I was asked today exactly why and how there are such things as high and low pressure areas in weather reports. The one asking (wrongly assuming I would know) asked if the “air stack” over a large area was taller, creating a high pressure area, if humid air weighs less causing a low pressure area, etc.
The questioner already assumes that our West Texas winds are high pressure air rushing to a low pressure area, but would like to know what causes the “heavy air/light air” variances from place to place in the first place. Thank you!
My atmospheric physics reference (Encyclopedia of Physics and Britannica) tell me that pressure differences are caused by unequal heating of the surface. This unequal surface heating results in different amounts of heat being transferred to the atmosphere above the surface and that results in pressure differences. The pressure differences persist because of Coriolis effect resulting from the rotation of the earth. This causes the air to flow from high to low pressure regions following a long, circuitous path instead of a direct flow from high to low pressure.
Hint:
ALL weather is the result of uneven heating of the earth’s surface by the sun.
Take it from there…
Land hot.
Sea cool.
Hot air over land rises causing low pressure.
Cool air from sea moves to land to equalise pressure.
Result, sea breeze. This is but one localised example and remember the air doesn’t actually need to be hot it just needs to be warmer than the neighbouring air.
The three rules for wind:
- Hot air rises
- Cold air falls
- Nature abhors a vacuum
As has been mentioned, localised heating of the Earth’s surface causes (relatively) warmer air to rise, while in another place, the cooler air falls. The rising air produces a low pressure area at the Earth’s surface, the falling air produces high pressure. The high-pressure area produces wind that moves toward the low-pressure area. The proximity and intensity of these pressure differences determines whether you get a mild breeze or a hurricane.
This might seem logical, except that several sources on barometric pressure explicitly state that pressure is determined by the height of the stack of air over a location, implying (to me, at least), that the actual height of the air in high pressure regions is greater than an equivalent height over low pressure regions.
Now cold air should be denser than warm air, and perhaps the measured “stack” is simply the apparent height of the column of air (recalling that barometers are used in measuring altitude), however, the explanations thus supplied seem to avoid any actual reference to height and air “falling” does not seem to be an adequate explanation, either.