Flight Training - March 2015 - 52

WEATHER

By Jack Williams
JET STREAMS are usually found
somewhere between six and nine
miles above the Earth's surface.

A LOW-LEVEL SURPRISE

JET STREAMS DOWN WHERE YOU FLY

R

egular 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.

52 /

FLIGHTTRAINING.AOPA.ORG

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



http://FLIGHTTRAINING.AOPA.ORG

Flight Training - March 2015

Table of Contents for the Digital Edition of Flight Training - March 2015

Contents
Flight Training - March 2015 - Cover1
Flight Training - March 2015 - Cover2
Flight Training - March 2015 - Contents
Flight Training - March 2015 - 2
Flight Training - March 2015 - 3
Flight Training - March 2015 - 4
Flight Training - March 2015 - 5
Flight Training - March 2015 - 6
Flight Training - March 2015 - 7
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Flight Training - March 2015 - Cover3
Flight Training - March 2015 - Cover4
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