The gusty weather over the past couple of weeks piqued my interest in where wind comes from.

From the zeroth law of thermodynamics we can generate a simple statement of why the wind blows. This law in simplified form states that heat always and only moves from warm to cold.

On average, energy input from the sun equals energy radiated into space by Earth. The sun heats the equatorial regions more than the polar regions, so heat must be moving from the equator toward the poles. Otherwise, the equator would heat up at the expense of the poles.

On a non-rotating planet with a gaseous atmosphere, heat transfer occurs via convection as the warm, less dense equatorial atmosphere rises and moves toward the poles. On such a planet, surface wind would always blow toward the poles from the equator.

A rotating planet changes the motion of the air in the same way that a rotating carousel affects the path of a thrown ball. This is the Coriolis effect.

Because of the rotation, the ball’s target will not be there when it arrives. To a person on the carousel, the ball appears to deflect in a direction opposite to the motion of the other side of the carousel. For counter-clockwise rotation, this apparent deflection will always be to the right.

The diameter and speed of rotation of the carousel and the speed of the ball all affect the amount of deflection. An observer not riding on the carousel would see that the ball actually travels in a Newtonian straight line.

For Earth, rotating once every 24 hours, the Coriolis effects breaks the convection into three cells twisting around the globe like coils each turning in opposite directions. These three cells are responsible for prevailing winds on the surface, and jet streams high in the atmosphere.

Convergence and divergence of air at the cell boundaries causes air to pile up, creating a region of higher pressure, or to leave a dent, creating a region of low pressure. Search online for “Hadley cells” or “global circulation” for illustrations.

Water and land add more layers of complexity. Water vapor carries heat that goes back into the atmosphere when the vapor condenses. Land warms and cools much faster than water, so heating and cooling are different over continents and oceans.

This allows for the formation of huge, interacting, continent-size air masses that can vary in temperature, humidity and density: hot and dry, warm and moist, cool and dry, etc.

The result is a turbulent mixing of the winds that leads to a jumble of highs and lows that move eastward around the globe. The size of the highs and lows is large compared with the speed of the wind, and the sun keeps adding energy, so they persist and interact as they move.

As air flows away from the center of the high, the Coriolis effect deviates it to the right (left in the Southern Hemisphere). The Coriolis effect depends on speed, so the effect becomes greater as the air gains speed.

Eventually the air will come to equilibrium, circulating in a clockwise motion around the center of the high, parallel to pressure contours. The slope of the hill is the pressure gradient, which determines the wind speed.

Our January wind was the result of an unusually steep pressure gradient from a high-pressure center to the northeast.