How nighttime, low-level jet streams form
A fast-moving river of air known as a "jet" sometimes forms at about 1,000 feet above the ground after sunset.
On a clear evening the atmosphere cools down. And if conditions are calm, a stable temperature "inversion" sets up where cool air aloft, which is heavier than warm air, sinks to the ground and any leftover warm air sits on top of it. The stable air in an inversion acts like a nearly solid object and allows the air above it to flow rapidly past the inversion like wind blowing over water.
Differences in air pressure on either side of the developing low-level jet help to concentrate the flow of air into a corridor or stream less than several hundred miles wide. Winds in the stream can flow at speeds of 60 mph or more. Mountain ranges can further enhance low-level jet stream winds.
Nighttime, low-level jet streams are marked by a rapid change in wind speed with height. The sudden shift in wind can catch late-night fliers by surprise and be hazardous to landing aircraft. And low-level jet streams can form other hazards for pilots.
When strong winds blow across a body water waves form. The atmosphere reacts in a similar manner forming waves of air on the edges of the inversion layer called turbulence. Pilots try to avoid flying into air turbulence because at the very least it causes a bumpy ride. Turbulence can become quite severe and some aircraft have been damaged by turbulence.
On the central Plains of the USA, the nighttime, low-level jet stream also feeds developing thunderstorms with warm, moist air. Large, nocturnal thunderstorm clusters called mesoscale convective complexes, or MCCs, can span areas as large as whole states, blocking flight paths with torrential downpours, deadly lightning, damaging winds and even tornadoes.
The light of day kills the low-level jet. Once the sun begins to heat the land, the lower atmosphere begins to mix as the warm air rises, breaking the inversion. As this happens,the jet rises in some places and sinks in others like a giant roller coaster. Without a smooth surface to glide over, the jet encounters friction and slows down. But the same conditions the next night could allow the low-level jet to reform with equal strength and similar consequences.
Nighttime, low-level jets aren't confined to the Plains states. In 1991, Doppler radar in Sterling, Va., thoroughly documented a low-level jet for the first time east of the Appalachians. As daylight faded, forecasters noticed winds increasing rapidly with height to the 1,000 foot level while surface winds remained at a tranquil 6 knots. Pilots reported low-level wind shear with no thunderstorms in the area.
The nighttime, low-level jet stream isn't unique to the United States either. It's been detected in Canada, China, Australia, Argentina and western Europe.
(Written by Chris Cappella, USATODAY.com)