The main masses of air, in which we shiver or sweat, are either tropical or polar and either continental or maritime.
During the long winter night — over Greenland, Canada, Alaska, Siberia, and in its season, Antarctica — when the stars are bright and the air is calm but bitterly cold, this continental polar air spreads over Europe and much of China. Sometimes it is funneled by way of the Rocky Mountains all the way to Texas. When this happens, it is said that the only thing between Texas and the North Pole is a barbed wire fence.
In summer the ever-present sun warms the ground, and the ground warms the air above it, although the air is still cool enough to relieve a summer heat wave and dry enough so that daytime cumulus clouds never mature enough to produce rain.