A jet stream is a narrow band of extremely fast-moving wind high up in Earth's atmosphere, typically five to nine miles above the surface. These winds race from west to east at speeds that can reach 275 miles per hour, faster than the fastest race car.

Jet streams form where cold polar air meets warmer air from the tropics. The temperature difference creates these rivers of wind that flow around the planet like invisible highways in the sky. Earth has several jet streams, but the most important ones for North America are the polar jet stream and the subtropical jet stream.

Meteorologists track jet streams carefully because they steer weather systems across continents. When the jet stream dips south, it can pull Arctic air down into the United States, causing cold snaps. When it shifts north, warm air flows up from the south. A wavy jet stream might trap weather in one place for days, causing droughts or flooding rains.

Airplane pilots pay close attention to jet streams too. Flying with a jet stream can cut hours off a cross-country flight and save thousands of gallons of fuel. Flying against one makes the trip longer and bumpier. On a flight from Los Angeles to New York, a strong jet stream might push your plane along at over 600 miles per hour relative to the ground, even though the plane's engines alone would carry it at about 500 miles per hour.