Threshold level is an altitude, below which the atmosphere is moist so that water vapour absorbs all the radiation coming from the surface. In practise it is the level below which the satellite does not see in water vapour bands.
Assumptions of the topographic presentation:
A vertical cross section of a moisture terrain:
Threshold height depends on temperature. In cold areas the threshold is lower than in warm areas (lower and upper troposphere, respectively).
Usually the first assumption is valid in the troposphere. Therefore, the interpretation of images depends on layered moisture, inversions, and clouds. Letæ„€ look closer at the layered moisture.