Thursday, February 22, 2007

What drives climate? Climate dynamics I

In a word, differences - differences drive climate. The most basic is the distinction among up, down, and sideways, determined by the direction of gravity.

Differences in pressure from point to point give rise to forces that push the atmosphere in the direction of lower pressure. They are one cause of atmospheric winds, as well as the vertical circulation that defines the "high" (higher) and "low" (lower) pressure systems on weather maps. Air is falling at the centers of "highs" and rising at the centers of "lows."

The differences between air and ground are manifold; one result is friction between air and ground (and between ocean and seabed) when the fluid slides over the solid.

A more abstract difference - between inertial and non-inertial (accelerated) frames - arises from the fact that the Earth is rotating. Because of air-ground friction, the air more or less rotates with the Earth. But the air then lives in an accelerated (rotating) frame. In a rotating frame, the Coriolis effect or pseudo-force accelerates anything moving in any direction other than parallel to the rotation axis itself. In the northern hemisphere, something moving northward is deflected eastward (or southward and westward). In the southern hemisphere, something moving southward is deflected eastward (or northward and westward).

The most important set of differences in climate are temperature differences, which drive the flow of heat in the air. Heat moves around by three mechanisms:
  • Conduction
  • Convection
  • Radiation
The last two are at work in the Earth's atmosphere, while the first is not effective in air. (It is important in the water and ground.) The conduction of heat happens by diffusion; slightly hotter molecules bounce off of slightly cooler neighbors and transfer heat in the colder direction. In convection, big parcels of air (or water) hotter than their surroundings move to lower temperature surroundings, giving off their heat as they go. Radiation transports heat because electromagnetic waves have energy; that energy can have a temperature, and radiation can transfer that heat to (or remove it from) material by absorption or emission. A later posting will talk about radiation in more detail.

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