Convection heats by transporting entire groups of molecules from one place to another with a substance. The substance is usually a fluid that can move freely, such as water or air. Think of a pot of thick soup on a stove. By conduction, the soup at the bottom of the pan heats first. Then it begins to rise (because heat rises) to the top of the column of soup. As it rises, cooler soup up above sinks to the bottom to take its place. That portion then heats and rises. Soon, a circular pattern of heating has begun in the pot, heating the soup throughout. This is convection.
Convection also occurs in the atmosphere. It can include both large- and small-scale rising and sinking of air masses. These vertical
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In a simplistic view, the warm, light air at the equator rises and spreads northward and southward, and the cool dense air at the poles sinks and spreads toward the equator, forming two convection cells.
motions can be large enough that they are able to actually distribute heat and moisture throughout the entire atmospheric column. This motion and transfer of heat energy is what allows clouds to form and storms to develop.
Storm systems are one scale of convection. There are also very large scales of convection that are able to effectively distribute heat all over the surface of the Earth. The Earth's landmasses, ocean distributions, rotation, and other features can make this somewhat complicated, but to understand the basic flow of convected energy in the atmosphere, think of the Earth in a more simplistic way, such as in the pot of soup example. Most of the Sun's heat is deposited in the Tropics of the Earth. This is because the Earth's rotational axis is an almost perpendicular plane to the plane of the Earth's orbit around the Sun. The polar latitudes receive much less solar heating than the equator. Incoming sunlight warms the equator more than it does both the North and South Poles. This makes the air at the equator warm faster and begin to rise. As it rises, it diverges, and some travels to the North Pole and some to the South Pole.
Because the air at the North and South Poles is cooler and more dense, it sinks and is forced to move toward the equator. At this point, two basic continuous convection cells have begun: one traveling from the equator to the North Pole and back to the equator again in an endless loop; and another one just like it but traveling from the equator to the South Pole and back again. Things become a little more complicated when the rotation of the Earth is added in. Solar heating of the Earth and its atmosphere drives the large-scale atmospheric circulation patterns, and even the seasons. This will be looked at in more detail in chapters 5 and 6.
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