Forecasters can easily identify areas of developing clouds. For example on a sunny summer afternoon, the forecaster can watch small cumulus clouds develop. If atmospheric conditions are right, these small cumulus may develop into thunderstorms. Essentially, the satellite is sensing heat. Different shades are assigned to the various temperature ranges.
Some computers create enhanced images, which displays colors for certain temperature ranges.
This allows to the forecaster to quickly see where the clouds are getting colder, which are areas of precipitation. The taller or higher the clouds, the colder they become, hence the coldest clouds are often high up in the atmosphere. On the opposite side of the spectrum, the lower the clouds are to the ground, the more dark they appear. Since the ground heats up the most, the darkest areas will be the areas of no clouds where the ground can heat rapidly. Imagine a column of air extending from the surface to the highest part of the atmosphere.