Windswept_ The Story of Wind and Weather - Marq de Villiers [40]
Exactly why some mixes of cold and warm air deepen explosively and others remain benign is still unknown. It all depends, says Peter Bowyer of the Canadian Hurricane Centre, on how fast the cold dry air and the warm moist air masses are forced together. What happens after they deepen is much better understood. Bowyer tells me, "Our computer models are quite adept at handling these storms. The physics of winter storms is better understood than that of hurricanes, so our models do quite well, and we can predict many days ahead of time when a storm is likely to form over, say, the Carolinas, and how long it will likely take to reach our region."
High overhead in both hemispheres are the stratospheric gales, and the jet stream of popular weather forecasting. This jet stream, which is essentially a standing wave of high pressure pushed eastward by the Coriolis force at the boundary between cool polar air and warm tropical air, flows at speeds up to 240 miles an hour, and sometimes more, at elevations around 30,000 to 35,000 feet, a little over five miles. Jet streams are generally the reason why long-distance flights are faster going eastward, because airline bosses like pilots to ride the jet stream to conserve fuel. Not that this is always easy; jet streams do meander, and substantial vertical wind shear, which can cause clear-air turbulence, can often be encountered at their edges; passengers who'd like a quick crossing don't necessarily sign on to a roller coaster and, as cabin crews can attest, are easily irritated by bumpy rides. Jet streams were first discovered during the Second World War, which was when high-altitude transatlantic flying first became commonplace. They were dubbed jet streams because they seemed to flow in narrow ribbons at high speeds—and jet aircraft had just been invented. More than one jet stream exists—the midlatitudes jet stream, the polar jet stream, and the polar vortex—and they are found above all oceans and all continents. The first to exploit them were the Japanese military, which in 1944 and 1945 launched experimental bombs suspended from balloons into the Pacific midlevel jet stream; some of these weapons traveled five thousand miles in three days; one reached the coast of Oregon, exploding near a Sunday school picnic, killing five children and the minister's wife.15 Another made it over the Rockies and reached the thinly populated Canadian prairie province of Saskatchewan, a most unexpected assault.
A jet stream, too, can be deflected by pressure systems and is generally closely watched by meteorologists in winter—a jet stream represents an area the scientists call a zone of baroclinic instability, and a deflected jet stream in midlatitudes can mean the difference between mild temperatures and severe ones.16 Jet streams that dive southward into the United States typically mean intense cold over much of the continent; when they retreat into midlevel Canada, the weather will be unusually mild. In addition, a jet stream pattern that swoops down toward the southern United States in late summer can cause a flow pattern that can steer hurricanes my way; I have learned to keep a wary eye on its position. The jet stream boundary is the locus of squalls, cyclones, and storms.17
Jet streams can work as a steering mechanism for short-term winter weather too. In January 2005 warm, moist air from the southern Pacific flowed up the jet stream all the way to the Aleutians, then dumped massive quantities of rain on British Columbia, where they called it the "Pineapple Express." The same system followed the jet stream across the Rockies, where it became a clipper— Montana clipper in the United States, Alberta clipper in Canada. Snow and snow squalls duly followed. But that wasn't all: The system followed the jet stream into the southern Great Plains, then back up to the Ohio valley and out to sea somewhere around Virginia. After which, yes, it rode the jet stream back up past New England and Maritime Canada, bringing snow squalls and high winds. Thank you, Hawaii.
At high altitudes