We take the wind for granted. Sometimes it goes back and forth with the seasons; sometimes it varies on a daily basis. But over most of the globe, the wind has a "prevailing" direction. This direction is determined by the distribution of high and low pressure systems.
Pressure Systems Around the World
Our global atmospheric circulation is dominated by four major pressure zones: high pressure at the poles; low pressure at 60 degrees latitude; high pressure at 30 degrees; and low pressure at the equator. A detailed explanation of why this is so would get into the weeds of complicated physics. However, it is possible to make a general argument that substantially explains things.
If the earth didn't spin, but could somehow be heated at the equator (all the way around) and cooled at the poles, it isn't hard to see that air would rise at the equator and sink at the poles. Surface pressure would be low at the equator and high at the poles, and winds at the surface would always and everywhere be from the north in the northern hemisphere and south in the southern.
But the earth spins -- and this changes everything. The spin of the earth introduces a force, the Coriolis force, on moving air. What we might expect is that the north and south winds will turn and be east winds, with an equatorward component in order to continue the transport of heat between equator and pole. And yet this is not the case.
The Physics of the Atmosphere
Without getting into complicated science, the atmosphere can be characterized by 7 equations in 7 unknowns. They have a solution, but nobody knows how to find that solution. If a solution were found, the atmosphere would surely prove to be theoretically exactly as it is physically. So we can conclude that the system we see is just the simplest way for Mother Nature to comply with all the physical constraints.
The General Circulation of the Atmosphere
The circulation on a spinning earth is not really so much different from what we'd expect -- just a little more elaborate. The overall pattern is not a single circulation system driven by one high pressure system (at the poles) and one low pressure system (at the equator). Instead, in addition to these two, we find two new systems: high pressure at 30 degrees latitude and low at 60 degrees.
The resulting wind systems are: easterlies between 60 degrees and and the pole, known appropriately as polar easterlies; westerly winds from 30 to 60 degrees, the mid-latitude westerlies. And the tropical easterlies, commonly known as trade winds.
Historically, the general wind patterns have been known and have been important for a long time. Just as one example, explorers from Europe rode the trade winds (all they had for power was the wind in their sails) to their discovery of the New World.
Jon Plotkin - The author was a math major at Cornell and has a master's degree in meteorology from MIT.
