Sunday, September 09, 2007

Atmosphere in motion: Hadley cells

A permanent, year-round feature of the Earth's current climate regime is the movement of warm air away from the equator and toward the poles, with the convective motion broken into a set of three distinct latitudinal bands or "cells", one set per hemisphere, for a total of six cells all told. These cells now carry the name of the British natural philosopher George Hadley who, in the 18th century, first described these cells and grasped the associated wind patterns that so influence weather at different latitudes and played a critical role for seafarers in the Age of Sail.

It's difficult to describe these cells in words and better to see them in pictures or, preferably, as dynamic animations. Here's a static picture. And here's a more detailed diagram.

The Hadley cells originate from the warm air that rises at the equator (0o lat), sinks at ±30o lat, rises at ±60o lat, and sinks at the poles (±90o lat). The basic north-south movement is:
  • Between 0o and ±30o lat, southward at the surface and northward aloft (tropical zone)
  • Between ±30o and ±60o deg lat, northward at the surface and southward aloft (temperate zones)
  • Between ±60o and ±90o deg lat, southward at the surface and northward aloft (polar zones)
The consequence is a permanent tendency to form low-pressure systems at 0 and ±60o lat and high-pressure systems at ±30o and ±90o lat. Tropical cyclones (such as hurricanes and typhoons) form close to the 0o lat line, while subarctic (extra-tropical) cyclones form near the ±60o lat lines.

Now add in the effect of the Earth's rotation and the resulting Coriolis force (or pseudoforce, a feature of rotating, noninertial reference frames). Poleward motion at the surface is deflected eastward, while equator-ward motion is deflected westward. This causes the basic wind patterns we see at the surface: "westerlies" in the temperate zones (coming from the west, but headed east - I know that's confusing) and "easterlies" in the tropical and polar regions (coming from the east, but headed west). Hence, in the temperate zones, weather systems tend to move SW to NE in the northern hemisphere and NW to SE in the southern. In the polar and tropical regions, weather systems tend to move NE to SW in the northern hemisphere and SE to NW in the southern.

The Hadley cells, together with the Coriolis force and seasonal cycle, explain most of what you need to know about geographic weather patterns. The calm region of high-pressure, sinking air at ±30o causes the "horse latitudes," where ships in the Age of Sail were becalmed from lack of wind and had to throw their horses overboard to conserve on water. The tropical easterly "trade winds" were discovered and exploited by Columbus and other European explorers in the Age of Sail as the basic means of getting east to west, while the temperate westerlies (familiar to us living in the North America and Europe) were the basic means of getting west to east.

Local weather phenomena near the surface tend to move SW to NE in the northern temperates. But weather systems with strong vertical structure move NW to SE in the same latitudes. The controlling flow (steering wind) in such cases is the southward Hadley flow aloft, not the northward surface flow.

Hadley cells on other planets. The terrestrial inner planets Venus and Mars also feature atmospheric convective cells. Venus' atmosphere is so thick and stable that it only has one Hadley cell per hemisphere. Mars' atmosphere, OTOH, is so thin and unevenly heated that it apparently has no stable configuration of Hadley cells.

The outer gas giants of our planetary system also have convective cells in their atmosphere. However, these gas giants lack a significant solid surface and their atmospheres are very opaque. So the pattern of heating and convective "rolls" in their atmospheres are essentially different from those of the terrestrial planets. Our sort of Hadley cells arise from having a mostly solid planet with a thin atmosphere that is mostly heated from below.

Hadley cells in your kitchen. You can form Hadley cells in mildly heated water, if you introduce differential heating; that is, more heating at one end of a pan and less at the other. The exact pattern is a function of liquid density and viscosity. But you should expect rising liquid at the more heated end and falling liquid at the less heated end.

Of course, it's pretty hard to getting the Coriolis rotational effect in your kitchen. But you can get something like it if you use fancy lab equipment.

Rayleigh-Bénard convection. Hadley cells are an example of a well-studied phenomenon in fluid dynamics called Rayleigh-Bénard convection or the Rayleigh-Bénard instability. See here and here for more.

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