Atmospheric circulation pattern, I The equatorial zone
By the time air at tropopause reaches 30°N or S, it has cooled enough that is now more dense than the surrounding air, & so it sinks
Sinking air is compressed & heats up
Yields warm air masses with low relative humidity at 30°N or S
Regions at 30°N or S dominated by relatively high barometric pressure & little precipitation
We call this region of subtropical highs the horse latitudes