365 Days of Climate Awareness 36 - Global Wind Circulation 1: Basics Dynamics

The wind circulation of the planet, and therefore the weather, acts to distribute heat from the tropics to the poles. On average, over the course of a year, nearly 40% of the sun's energy which reaches the earth falls on the tropics (between 23.5° south and north latitude--corresponding to the tilt of the earth's axis. Likewise, the Arctic and Antarctic circles, at 66.5° N/S, are 90-23.5. The Tropics of Cancer and Capricorn represent, respectively the furthest line of north (south) latitude over which the sun shines directly during the year. The Arctic/Antarctic Circles represent the maximum reach of the sun's rays at the winter solstices for each pole).

Incoming solar radiation (insolation) and the convective heating response.

Returning to insolation. Nearly half the sun's heat energy impacts the tropics, and that heat warms the atmosphere, land and ocean. The land doesn't move, to any appreciable amount, but the atmosphere and ocean do. We will examine the ocean's activity in future posts. Regarding the wind, heat causes the air to expand, losing density and rising. If the air is near or over water, the evaporation of water will contribute to the lightness and upward motion of the air.

Insolation, and convective circulation (heating and cooling).

When less dens air begins to move up, cooler, higher pressure air moves laterally--across the land or sea--to take its place. This is the creation of wind. After the rising air has cooled adiabatically--due solely to expansion--to the temperature of its surroundings, that air too begins to move laterally, away from the upward convection current.

At elevation the air loses infrared heat energy, and ultimately begins to sink. This creates a downward current of air--not as violent as the downdraft of a thunderstorm, and much more stable--which reaches the surface of the planet and spreads back out. The air is warmed by contact with the surface, and so resumes the cycle.

It is this basic cycle of warmed air moving upward, and colder, higher-pressure air moving to fill the relative vacuum left by the warmer, lower-pressure air, which drives much of the weather we experience. When a difference in pressure--high to low--exists laterally across the surface of the planet, it is called a pressure gradient. Air generally moves in such a way as to reduce or eliminate the gradient. But in future posts we will see why this is not always possible.

Tomorrow: daily and annual heat/pressure cycles of wind.

Be well!