But first, users must remember that water vapor is the main absorbing constituent in this channel, and that variations in water vapor are detected through variations in the effective temperature measured in this channel. The effective temperatures are of the atmospheric layer which is most radiatively active, depending on the total amount of water vapor in the scene; basically the temperature of the layer from which the maximum radiation comes. In areas with warmer effective temperatures the radiation comes from lower layers of the atmosphere (generally those areas are drier than areas with colder effective temperatures). But this is only true if the actual temperature profiles are the same. The moisture effect is intertwined with real temperature variations as well, so that this channel sees both real temperature variations and variations in temperature due to changes in the amount of water vapor in the vertical column. Also, this channel, being infrared, does not see through clouds and the coldest temperatures observed are those from cloud tops.