Atmospheric scientists are interested in low-level jets because in the summer they feed moisture into huge, nighttime clusters of thunderstorms that bring the Great Plains and the Mississippi Valley around half of their summer rain. These same low-level jets haul in moisture from the Gulf of Mexico and the Gulf of California to feed �monsoon� showers and thunderstorms in the southwestern United States in the summer.
While the low-level jets that race above the Plains and desert Southwest occur in the summer, the West Coast has its own low-level jets that help to bring the rain of winter storms that move in from the Pacific Ocean. These jets can bring extremely heavy rain, leading to mud slides and floods along the coast.
Meteorologists have found low-level jets around the world and discovered that understanding and forecasting them will help in other kinds of forecasts, such as for overnight thunderstorms on the U.S. Plains.
Pilots should be interested in low-level jets because they share the air with these winds. The low-level jets associated with West Coast storms occur when the weather is windy, cloudy, and rainy�times when few pilots are likely to be on recreational or training flights. But the low-level jets that blow in the summer across the Plains, in the Southwest, and probably over other parts of the country tend to occur on clear nights when surface winds are calm�the kind of night than can encourage night flying.
These low-level jets:
- Have winds up to 60 knots, although 30-knot speeds are more common.
- Are usually from around 300 feet to 1,000 feet above the ground.
- Can be a stream of wind more than 100 miles wide and 1,000 miles long.
- Frequently occur during the nighttime hours, when winds aloft �decouple� from lower-level winds.
As you can imagine, running into a 60-knot wind only 300 feet above ground level after calm surface winds lulled you into being relaxed could lead to more excitement than you really wanted.
Even climbing into a more common 30-knot, low-level jet could give you a few bad moments, especially if the wind is traveling in the same direction you are. In this case, you would have a sudden 30-knot tailwind. With the resulting loss of lift, you would probably lose some altitude before regaining airspeed.
You could also expect turbulence because you are running into wind shear�a quick change in wind speed or direction creates turbulence.
If the National Weather Service knows about the low-level jet, the information will show up when you use DUATS or telephone a flight service station to get a standard weather briefing. Thanks to the network of Doppler weather radars that covers almost all of the United States, forecasters today are far more likely to know when low-level jets are blowing than they were even a decade ago. In fact, detection of low-level jets was one of the unexpected benefits of the Doppler radar system when it was installed all over the country.
Doppler weather radar was developed in Oklahoma, where it proved its value by increasing the warning time for severe thunderstorms and tornadoes in Tornado Alley. In 1991, the second Doppler weather radar in the East went into operation at the Weather Service�s Washington-Baltimore area forecast office in Sterling, Virginia, just north of Dulles International Airport.
On a quiet Saturday evening in early October 1991, the surface winds were at about six knots when meteorologists at Sterling noticed that the wind readings from the Doppler radar were showing increasing wind speeds about 1,000 feet above the surface while the surface winds remained at around six knots. When the 1,000-foot winds reached 25 knots, the office issued new terminal forecasts calling for low-level wind shear for Dulles, Washington National, Baltimore-Washington International, and other airports in the region. As the winds 1,000 feet above the ground increased to 40 knots�with no change in the surface winds�scores of pilot reports of low-level wind shear began coming in.
Pilots learn to expect low-level wind shear when thunderstorms are around, but not on calm, clear evenings.
The clear skies and calm surface winds are all part of the pattern that makes nighttime low-level jets possible. During the day the sun heats the ground, which in turn warms the air next to the ground. (The air is warmed very little by sunlight passing through it; it�s said to be �transparent� to most solar radiation.) As air near the ground warms, it begins rising, carrying heat aloft. Even if the convection�or up and down movement of the air�doesn�t lead to showers or thunderstorms, it manages to warm the first few thousand feet of the air.
After sunset on a clear night, the ground�s heat begins to radiate away into space. If clouds are in the sky, they absorb some of the heat leaving the Earth and radiate it back down, which keeps the ground from cooling as much as on a clear night. On a clear night, however, the ground cools and cools the air next to it while the air aloft stays warm.
Eventually, an inversion forms, which means the air aloft is warmer than the air below. It also means that the chilly air next to the ground is more dense than the air above the inversion. Fluids of different densities, such as air and water, or even warm air and chilly air, don�t mix easily. This means that any winds blowing in the warm air will have little effect on the cold, dense air below. In other words, the inversion makes it possible for winds only a few hundred feet above the ground to be blowing at high speeds while the air below remains almost calm.
While the air near the ground might be calm, the zone where the cold and warm air meet is likely to be anything but calm. Like wind blowing across water, the warm air blowing across the denser cold air creates waves, a kind of wind-shear turbulence. This is why wind shifts and turbulence can be expected whenever you climb through an inversion, whether there is a low-level jet blowing or not.
The failure of cold and warm air to mix at the inversion level actually helps low-level jets to blow faster.
Winds are caused by differences between high and low air pressure, creating a force that pushes the air. As the air begins moving, friction with the ground slows it. When you take all of the forces into account, including friction, the wind blows at a certain speed. As you climb, the force of friction acting against wind flow decreases, which is why winds aloft are generally faster than at the surface. But, friction does affect wind speeds for a few hundred feet up. When an inversion forms, the winds aloft have been �decoupled� from the force of friction; they are free to move faster with the same amount of air pressure difference as earlier in the day. After the sun comes up and begins heating the ground, the inversion begins to fade as the air near the ground warms up. Without the inversion, the low-level jet will eventually die out, but it can last until 9 or 10 a.m.
In addition to worrying about the winds themselves, pilots�especially those flying in the central United States during the summer�should be concerned about low-level jets because they bring in the huge amounts of Gulf of Mexico humidity needed to feed the clusters of thunderstorms that form in the evening and last most of the night. The Plains low-level jet begins forming in the spring and is usually apparent for a few nights in May, becoming more regular from June through August.
Humans aren�t the only fliers interested in the low-level jet. Scientists studying the black cutworm moth in the Midwest discovered that the moths catch the low-level jet as they migrate north. Not only does the jet speed them on their way, but the jet�s warm, humid air also is more comfortable for them. The moths took eons to evolve the �knowledge� that enables them to make use of low-level jets. A few minutes with a DUATS weather briefing or a call to a flight service station can supply a human pilot the knowledge needed to fly safely when low-level jets are howling.
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