2.3. Derivation of brightness temperature
Satellite TIR sensors measure radiances at the top of the
atmosphere (TOA), from which brightness temperatures (also
known as blackbody temperatures) can be derived by using
Plank's law (Dash et al., 2002). It is assumed that the water vapor
content of the atmosphere is constant for a relatively small
region, so that the atmospheric condition could be considered as
uniform, and the influence of atmosphere on radiance temperature could be neglected. Therefore, the at-satellite brightness
temperature can be used to reflect the distribution of the surface
temperature fields. It is recognized that the water vapor content
does vary over time due to seasonality and inter-annual variability of the atmospheric conditions. It is not appropriate to
directly compare temperature represented by the at-satellite
brightness temperature between multiple time periods. Therefore, we focused on the UHI intensity and its spatial patterns
across the study region on the image acquisition dates. UHI
intensity is measured as the difference between the peak temperature found inside the urban area and the background rural
temperature (Oke, 1987). In this way, the UHI effect can be
measured for the individual thermal images and then compared
between different time periods. The retrieval methods of brightness temperature from the TM and ETM+ images are different,
which are discussed as follows.