Weather

Regular viewers of television weather forecasts are used to hearing about the jet stream and its effects. A jet stream that dips to the south brings cold air in winter, while warmer temperatures accompany one that bulges toward the north.

These jet streams are wide rivers of wind—maybe 200 to 600 miles across—reaching upward from around 20,000 feet to above 40,000 feet.

Meteorologists began talking about atmospheric, high-altitude jet streams in the 1930s. The fast-moving streams of air appeared somewhat like airflow forced through a nozzle, and which is moving faster than the surrounding air.

AT LOWER ALTITUDES. Some jet streams—relatively narrow bands of fast-moving air—are found at lower elevations as well. These low-level jet streams are most common on the Great Plains and in the eastern United States, but they also occur in many other places. Low-level jet streams, which are usually 1,500 to 3,000 feet above the ground, are important to general aviation pilots because an encounter with one could cause unexpected turbulence or headwinds.

MOST WORK AT NIGHT. Low-level jet streams rarely form during the day; friction with the ground slows the speed of low-level winds. Up-and-down air currents caused by the ground warming the air on sunny days—which rise as cooler air from aloft sinks to replace the rising air—generally ensures that winds close to the ground remain close to the speeds and directions of winds a few hundred feet aloft.

On clear nights when the atmosphere is stable, heat from the ground radiates into the sky, while the air near the ground grows colder than the air a few hundred feet above. Such a layer of warm air between cooler air above and below is called an inversion.

An inversion is stable, which means it suppresses rising and sinking air motions. The layer of stable air acts like a nearly solid object and allows the air above it to flow rapidly over the inversion, without picking up bubbles of warm, rising air moving forward at slower speeds. When this happens, atmospheric scientists say the winds aloft are “decoupled” from the winds close to the ground—surface winds and those aloft don’t affect each other.

When the sun rises, it begins to stifle a low-level jet stream as it heats the land. As warmed air near the ground begins to rise, some air aloft begins to sink, breaking the inversion, and the low-level jet stream fades away. But the same conditions could allow another low-level jet stream to form on the following night with equal strength and similar consequences.

THE RISKS. Nighttime, low-level jet streams create rapid changes in wind speed and sometimes direction with height. The sudden shift in wind can be hazardous.

Climbing into a 30-knot, low-level jet stream could give you a few bad moments, especially if the wind is traveling in the same direction as you. 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 also should expect turbulence, because you are running into wind shear—a quick change in wind speed or direction over a short distance—which creates turbulence. If you are taking off or landing when a low-level jet stream is around, be ready for quick wind changes aloft. When strong winds blow across water, it creates waves. The atmosphere reacts in a similar manner along the boundaries between layers of air with different temperatures, such in a low-level jet stream, forming invisible turbulent waves, in an inversion.

LIKELY ENCOUNTERS. In the United States, most commonly low-level jet streams blow from south to north across the Great Plains, from the Mississippi Valley to the Rocky Mountains, in the spring and summer. During winter, low-level jet streams can sock the West Coast especially hard. The “pineapple express”—when flooding rain falls on the coastal mountains of California, Oregon, and Washington—refers to a special kind of low-level jet stream known as an atmospheric river. These bring humid air to the coast from the tropical Pacific around Hawaii.

The low-level jet streams that blow in the summer across the Plains, in the Southwest, and over other parts of the country tend to occur on clear nights when surface winds are calm.

WHY HAVEN’T I HEARD OF them? Unless you’ve taken college-level courses in meteorology you probably haven’t heard of low-level jet streams. They were studied and measured after the national network of Doppler weather radars, which measure wind speeds and directions, was deployed across the United States in the 1990s. Experiments that began in 1971 had conclusively shown that they would greatly improve weather forecasts and warnings, especially for severe storms.

Unfortunately, pilot weather training based on FAA materials and knowledge tests hasn’t completely caught up with ongoing meteorological research that began in the 1980s. Much of this research is based on technologies, especially Doppler weather radar, that researchers began using in the early 1970s.

As Doppler radars were installed at National Weather Service offices across the United States and at some overseas military bases, forecasters and researches began learning new things about weather, including how often low-level jet streams occur. What this means for pilots was illustrated in 1991 after the first NWS Doppler radar went into regular operation at the Sterling, Virginia, NWS office.

In October 1991 this new radar showed that while surface winds remained at around six knots, the 1,000-foot winds reached 25 knots in a low-level jet stream. The NWS office issued new terminal forecasts calling for low-level wind shear.

If you’re taking off at night when the air near the ground is cooling off and is calm or nearly calm, think about the possibility of encountering a sudden shift in wind speed and some turbulence as you climb.

If you took off in the afternoon and are flying into the night with reports of nearly calm air from weather stations along your route, be prepared for wind shifts as you descend to land, even if you don’t hear anything about low-level wind shear or low-level jet streams.

If you do run into signs of low-level jet streams on climbout or while descending to land, contact the tower or Flight Watch (122.0 MHz) as soon as things calm down with a pilot report of the wind shift or turbulence. Your report could help other pilots avoid trouble and help forecasters improve their reports of current weather, as well as predictions of what the rest of the night could bring.