Clearing the Air On the Meaning Of the `Jet Stream'

Discover wind flows no one knew existed. Describe them by a catchy metaphor. Use it to ``explain'' the latest heat wave or cold snap, and you generate a new household word. That's how we got the famous ``jet stream.'' And this month is the 50th anniversary of its initial discovery.

Actually, it took meteorologists many months to learn what they were dealing with and give it a name. For American air crews assembled on Saipan a half century ago, it was an unexpected invisible enemy. They were prepared to bomb Tokyo from altitudes of 30,000 to 35,000 feet, where few aviators had gone before. They were not prepared for the 200-mile-an-hour westerly winds that awaited them.

The duty to forecast winds for the first mission fell to William Plumley and Reid Bryson at the 20th Air Force Weather Central on Guam. Recalling that challenge in a recent conversation, Dr. Bryson said it was an occasion where atmospheric science made up for lack of weather data.

At the University of Wisconsin's Center for Climatic Research in Madison, where Bryson now works, a lobby display shows the current satellite views of regional and global weather. We take such knowledge for granted now. Sitting on Guam 50 years ago, all the two forecasters had to go on were their own weather observations and a few scattered reports from land-based and sea-based observers.

That's where the science came in. They knew that cold air was coming down from the north over Japan, while warm moist air prevailed to the south. Putting all their data together, they sketched the probable temperature distribution through the lower atmosphere. Then they used what meteorologists call the thermal-wind relationship.

In simplistic terms, this states that if you stand with your back to the wind with colder air to your left and warmer air to your right, the wind will get stronger with altitude.

The forecasters made the calculations and predicted 168-knot westerly winds over Tokyo. Bryson said the commanding general refused to believe such a ``fantastic'' forecast. When the mission failed because of 170-knot winds, Bryson said, the general apologized. As an official report from that time explains:

``Attacking Japan's best-defended cities in the teeth of a 200-knot [head] wind was unthinkable; going downwind, the B-29s reached ground speeds of 500 miles an hour, in which case neither bomb sights nor bombardiers could function properly. Moreover, the high winds made it impossible for crews to make a second pass ....''

As more planes - including German reconnaissance aircraft - began flying at high altitudes, such incredible winds were found over many parts of the Northern Hemisphere. Carl-Gustaf Rossby at the University of Chicago - the preeminent weather guru of the day - named them jet-stream flows.

However, they aren't like jets from an air hose. There is no analogous jet-producing mechanism in the atmosphere.

What meteorologists mean by the term are narrow high-speed air flows that are a normal part of the general circulation. This is the circulation that distributes the solar energy that heats the tropics to colder regions.

Actually, there are several jet streams that wax and wane at different heights and latitudes throughout the year. The one of greatest concern for North America is the polar-front jet that you see on television. Weathercasters are correct to use that jet to help explain how storm systems move.

But none of these jets cause particular weather patterns. They are part of the patterns.

As Bryson notes, people who try to explain weather as caused by the jet stream turn around and try to explain the jet stream as caused by the distribution of warm and cold fronts and of high and low pressure regions. It's circular reasoning. But after 50 years of wowing the public, the jet-stream metaphor is here to stay. After all, it is a handy way to think about the weather.

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