Stupid Question ™
Jan. 9, 2003
By John Ruch
Q: How does wind start?
A: All weather begins with wind, and all wind begins with the Sun.
We live at the bottom of an ocean of air, a light gas that flows like any fluid. (It would be more clear if we said the wind “flows” rather than “blows.”) Its flow relative to the ground is what we call wind.
Sunlight heats air molecules, causing large air masses to heat up. Heating makes it more active, causing it to expand in volume, decrease in density, and thus float upwards. Air masses that receive comparatively less heating are cooler, less active and denser, and thus sink downwards.
Hot invariably flows to cold in nature, and pressure inevitably tries to equalize. Thus, the warmer, less dense air (a low-pressure area) tends to move into the space left by the cooler, denser air (a high-pressure area). This movement is what we experience as wind. Because air heats and cools quite rapidly, the system is volatile.
The Sun does not heat the Earth equally, resulting in this volatility, and the areas it heats vary with the seasons.
However, this heating system results in some fairly regular areas of high and low pressure on the global scale. The equator is famed for its doldrums, the areas of strong low pressure where sun-heated air is rising and expanding, and within which there is little wind.
Around 30 degrees of latitude are the “horse latitudes,” where the air sinks back down again after being cooled by condensation of the moisture within it. This sinking is generally quite gentle and results in little wind.
The polar regions tend to have high pressure due to the extreme cold of the ice caps. Between 30 and 60 degrees latitude, however, the so-called polar front stands as a border between polar and subtropical air, where many winds are born.
Winds blow between these various rings of generic high and low pressure. They don’t blow directly north or south due to the Coriolis effect, a deflection caused by the rotation of the Earth itself. This deflects large-scale winds to the east in the Northern Hemisphere and to the west in the Southern Hemisphere, setting up regular currents known as the trade winds.
On a lesser scale, differences in temperature between land and sea can cause winds between them, most dramatically including the monsoon. Water also retains a steady temperature better than land. This results in the regular “sea breeze” blowing toward the cooler sea during the day, and the “land breeze” blowing towards the cooler land at night.
Storms can create impressive localized winds. The tornado is a vortex of plunging cold air and skyrocketing hot air moving between two larger, rotating layers of hot and cold air. Complex—but still just wind.
On the local scale, surface objects can halt or intensify wind. Blocks of tall buildings, for example, can channel the wind, increasing its speed and pressure in a relatively small area.