The Why Files The Why Files --

A Winter's Tale


Blowhard: The jet stream story
If they're good for nothing else, cirrus clouds are excellent markers for a high-powered blast of air called the jet stream. Blowing from 100 kilometers per hour (kph) to more than 200 kph, Behemoth football players exhale huge gusts of steamy air during a game.these winds help set the misery index of a Wisconsin winter.

The Green Bay Packers battle Detroit on the frozen tundra. Blame these frigid conditions on the jet stream, and call them the Packers's home-field advantage. Photo: Green Bay Packers

When the polar jet is dipping southward, near-Arctic conditions give those nutty Green Bay Packer players a home-field advantage over interlopers from sensible climates like Tampa Bay. When the jet stream moves further north, cheeseheads can forget their handwarmers. In honor of oversize Homo sapiens who chase pigskin across snowy fields, smear some camouflage on your mug and buckle your helmet. Let's kick off the football fanatic's guide to the jet stream.

The jet stream, which blows between 8 to 12 kilometers above the surface, was discovered during World War II -- back when football players wore leather helmets and the United States wasn't the only country to measure with feet and inches. Crews of high-flying B-29 bombers heading west across the Pacific Ocean ran into ferocious headwinds that prevented them from reaching their bombing targets on schedule.

Red and blue arrows show the flow and ebb of the jet stream of the US.

Here we see the Jet Stream affecting the US. When the jet stream is farther north, the weather to its south tends to be mild or at least less cold. When the stream swings south (which is usually during the winter) it turns very cold and harsh weather prevails at the surface on the northern side. This diagram shows two typical positions at the height of Summer and of Winter. Graphic: NASA

Because the rapid winds were reminiscent of the new jet airplanes, they were dubbed jet streams. (They were not, contrary to rumor, named for a New York football squad.) The Japanese used the jet stream to carry incendiary balloons eastward, intent on igniting forest fires along the Pacific Coast.

Steamin' 'n Streamin'
Like other global atmospheric phenomena, the jet streams distribute heat from the equator to the poles. In fact, the atmosphere and oceans are actually a giant heat engine that moves heat from low altitudes near the equator to high altitudes near the poles, where heat is returned to space.

Still, like a fullback dragging tackles down a mucky field, the atmosphere is an awfully inefficient machine. According to rough calculations, less than 1 percent of the incoming solar energy is translated into motion.

Two jet streams girdle the world in each hemisphere, but since winter has arrived, we'll focus on the one that brings polar peril to the United States and Canada -- the northern polar front jet stream.

 Cottony clouds circle Earth above the latitude of Central America and Brazil. The cirrus clouds meander but are essentially parallel to the equator.The two subtropical jet streams girdle Earth north and south of the equator. The polar front jets are less apparent in this infra-red image from 4 Dec. 2000, which shows cirrus clouds in the jet streams. Courtesy CIMSS.

Jets kiddin'
What causes the jet stream? Why does it meander from place to place?

Temperature creates pressure differences in the atmosphere, because warm air is less dense than cold air.

When under pressure, quarterbacks skedaddle. Likewise with air -- it "wants" to move from high pressure toward low pressure. Whether it's high in the atmosphere or down on the field, air is always being driven by pressure differentials.

Now consider two side-by-side columns of air, one warm and one cold. Say you want to watch a game from those ridiculous blimps that lurk above football games. If you rise 1,000 meters in cold air, you would rise above more air molecules than you would in warm air. Translated: pressure falls faster in colder air.

Eventually, this change in pressure makes a big difference. At the high altitudes of the jet stream, pressure is much higher in warm air. And just as a 380-pound lineman shoves aside a stripling weighing only 295, the warmer, denser air pushes toward the cold air. The result is wind.

Shove from above
When you travel horizontally in the atmosphere, pressure can change slowly or rapidly. When it changes rapidly -- when those squiggly pressure lines are close together on a weather map -- higher pressure pushes air harder toward the lower pressure. That translates into higher wind speeds.

Weather map with squiggly lines showing the altitude of 300 millibars pressure. <ALT>  Weather map with squiggly lines showing the altitude of 300 millibars pressure.The blue lines show the altitude, in meters, for 300 millibar air pressure. Pressure differences drive wind; the closer the lines, the faster the wind. The jet stream is avoiding New England, but is blindingly fast over the plains of Western Canada and the United States. The wind arrows are parallel to the altitude lines, not across them, due to the Buys-Ballot effect. Huh? © Meteorology Program, Department of Geosciences, San Francisco State University.

Not following? Then consider a fullback staggering downfield against a mob of defenders. If he gets one forward push from a teammate every yard, he'll go much faster than if he gets a push every 5 yards.

In the atmosphere, a quick drop in pressure -- marked on a weather map by areas with close pressure lines -- is equivalent to those multiple shoves. And because the pressure changes rapidly near low-pressure zones, that's where strong winds are usually found.

If you've been paying attention, you probably figure the winds flow across the pressure lines. That answer is logical -- and wrong.

When things travel across a spinning globe, they don't always end up where you expect. Say Cowboys QB Tony Romo is playing in the Minnesota Vikings stadium and finally succumbs to all those concussions and throws a long pass south toward the end zone -- in his home stadium in Dallas. Even if the ball speeds southward at 1,000 miles per hour, by the time it reaches Dallas an hour later, Earth will have rotated 1/24 turn toward the East, and the ball may land in the Arizona Cardinals' stadium.

Twistin' 'n turnin'
That apparent deviation (the ball actually travels in a straight line, when seen from above) reflects the Coriolis force. Also called the Coriolis effect, it affects wind, not just addle-pated quarterbacks, making wind seem to deviate to the right in the Northern Hemisphere and the left in the Southern.

The wind deviation obeys the fortuitously named "Buys Ballot law." Christopher Buys Ballot was a Dutch (not a Floridian) meteorologist who observed in 1857 that if you stand with your back to the wind in the Northern Hemisphere, low pressure will be on your left.

still of movie of diagrams on blue backgroundThe Coriolis effect does not affect winds parallel to the equator, and it's stronger near the poles. As a result, the polar front jet stream does not flow directly from high to low pressure, but nearly parallel to the equal-pressure lines.

Check it out! This Java applet relates wind speed, temperature gradient and pressure gradient. Steven Ackerman and Thomas Whittaker, Cooperative Institute for Meteorological Satellite Studies.

Got a job
If the polar front jet stream is driven by differing temperatures and pressures at various altitudes, it also acts as a global mixing device, says Steve Ackerman of the Cooperative Institute for Meteorological Satellite Studies. "When the jet stream meanders toward the equator, it brings cold air toward the equator. When it meanders north, it brings warm air north toward the pole."

Eventually, this mixing might make the atmosphere as boring as incessant instant replays a football game. Fortunately, the sun continually delivers an enormous amount of heat that sustains the temperature differential, keeping weather interesting (and meteorologists in business).

Without the sun, entropy -- a unit describing a similarity of energy states -- would increase, as the laws of thermodynamics require for a system that receives no outside energy.

Long white bands of clouds against a dark background.Astronauts snapped this pic of the polar front jet stream above Canada's Maritime Provinces in May, 1991. The airflow is driving those streaks of high, cirrus clouds, blowing generally to the east at 90 to 180 miles per hour (145 to over 290 km/h) and above. Caution: this is one frigid breeze! Courtesy NASA.

Whar she blows?
So much for the causes of the jet stream. What about its effects? The polar front jet stream is often accompanied by the winter storms and frigid temperatures. It can also start or intensify summer thunderstorms and steer hurricanes toward or away from vulnerable coastlines.

Finally, jet streams can even defuse cyclonic storms, including thunderstorms and hurricanes. Strong winds aloft help remove air from the top of these storms, allowing the storm to pull more air in to its base, perpetuating itself.

But an overactive jet stream can "blow the top off" a hurricane and reduce its intensity.

For all these reasons, it's helpful to predict the jet stream, which may remain stationary from day to day -- or move hundreds of miles. "One of the challenges of making accurate three-day forecasts is predicting the location and strength of the jet stream," Ackerman says.

Snow may be a messy sight on a football field. But skiers proudly "Think snow." Wut's so byootiful about snow and ice?


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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