23a. The Centrifugal Force
24a.The Rotating Earth
24b. Rotating Frames
S-1. Sunlight & Earth
S-1B. Global Climate
S-3.The Magnetic Sun
S-4. Colors of Sunlight
S-5.Waves & Photons
and from its circuits returns the wind.
Ecclesiastes, ch. 1, v.6
Pressure and ConvectionThe heating of the ground by sunlight also causes air to flow. If on a clear hot day you stand on the ocean shore, chances are you will be cooled by a breeze from the ocean. What is happening is that while sunlight heats both the water and the shore, the shore warms up faster, because the top layers of the water are stirred up by wind, causing the heat to be shared through a thicker layer. Thus the shore warms up faster, and so does air above it, which expands, becomes less dense than the surrounding air and buoyant, and rises like a hot-air balloon (or like a drop of less-dense oil in a bottle of water).
At the higher levels of the atmosphere, this warm bubble again gives up its heat (to other flows or perhaps to cold space), cools down, and other bubbles coming from below push it to the side, where it descends again. (diagram above). Such a circulating flow is called convection.
More generally, convection is any flow which
The important thing to remember when dealing with convective flows, is that the higher one is in the atmosphere, the lower are the pressure and density of the air. What compresses it is the weight of air above it which it must support. On top of Mt. Everest, less air is piled up on top and therefore the pressure is lower.
At ground level, the compressing weight of the atmosphere amounts to about 1 kilogram on each square centimeter. That pressure does not bother our bodies, because the air inside us is at the same pressure, and the fluids of the body (like blood) do not compress easily. For the same reason, fish have no problems with depth--even at a depth of 100 meters, with a pressure 11 times larger (10 kilogram water above each square centimeter, plus the weight of the atmosphere) they feel no discomfort.
At an altitude of about 5 kilometers, only half the atmosphere is above us, the other half is below, so only the weight of half the atmosphere must be supported, and the pressure is reduced to one half.
By "Boyle's law" (named for Robert Boyle, 1627-91), the density is also reduced to one half (for the same temperature). Rising an additional 5 kilometers, the pressure again falls by half, to 1/4 of what it was on the ground, and at 15 kilometers, it is halved again to about 1/8. All this is approximate and depends on temperature, but the trend should be clear.
The cabin of a jetliner flying at 10 km must be sealed and pressurized, because passengers breathing air at 1/4 the sea-level density would be starved for oxygen and might lose consciousness. On the very rare occasions when a jetliner loses its pressure, masks connected to oxygen canisters drop down automatically, allowing the passengers to breathe normally while the pilot quickly descends to a lower altitude.
The Higher We Go, the Cooler it Gets--Why??
When the atmosphere is stable, the higher we go, the cooler the air is.
Air is warmest near the ground, which absorbs receives heat from sunlight. It is coldest above the level where jetliners fly, at 10-15 kilometers, the region from where it radiates most of its heat into space. That is why mountaintops are cold and the highest mountains have snow on their tops.
How exactly does this happen?
But it is still interesting to see how it happens.
Suppose some "parcel of air" (dry air, for now--humidity is an additional factor, considered later) is heated by the ground and rises. Higher up pressure is lower, so the air expands: but expansion cools it.
Similarly, if for some reason the parcel is blown down, it is compressed again and heated by the compression. Such up-and-down motions happen all the time, and the net result is that when conditions are stable, the temperature drops at a steady rate as we go higher.
The motion of the rising parcel of air depends on its surroundings. It always cools by expansion--but is it still warmer than the still air around it? If it is, it continues to rise; if not, it stops. As will be seen, this is where the humidity of air has an important effect.
Instead of heating the Earth, sunlight can evaporate water from it--especially from the oceans, which cover most of the Earth's surface. Humid air may be viewed as containing additional energy, invested by the Sun when its heat evaporated the water. While heat drives convection, humidity may amplify it.
Hot humid air is what drives thunderstorms, and a warm ocean surface is also the traditional birthplace of violent tropical storms, known as hurricanes in America and typhoons in Asia.
We look at two examples of humidity in action.
Who first wrote "Everybody talks about the weather, but nobody does anything about it"? Most would claim it was Mark Twain (just type the first 5 words into a search engine and see!), but it ain't necessarily so. It reads like Twain's style, but actually the words first appeared in an editorial in the Hartford (Conn.) Courant on 24 August 1897, written by Charles Dudley Warner.
Warner was a good friend of Twain, who himself had lived in Hartford for many years (he left before 1897). He was a newspaperman in Hartford and the two had collaborated on an 1873 book "The Gilded Age." Warner is also remembered for other quotes, e.g. "Politics make strange bedfellows."
More about Charles Dudley Warner (and Twain) at
Questions from Users: "Doesn't heat rise?"
*** Sudden decompression, 5 miles up
*** What does "lapse rate" mean?
*** The lowest 700 km of our Atmosphere
*** Precession, Greenhouse and more...
*** Imagine a non-rotating Earth
*** Why doesn't gravity overcome buoyancy?
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Updated: 9-22-2004 ; Re-formatted 26 March 2006 ; Edited 15 October 2016