Winds and Weather

All winds and ocean currents are driven by Earth's heat imbalances -- the dramatic difference in sunshine between the Equator and the poles (especially in winter). At the same time, they are profoundly affected by Earth's rotation from west toward the east. Every little breeze or ocean eddy is part of a vast system that redistributes heat and cold in response to geographic disparities in radiant energy.

Abundant sunshine and the retention of outgoing infrared heat causes tropical air to expand and become less dense. The buoyant air rises and cools, which condenses water vapor and leads to torrential tropical rains. Latent heat released by all those condensing raindrops juices the updrafts even more, like rocket fuel! Tropical thunderstorms shoot upward with such momentum that they often pierce the tropopause and propel raindrops all the way to the lower stratosphere. Those fountaining tropical updrafts shoulder the upper air aside, pushing it toward the poles. 

On the other hand, the winter pole receives no sunlight at all for months at a time. Air in the polar night gets extremely cold and contracts, hunkering down near the frozen surface. Sinking air at the pole makes way for currents of upper air pushed  poleward from tropical updrafts.  The tropics-to-pole flow aloft, coupled with the sinking cold air at the winter pole, leads to an accumulation of air at thet highest latitudes. This air then must flow back toward the tropics near the surface. This equator-to-pole flow of warm air constitutes one half of the global "ventilation system" that allows the tropics to equalize Earth's temperatures. The other half is the gigantic gyre current systems in the oceans.

Without rotation, warm tropical air would flow poleward aloft and cold polar air would flow equatorward at the surface, more strongly in winter and less strongly in summer. Think of the supermarket conveyor belt that moves your groceries to the cash register. The atmospheric conveyor lifts warm air in the tropics, pushes it poleward where it gives up its heat and sinks, then carries the cold air back toward the equator.

The rapid spin of our planet complicates this picture, twisting the invisible atmospheric heat conveyor into complex 3D swirls that constitute Earth's familiar patterns of wind and weather. The tendency of moving objects to pushed to the right in the Northern Hemisphere and to the left in the Southern Hemisphere is called the Coriolis Effect.

Warm air aloft flowing toward the winter pole gets shoved eastward by the Earth's rotation, becoming the prevailing Westerly winds familiar to those of us who live in the "middle latitudes" between the poles and tropics. At the same time, the return flow of cold air near the surface is pushed westward as it approaches the Equator to become the Easterly Trade Winds that prevail in the tropics.