You’ve probably heard this term on weather reports when a meteorologist talks about the jet stream, but what is it exactly? The jet stream, which is considered a “river” of air in the upper atmosphere, is essentially a very narrow tube of wind between 20,000 feet and 50,000 feet.
It flows from west to east in the upper atmosphere and can reach speeds of more than 275 mph. As a comparison, a Category 5 hurricane (the strongest hurricane) has winds above 156 mph. Jet stream winds are very powerful, but how do they develop, and why are they important, specifically for air travel?
The jet stream blows from west to east because of the rotation of the earth. As the air moves away from the equator, it doesn’t simply rise to the north or south, and it’s affected by the location where the air is located. A position on the equator moves faster relative to a point farther north or south as the earth rotates.
To put it simply, as the air moves toward the poles, it continues to move in an easterly direction, but the earth moves slower in areas toward the poles (relative to a spot on the earth’s surface). This makes the air move even faster in an easterly direction when it gets farther from the equator.
Jet streams are related to the differences between hot and cold air. These differences can be seen the most during winter, where variations between hot and cold air are the most in the northern and southern hemispheres. When you add temperature differences between hot and cold air, the wind strength increases even more in the upper atmosphere. This is where two bands of wind form.
One is called the Polar Jet, which is located closer to the north and south poles, and the other is called the Subtropical Jet, which is located between the Polar Jet and the equator. These bands of strong air can vary in position based on the season, air temperature changes, pressure systems, and the sun. Jet streams are very important in air travel as airplanes can “ride” these strong winds from west to east to decrease time and fuel costs, thus saving the airline money.
One example of the power of the jet stream occurred in January 2016 when a British Airways Boeing 777 flying from New York to London recorded a ground speed of 745 mph as it rode the jet stream that at the time was producing a wind of more than 200 mph. The airplane even got close to supersonic speeds, which is a ground speed of 761 mph. The flight was an hour and a half ahead of schedule when it landed at Heathrow Airport in London.