What is the jet stream?
A narrow river of wind about 30,000 to 40,000 feet up, often blowing 100 to 200 mph from west to east. It runs along the border between cold polar air and warm air, and its waves steer storms across the map.
You don't feel the jet itself; you feel the storms and heat it steers.
A stuck ridge can trap heat and ozone; a trough brings fresh, cooler air.
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Why there's a river of wind up there
The jet stream forms where cold polar air and warm subtropical air sit side by side. The bigger the temperature difference across that border, the faster the wind on top of it, which is why the jet is strongest in winter, when the contrast is sharpest. Earth's spin turns that flow into a west-to-east current.
There are two main ones in each hemisphere: the polar jet, the one that matters most for US weather, and the higher subtropical jet closer to the tropics. A jet can run thousands of miles long but only a few hundred miles wide and a few thousand feet deep. Airline pilots ride it east to save time and fuel, and avoid it flying west.
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Waves: troughs and ridges
The jet rarely flows straight. It meanders north and south in big waves. Where it dips south (a trough) it lets cold air spill down behind it, and storms tend to form on its downstream side. Where it bulges north (a ridge) warm air builds underneath, skies clear and heat can pile up.
When those waves grow large and stall, the same weather sits in place for days: a heat dome under a stuck ridge, or cold and storms under a stuck trough.
Your air vs. the jet
The jet sits far above the air Air Story traces near the ground. That's why your air's trail can come from the east, a Santa Ana, while the jet overhead still blows from the west. Low-level air circles highs and lows, and the jet steers those highs and lows along.
Questions people ask
- How fast is the jet stream?
- Usually around 100 mph, often over 150 mph in winter, and occasionally above 200 mph. It has to blow at least about 70 mph to be called a jet.
- Does the jet stream move with the seasons?
- Yes. It shifts toward the equator in winter, bringing storms farther south, and retreats toward the pole in summer.
- Why do flights east take less time than flights west?
- Eastbound flights ride the jet stream's tailwind; westbound flights fly into it or route around it.