The Universe in the Classroom

The topsy-turveying of planets, stars, and lava lamps

Oh, Smoggy Day

Not all hot fluids convect. A smoggy day in Los Angeles or any other big city illustrates what happens when things don't convect. The air just sit there and doesn't get stirred. Why not?

Convection can't occur unless certain conditions are right. The most important condition is the temperature profile. Normally, the air is hottest near the ground and gets cooler as you go up. Outside an airplane window at 30,000 feet, the air temperature drops to -40 degrees F. The reason is straightforward: Because air is transparent, sunlight passes right through it and heats the ground. The ground, in turn, heats the air near the ground. Air further away from the ground doesn't get heated directly by the Sun.

Because it gets colder as you go up, the atmosphere wants to convect. Cold air is sitting on top of hot air. The hot air wants to rise, the cold air wants to sink. A convection cycle develops.

Sometimes, though, the ground manages to become cooler than the air above it. This can happen after a clear night during which the ground cooled more than usual, and it can happen in valleys where the walls shade the floor and trap cool air close to the ground. In these cases, the air is cold near the ground and hotter as you go up. This situation is a temperature inversion. The air doesn't want to convect, because hot air is sitting on top of cold air. The cold air has already sunk, the hot air has already risen. The atmosphere is happy; it has no reason to change the status quo.

Without convection, the air stagnates -- as does anything in the air, like smog. Usually, convection whisks away car fumes. When a temperature inversion occurs, car fumes have no place to go, except into your eyes and lungs.

Fortunately, this is unusual. The lower 6 miles of the Earth's atmosphere -- the air below you when you fly in an airplane or climb Mount Everest -- is almost always convecting. This churning region, known as the troposphere, is where the weather takes place (see Figure 5). All of the planets have tropospheres.
temperature profile
Figure 5
The temperature profile of the Earth's atmosphere. Tornadoes, thunderstorms, and other weather patterns take place in the lowermost layer of the atmosphere, the troposphere. Convection is responsible for much of the meteorology. If it weren't for the temperature profile, convection wouldn't occur: Because the temperature decreases with altitude, cold air sits on top of hot air, allowing convection. But if for some reason the temperature increases with altitude (dotted line), convection grinds to a halt. This happens on smoggy days in urban areas and everyday in the stratosphere.

Many weather patterns in the troposphere occur because of convection. Sea breezes are a good example. During the day, the land warms up faster than the sea. The air above the ground is hot, the air above the water is cold. The hot air above the land rises, and the cold air above the sea swoops in to take its place. This causes a wind to blow from the ocean toward the land, a sea breeze. At night, the situation reverses. The land cools down faster than the sea, so the air above the ground is colder than the air above the water. The hot air above the water rises, the cold air from the land rushes in, and you have a land breeze.

The same thing happens on a global scale. The tropics are warmer than the poles, and this discrepancy powers giant convection cycles called Hadley cells. In such a cycle, air rises in the warm low latitudes, moves toward the pole, sinks at higher latitudes, and rushes along the ground back toward the equator. (These convection currents don't quite go due north or due south; they veer off course because of the Earth's rotation, creating the easterly trade winds in the tropics and the prevailing westerlies at temperate latitudes.) The Northern and Southern hemispheres each have three sets of Hadley cells, one set for the tropics, one for temperate latitudes, and one for the polar regions. In the Northern Hemisphere, the boundary between the Arctic and temperate cells, called the polar front, seldom stands still. It weaves north and south, and we have its fidgeting to blame for the weather patterns in Europe, North America, and northern Asia. Venus and Mars also have Hadley cells, but only a single set.

Above the restless troposphere is the subdued stratosphere. In the stratosphere, unlike the troposphere, the air gets hotter as you go higher. On the Earth, this is because the ozone layer absorbs solar ultraviolet radiation. Not only does the ozone preventing these deadly rays from reaching the surface, it also heats up the stratosphere. As a result, the stratosphere is stagnant. The air doesn't circulate much.

Jupiter, Saturn, and Titan also have stratospheres. On those worlds, the stratosphere owes its existence to the absorption of solar radiation by methane and dust, rather than by ozone. Venus has a sort of stratosphere -- the temperature remains roughly constant with height, instead of increasing with height -- due to nasty clouds of sulfuric acid at an altitude of 40 miles. These thick clouds absorb nearly all of the sunlight falling on Venus and prevent us from seeing the surface.

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