The Jet Stream: What It Is and How It Affects Our Weather

How does the jet stream affect weather?

A side-view of Earth's Northern Hemispheric jet stream. NASA/GFSC

You've probably heard the term "jet stream" many times while watching weather forecasts on television. That's because the jet stream and its location is key to forecasting where weather systems will travel. Without it, there would be nothing to help "steer" our daily weather from location to location.

Bands of Rapidly Moving Air

Named for their similarity to fast-moving jets of water, jet streams are bands of strong winds in the upper levels of the atmosphere that form at the boundaries of contrasting air masses. Recall that warm air is less dense and cold air is more dense. When warm and cold air meet, the difference in their air pressures causes air to flow from higher pressure (the warm air mass) to lower pressure (the cold air mass), thereby creating high, strong winds.

Location, Speed, and Direction of Jet Streams

Jet streams "live" in the tropopause—the atmospheric layer closest to earth that is six to nine miles off the ground—and are several thousand miles long. Their winds range in speed from 120 to 250 miles per hour but can reach more than 275 miles per hour.

Additionally, jet stream often house pockets of winds that move faster than the surrounding jet stream winds. These "jet streaks" play an important role in precipitation and storm formation: If a jet streak is visually divided into fourths, like a pie, its left-front and right-rear quadrants are the most favorable for precipitation and storm development. If a weak low-pressure area passes through either of these locations, it will quickly strengthen into a dangerous storm.

Jet winds blow from west to east, but also meander north to south in a wave-shaped pattern. These waves and large ripples—known as planetary waves or Rossby waves—form U-shaped troughs of low pressure that allow cold air to spill southward as well as upside-down U-shaped ridges of high pressure that bring warm air northward.

Discovered by Weather Balloons

One of the first names associated with the jet stream is Wasaburo Oishi. A Japanese meteorologist, Oishi discovered the jet stream in the 1920s while using weather balloons to track upper-level winds near Mount Fuji. However, his work went unnoticed outside of Japan.

In 1933, knowledge of the jet stream increased when American aviator Wiley Post began exploring long-distance, high-altitude flight. But despite these discoveries, the term "jet stream" was not coined until 1939 by German meteorologist Heinrich Seilkopf.

Polar and Subtropical Jet Streams

There are two types of jet streams: polar jet streams and subtropical jet streams. The Northern Hemisphere and the Southern Hemisphere each have both a polar and subtropical branch of the jet.

  • The polar jet: In North America, the polar jet is more commonly known as "the jet" or the "mid-latitude jet," so-called because it occurs over the mid-latitudes.
  • The subtropical jet: The subtropical jet is named for its existence at 30 degrees north and 30 degrees south latitude—a climate zone known as the subtropics. It forms at the boundary of the temperature difference between air at mid-latitudes and warmer air near the equator. Unlike the polar jet, the subtropical jet is only present in the wintertime—the only time of year when temperature contrasts in the subtropics are strong enough to form jet winds. The subtropical jet is generally weaker than the polar jet. It is most pronounced over the western Pacific.

Jet Stream Position Changes With the Seasons

Jet streams change position, location, and strength depending on the season.

In the winter, areas in the Northern Hemisphere may get colder than in other periods as the jet stream dips "lower," bringing cold air in from the polar regions.

In spring, the polar jet starts to journey north from its winter position along the lower third of the U.S. and back to its "permanent" home between 50 and 60 degrees north latitude (over Canada). As the jet gradually lifts northward, highs and lows are "steered" along its path and across the regions where it's positioned.

Why does the jet stream move? Jet streams "follow" the sun, earth's primary source of heat energy. Recall that in spring in the Northern Hemisphere, the sun's vertical rays go from striking the Tropic of Capricorn (23.5 degrees south latitude) to striking more northerly latitudes (until they reach the Tropic of Cancer, 23.5 degrees north latitude, on the summer solstice). As these northerly latitudes warm, the jet stream—which occurs near boundaries of cold and warm air masses—must also shift northward to remain at the opposing edge of warm and cool air.

Although the height of the jet stream is typically 20,000 feet or more, its influences on weather patterns can be substantial. High wind speeds can drive and direct storms, creating devastating droughts and floods. A shift in the jet stream is a suspect in the causes of the Dust Bowl.

Locating Jets on Weather Maps

On surface maps: Much of the media that broadcast weather forecasts show the jet stream as a moving band of arrows across the U.S., but the jet stream isn't a standard feature of surface analysis maps.

Here's an easy way to eyeball the jet position: Since it steers high and low pressure systems, simply note where these are located and draw a continuous curved line in between them, taking care to arch your line over highs and underneath lows.

On upper-level maps: The jet stream "lives" at heights of 30,000 to 40,000 feet above Earth's surface. At these altitudes, atmospheric pressure equals around 200 to 300 millibars; this is why the 200- and 300-millibar-level upper air charts are typically used for jet stream forecasting.

When looking at other upper-level maps, the jet position can be guessed by noting where pressure or wind contours are spaced close together.