You’ve probably heard the words “jet stream” many times while watching weather forecasts on TV. 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.
Rivers 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. Jet streams form at the boundaries of contrasting air masses. When warm and cold air meet, the difference in their air pressures as a result of their temperature differences (recall that warm air is less dense, and cold air, more dense) causes air to flow from higher pressure (the warm air mass) to lower pressure (the cold air mass), thereby creating high winds. Because the differences in temperature, and therefore, pressure, are very large, so too is the strength of the resulting winds.
Jet Stream Location, Speed, Direction
Jet streams “live” at the tropopause (about 6 to 9 miles off the ground) and are several thousand miles long. Jet stream winds range in speed from 120 to 250 mph but can reach more than 275 mph. Oftentimes, the jet houses 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, or Rossby waves) form U-shaped troughs of low pressure that allow cold air to spill southwards and upside-down U-shaped ridges of high pressure that bring warm air northwards.
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. Despite these discoveries, the term “jet stream” was not coined until 1939 by German meteorologist Heinrich Seilkopf.
Meet the Polar and Subtropical Jets
While we typically talk about the jet stream as if there was only one, there are actually two: a polar jet stream and a subtropical jet stream. The Northern Hemisphere and the Southern Hemisphere each have both a polar and a 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°N and 30°S latitude—a climate zone known as the subtropics. It forms at the boundary 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 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 normal periods as the jet stream dips “lower” bringing cold air in from the polar regions. Although the height of the jet stream is typically 20,000 feet or more, the influences on weather patterns can be substantial as well. 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.
In spring, the polar jet starts to journey north from its winter position along the lower third of the U.S., back to its “permanent” home at 50-60°N latitude (over Canada). As the jet gradually lifts northward, highs and lows are “steered” along its path and across the regions where it’s currently positioned. Why does the jet stream move? Well, 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° south latitude) to striking more northerly latitudes (until it reaches the Tropic of Cancer, 23.5° 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.
Locating Jets on Weather Maps
On surface maps: Many news and 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 mb; this is why the 200 and 300 mb 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.