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Venus provides a clue in the mystery of earth's weather

One of the underlying hopes of planetary scientists lies in the expectation that studying other atmospheres will help them learn more about atmospheres generally and understand better the winds and weather of Earth.

There are indications that this hope may eventually be realized.

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Donald M. Hunten of the University of Arizona noted last March that United States and Soviet probes have found "fairly convincing" evidence of lightning on Venus. This, together with the lightning photographed on Jupiter by one of the Voyager probes, suggests that lightning is a common atmospheric phenomenon.

More recently, Conway Leovy of the University of Washington pointed out that one of the great puzzles of the Venusian atmosphere may hint at an unsuspected basic fact of atmospheric circulation. Scientists have been hard put to explain how that atmosphere rotates once in four days while Venus itself takes 243 Earth days to make a turn. It may be, Leovy says, that there is an inherent need for an atmosphere to rotate regardless of what the underlying planet does.

Hints such as these are proding atmospheric scientists to think about the fundamental aspects of their subject more broadly then they did when they had only Earth to study.

Outlining the scientific challenge posed by lightning, Hunten observed in the journal Nature that "Theories of electrification are faced with the need to explain its [lightning's] presence under a wide variety of circumstances and atmospheric compositions."

Leovy made substantially the same point this October in commenting on the Venusian circulation during a symposium celebrating the 25th anniversary of the Geophysical Fluid Dynamics Laboratory of the US National Oceanic and Atmospheric Administration.

The Circulation of a planet's atmosphere is a heat distribution system. On Earth, it carries equatorial heat northward and southward. Theories developed primarily for Earth predict that such a circulation will give rise to so-called zonal winds that blow more or less, parallel to latitude circles (easterly or westerly winds). They assign a key role to the planet's rotation in establishing these winds. But on weakly rotating Venus, such winds blow as strongly as 360 kilometers an hour.

Leovy pointed out that several technically complex theories have been suggested to explain this. None can be confirmed by data gathered so far, nor can any be ruled out. Given heating at the equator, Leovy says, the balance of forces that govern atmospheric motion may demand zonal winds whether the planet rotates rapidly or not and these winds may arise in several different ways. Fundamentally, "there may be no such things as a nonrotating atmosphere," he says.

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If true, this would be a new theorem to add to atmospheric textbooks -- an important scientific dividend from the expense and effort of studying unearthly weather.