It's the 'jet' in jet streams
Because of jet streams, especially in winter when they blow the hardest, an airline flight from the East Coast to Denver may take four hours--while the return trip requires only three. Also, jet streams account for much, but not all, high-altitude turbulence.
Even if you never fly an airplane above 10,000 feet, jet streams are important because they help set the agenda for weather near the ground. Understanding weather requires some grasp of what jet streams are and how they work. In meteorology, a jet stream is defined as a "relatively narrow river of very strong horizontal winds (usually 50 knots or greater) embedded in the planetary winds aloft." As the name implies, planetary winds aloft are those that flow around the Earth high above the ground.
The name has nothing to do with jet aircraft. "Jet stream" refers to a high-velocity liquid or gas stream coming from a narrow opening. Pilots began encountering jet streams long before they started flying jet aircraft. One occurred in March 1935 when Wiley Post flew his single-engine Lockheed Vega from Burbank, California, to Cleveland, Ohio, climbing as high as 35,000 feet. His seven-hour, 19-minute flight set a record with the help of tailwinds averaging faster than 100 mph, and at one point blowing as fast as 160 mph. That flight prompted aircraft designers to think about the commercial and military advantages of high-altitude flight.
Russell Owens, a New York Times science writer, summed up this thinking in a March 24, 1935, article when he wrote that high-flying aviators had shown that "up to 15,000 or 20,000 feet, wind of great velocity, usually from the west, may be encountered" but above those altitudes winds blow from any direction and "there are no storms, and no bumps, only masses of air moving at moderate speed."
Today, of course, any time you hear the airline announcement to keep your seat belts fastened even when the seat-belt sign isn't illuminated, you see how mistaken the 1930s idea of calm air aloft was. Many more airplanes began flying at high altitude and reports by aviators of slow groundspeeds on westbound flights and unbelievably fast eastbound groundspeeds caught the attention of some of the world's leading meteorologists, who worked with the military during World War II. After the war the military helped to fund research that laid the foundations of today's understanding and forecasting of jet streams, both in their aviation role and their effects on all weather.
Air temperature differences cause changes in atmospheric pressures, which in turn cause all winds, including jet streams, to blow.
To see how this works, we first need to learn a couple of things about the measurement and mapping of air pressures aloft. The U.S. National Weather Service (NWS) records atmospheric pressures in millibars, which is a metric measure of pressure. The rest of the world, including Canada, uses the name hectopascals instead of millibars, but 1,000 hectopascals are the same as 1,000 millibars. Surface weather maps show patterns of atmospheric pressure with isobars, which are lines of equal air pressures. Regular upper-air maps, based on data from weather balloons, use contour lines to show the height above sea level of certain atmospheric pressures.
Figure 1 is an example. The map's blue lines show heights of 500-millibar pressure over the eastern half of the United States. The lower the height, the lower the pressure.
| Figure 1: On this day, the coldest air in the nation was centered on the Great Lakes. This upper-air map and others like it are available from the American Meteorological Society's DataStream Atmosphere Learning Files. |
This map's lowest pressures aloft are over the Great Lakes with the blue line across the western end of Lake Superior; across northern Illinois, Indiana, and Ohio; and back north across Lake Ontario showing where the 500-millibar pressure was 5,340 meters (17,519 feet) above sea level. The blue line across southern Florida shows the map's highest 500-millibar contour of 5,820 meters (19,094 feet) above sea level.
The map's black arrows are like those on surface weather maps: They show wind directions and speeds. Imagine they are arrows flying with the winds, with the "feathers" on the rear. A triangle on the rear of a wind arrow indicates 50 knots, a line is 10 knots, and a half line is five knots. For example, the wind arrow over Tampa Bay, Florida, shows a 55-knot wind, while the arrow along the North Carolina coast shows 90 knots.
Figure 2 shows how to read upper-air map numbers and the wind arrows.
| Figure 2: This key shows how to read numbers and the wind arrows shown on upper-air maps. |
Balloon data used for the Figure 1 map showed that at International Falls, Minnesota, on the Canadian border, the surface temperature was minus 20 degrees Celsius (minus 4 degrees Fahrenheit), and the 500-millibar temperature was minus 27 C. (This figure isn't clear on the map; the minus 47 is the dew point temperature.) The 500-millibar wind there was from the north at 75 knots. At Key West, the surface temperature was 20.7 C (70 F), the 500-millibar temperature was minus 10 C, and the dew point was minus 42 C. Winds were from the southwest at 45 knots.
Figure 3 shows how temperature differences such as those between Key West and International Falls on the map create different air pressures aloft. The "warm air" on the right would be Key West, and the "cold air" on the left would be International Falls.
| Figure 3: Temperature differences such as those between Key West, Florida, and International Falls, Minnesota, create different air pressures aloft. The "warm air" on the right is Key West, and the "cold air" on the left is International Falls. |
The surface air pressure at any location depends on the amount of air in a column above that location. At the time of the Figure 1 map, the surface air pressure was 980 millibars at International Falls and 1,015 millibars at Key West--a difference of less than 4 percent. In other words, virtually the same amounts of air were above the two stations.
Yet, as shown in Figure 3, the column of warm air over Key West expanded to be higher while the column of cold air over International Falls contracted to be lower. The dashed, average 500 mb height (approximately 18,000 feet) line in the figure shows that the air pressure 18,000 feet above Key West is higher than the air pressure 18,000 feet above International Falls because more air is above this altitude at Key West.
By the way, the 500-millibar level is the altitude with approximately half of the atmosphere's weight above and half below.
Such pressure differences, whether aloft or at the surface, create pressure-gradient forces that push air from high pressure toward low pressure. The Figure 1 map shows the result of higher air pressures in warmer air pushing air toward the lower pressures in similar altitudes in colder air.
If pressure is pushing air from places such as Key West toward places such as International Falls, why aren't the winds aloft blowing from south to north?
Whenever air begins moving across the Earth either at the surface or aloft, the Coriolis force, which is caused by Earth's rotation, pushes the wind toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. (By the way, the Coriolis force does not cause water to flow different ways in the two hemispheres. Draining water, or even small-scale weather phenomena such as tornadoes, don't last long enough to be affected by the weak Coriolis force.)
Since temperature differences through the depth of the atmosphere cause upper-air winds, the fastest winds--jet streams--are found roughly above the biggest temperature contrasts, and upper-air wind patterns, including the paths of jet streams, depend on the locations of warm and cold air.
In Figure 1, warm air is over the western United States north to the Canadian border. The 500-millibar air over northern Montana was the same temperature as the 500-millibar air over southern Alabama and Georgia. The coldest air in the nation was centered on the Great Lakes this day.
At the time of the map, the Coriolis force was turning winds blowing from over Montana toward Michigan to the right to blow from the north-northwest as shown by the arrows over Minnesota and Iowa. Warm air blowing from over Alabama and Georgia toward Michigan turned toward the east, while warm air blowing from over the Atlantic Ocean east of the Middle Atlantic states turned to blow toward the northeast, as shown by the arrows over North Carolina and Virginia.
| Figure 4: Subtropical jets (in red) are shown along the boundaries between tropical and middle latitude air; polar jets (in blue) are between middle latitude and polar air. The National Weather Service's Jetstream Online Weather School includes more information about these weather patterns. |
In the Southern Hemisphere, upper-atmosphere winds blowing from north to south, or from warm toward cold air, are turned to the left by the Coriolis force, creating middle latitude west-to-east winds as in the Northern Hemisphere. Figure 4 (at right) shows generalized jet stream patterns with subtropical jets (in red) along the boundaries between tropical and middle latitude air, and polar jets (in blue) along boundaries between middle latitude and polar air.
The continual pushing and shoving of warm air moving toward the poles and cold air moving toward the equator creates the ever-changing patterns and speeds of jet streams. You sometimes hear things such as: "The jet stream is swinging south over the Southeast, bringing us cold air." You could just as easily say that cold air is moving into the Southeast, bringing the jet stream south.
Jet streams help to steer storms and help determine where storms will form and strengthen and where and when they'll weaken. Storms, in turn, help to push cold air south and warm air north.
One thing you can tell by looking at the contour lines on upper-air charts is that if they are running more or less west to east across the country, the weather should stay generally calm for a while because the winds in the upper atmosphere are not hauling in warm air from the south or cold air from the north. But, when the contour lines turn to head south and then swing around to head back north, watch out. This means cold air is flowing down from the north and warm air is moving up from the south.
Since the clash of warm and cold air supplies the energy for middle-latitude storms, upper air maps with contours that curve a great deal are a sign that the weather could turn more interesting to meteorologists and become a hassle for pilots--including those flying at lower altitudes.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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