Remote Sensing - a tool for environmental observation

Theoretical balance can be defined as reflectivity

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theoretical balance can be defined as: reflectivity + absorptivity + transmissivity = 1 Reflectivity, absorptivity and transmissivity are determined by properties of the material and vary with wavelength of the incident radiation and with the temperature of the surface. For many materials, transmissivity is negligible and the previous equation reduces to (except a glass window for which transmissivity is fairly large): reflectivity + absorptivity = 1 The absorbed energy causes an increase in the kinetic temperature of the material. For the theoretical, ideal case of a blackbody absorptivity is 1. To understand the thermal behaviour of objects at the surface of the earth and hence, to interpret correctly thermal images four thermal properties of materials are important: - thermal conductivity - thermal capacity - thermal inertia - thermal diffusivity. Thermal conductivity (K) refers to the rate at which heat will pass through a material and is expressed as W/m*K (or as cal/cm*sec* ° C). It is the amount of energy that will pass through a 1 cm cube of the material in 1 second when the two opposite faces are maintained at 1 ° C difference in temperature. Rocks and soils are relatively poor conductors of heat. Thermal capacity (c) is the ability of a material to store heat. Thermal capacity is defined as the
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58 amount of calories required to raise the temperature of 1 g of a material by 1 ° C and is expressed in calories per gram per ° C. The thermal capacity of water is large. Consequently, much energy is needed to heat water one degree and it takes a lot of sunny days in spring to heat the swimming water. The thermal capacity of beach sand is pretty low. Hence, you can already burn you feet after one clear, sunny summer day. Thermal inertia (P) is a measure of the thermal response of a material to temperature changes and is expressed in cal/cm 2 *sec ½ * ° C). Thermal inertia is a function of thermal conductivity (K), thermal capacity (c) and the density ( ρ ) of the material and hence, thermal inertia is defined as: P = (K ρ c) ½ The density of the material is a very important property to determine the thermal inertia. Figure 4.4 illustrates the effect of differences in thermal inertia on surface temperatures. Thermal inertia cannot be determined by remote sensing methods because conductivity, density and thermal capacity must be measured by contact methods. Maximum and minimum radiant temperature however, may be measured from daytime and nighttime images. The measured temperature difference ( T) will be low for materials with high thermal inertia and vice versa. Using these temperature differences a property called apparent thermal inertia or ATI can be derived: ATI = (1-albedo)/ T A correction for the albedo is necessary to compensate for the effects that differences in absorptivity have on radiant temperature. Dark objects absorb more sunlight during the day than light materials. ATI values should be interpreted with caution because other factors than thermal inertia may influence
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