The Weather Journal / Explainers
The Jet Stream: The River That Runs the Weather
The jet stream is a narrow, wandering river of fast wind near the top of the troposphere, roughly five to nine miles up, flowing generally west to east. It forms along the boundary where cold polar air meets warm air from the south, and the sharper that temperature contrast, the faster it blows: NOAA puts typical core winds at 80 to 140 mph, with more than 275 mph possible. Storms ride along it like traffic on a highway, so its bends decide who gets a stormy week and who gets a quiet one. A northward bulge is a ridge, and usually quiet; a southward dip is a trough, and usually not.
A river with no banks
Take the air about five to nine miles up, near the , and it is not drifting. It is organized into narrow bands of wind moving at highway multiples: NOAA gives 80 to 140 mph as the usual range for winds, with peaks beyond 275 mph in the strongest winter jets. The bands are thousands of miles long, a few hundred miles wide, and only a mile or two deep. A river is the right picture, complete with meanders, except the banks are made of nothing but temperature difference.
Where it comes from
A jet stream forms along the boundary between with a serious temperature contrast, which is why the main one, the polar jet, lives along the line where cold polar air meets warmer air from the south. The bigger the temperature gap across that line, the harder the upper level winds blow along it. A second, weaker one, the subtropical jet, runs at lower latitudes.
That origin story explains the seasons of the jet itself. In winter the pole is dark and frigid while the tropics barely change, so the contrast is enormous and the jet is at its fastest, diving far south into the United States. In summer the contrast slackens and the jet weakens and retreats north toward Canada, taking its parade of storms with it. Why the flow ends up west to east at all is the same spinning-Earth bookkeeping that bends every long-distance wind, covered in why the wind blows.
Troughs, ridges, and your week
The jet rarely runs straight. It meanders in slow waves, and the waves have names worth knowing, because each one is a weather regime you can feel from the ground.
- A is a northward bulge. Beneath and downstream of it, air tends to sink, and sinking air means fair skies that can persist for days. Summer are usually a ridge that stopped moving.
- A is a southward dip. It swings cold air south, and the region ahead of it, the eastern side, favors rising air, which is the raw material of storms. Winter cold outbreaks are a trough doing exactly what troughs do.
Surface storm systems do not just sit under this river; they are steered by it and fed by it, growing on the rising-air side of a trough and getting carried along at the pace of the flow, generally west to east. This is the machinery behind a pattern every observer eventually notices and this site keeps a whole lesson about: in the middle latitudes, tomorrow's weather usually lives to your west today.
Fast planes and rough rides
The jet stream was a flight discovery as much as a science one, and aviation still budgets around it daily. Fly inside the river heading east and the tailwind hands you free speed, which is why the eastbound leg of a cross country round trip is routinely shorter than the westbound leg on the very same route. Fly along its flanks and you find its other signature: the edges of the jet, where fast air shears past slow air, are a common address for of cloud. Full sun.">clear air turbulence, the kind that arrives without a cloud in sight.
What the rock watches
You cannot see the jet stream from a lawn. But you can read its position in a week of weather: storm after storm arriving on the same track means you live under the river just now; a long stretch of unbudging heat means a ridge has parked on you; a sudden out of season means a trough dug deeper than usual. The rock has never been five miles up and does not intend to start, but it has noticed that its busiest weeks and the river's southern detours are the same weeks. It checked twice.
Words in this lesson
Every marked word above is here in full, because a definition that only appears on hover is no use to somebody reading on a phone, printing the page, or listening to it.
- Air mass
- A huge body of air, hundreds of miles across, that has sat in one place long enough to take on that place's temperature and moisture. Most day to day weather is the story of air masses shoving each other around.
- Clear
- Zero to one okta of cloud. Full sun.
- Cold snap
- A brief sharp turn cold.
- Heat wave
- Multiple consecutive days of excessive heat.
- Jet stream
- The high-altitude wind river steering weather systems.
- Okta
- One eighth of the sky. Sky cover is counted in oktas, from zero for clear to eight for overcast, which is why the familiar sky words are really just bands of a number.
- Ridge
- A northward bulge in the upper air flow, an elongated area of higher pressure. Air sinks beneath a ridge, bringing quiet, often warm weather that can overstay its welcome.
- Tropopause
- The boundary at the top of the troposphere where temperature stops falling with height. It acts as a lid, which is why the tops of tall thunderstorms flatten out and spread sideways.
- Trough
- A southward dip in the upper air flow, an elongated area of lower pressure. Air rises ahead of a trough, which is why one on the map usually means storms or a cold outbreak is coming.
This is a Working lesson
It belongs to the Weather School, which is free and always will be: twenty lessons from toddler to adult, every one with a printable worksheet and answer key that any classroom may copy. No account, no cost, no catch.
Another one at this level: why the wind blows.
For teachers and every lesson.
Definitions in this explainer follow the National Weather Service. The rock checked twice. The rock always checks twice.