Meteorological Society 1989).

The classical climatological research methods for surveying urban climate are mobile field surveys, both vehicular and pedestrian (cf. Maps 04.02 – 04.05). Another method is the calculation of individual surface element temperatures (roofs, streets, tree crowns, etc.) by means of Thermal-Infrared (IR) Imaging. It proceeds from the physical principle that all objects give off heat radiation corresponding to their surface temperatures (cf. Methodology).

Heat radiation, and thus surface temperature as component of an object’s heat balance, is of great importance as a control quantity for the heat balance of the earth’s surface. The primary daytime determinant is the short wave radiation spectrum, particularly the direct irradiation of solar energy onto an object’s surface, and the absorption or reflection of this energy (reflection = albedo, cf. Table 1). The only influence affecting the thermal radiation behavior of an object at night is the long wave spectrum and the soil heat flux.

Tab. 1: Albedo (Reflective Capacity) of Various Surfaces
Tab. 1: Albedo (Reflective Capacity) of Various Surface
Image: Nachbarschaftsverband - Neighborhood Association, Stuttgart 1992

Different types and compositions of surfaces can produce considerable differences in surface temperature (cf. Fig. 1), given equal irradiative (ingoing) and radiative (outgoing) conditions.

Link to: Vergrößern
Fig.1: Surface Temperatures of Various Structures
Image: Kessler 1971 in Mählenhoff 1989

The primary usefulness of thermal maps for (urban) climate analysis is their digitally-processable information regarding the total area. There is a differentiation between infrared photos taken by thermal imaging from aircraft, and data supplied from satellites. The Environmental Atlas Maps are based on satellite data.

The almost 2,000 km² size of Greater Berlin and its immediate surroundings means that only a satellite-based process is capable of the almost simultaneous recording of the longwave radiation of the earth (surface temperature) on consecutive nights and days. Satellite orbits and times cannot be changed however, and in this case they were regarded as not being optimal for the Berlin area (cf. Statistical Base).

The interpretation of IR thermal imaging also allows qualitative classifications of the thermal properties of individual surface elements and spatial units. The conversion of this data, however, requires extremely specialized knowledge of climate, as well as the use of other basic data, such as use maps and relief maps. Surface temperature in any given thermal image is influenced by various use structures. Surface temperature is always the result of complex physical processes, which include horizontal and vertical heat flows, and energy exchange turnovers such as evaporation and condensation. The inclusion of further climatological parameters, like air temperature and wind velocity, allows the use of surface temperature maps in determining climate function areas (cf. Map 04.07).