Jet streams

Free lesson · Meteorology · DGCA CPL and ATPL

Rivers of wind near the tropopause: what makes one, and the parts pilots talk about.

What is a jet stream?

Rivers of wind

The teacher's build-up. Tap Next.

  1. A long, thin, fast tube A jet stream is a strong, narrow current of wind along a nearly horizontal axis in the upper troposphere, present in both hemispheres all year. Its core speed is at least 60 kt (30 m/s). It is thousands of km long, hundreds of km wide and only a few km deep (the teacher: about 1500 NM by 200 NM by 12,000 ft), and its path snakes in waves.
  2. Strong shear all round The wind drops off quickly away from the core: vertical shear about 5 m/s per km, much stronger than the horizontal shear (about 5 m/s per 100 km).
  3. Inside the jet The axis is the line of maximum speed; the core is the tube of fast air around it. Along the axis the speed is not uniform: there are centres of even faster wind, jet streaks (often 100-200 kt), which move more slowly than the wind and sit over or near the ridges of a wavy jet.
  4. Warm meets cold Jets form where a large horizontal temperature difference sits under them: warm air on one side, cold on the other, so the thermal wind piles up with height. The bigger the contrast, the faster the jet. Flying with a jet raises your ground speed; flying against it costs time and fuel.

Memory hook: At least 60 kt. Thousands of km long, hundreds wide, a few km deep. Axis, core, streaks.

Takeaway: A jet stream is a narrow, fast river of wind (60 kt or more at the core) near the tropopause: thousands of km long, hundreds wide, a few km deep, wandering like a serpent. It is driven by a strong horizontal temperature difference; the bigger the difference, the faster the jet.

What are the parts of a jet stream?

Axis, core, streaks

A wavy westerly jet seen from above. Tap Next.

  1. A long, narrow river Jet streams blow in both hemispheres all year. Their path wanders like a snake: bends towards the pole are ridges, bends towards the equator are troughs. A jet is thousands of km long and hundreds of km wide.
  2. Axis and core The line of fastest wind is the axis. The tube of fast air around it is the core. Along a jet the speed can peak in one place or in several. To count as a jet, the core must blow at 60 kt (30 m/s) or more. The WMO set this limit.
  3. A few km deep Seen end-on, a jet is a flat tube only a few km deep, often less than 5 km. Above and below the core the wind drops quickly, by about 5 m/s for every km. That change with height (vertical shear) is much stronger than the change sideways.
  4. Jet streaks The speed along the axis is not the same everywhere. Patches of even faster wind, called jet streaks, sit along it with weaker wind between. A streak keeps its strength but moves slower than the wind blowing through it, by up to 25 kt. In a wavy jet the streaks lie over or near the ridges.
  5. Breaks and splits Where the speed falls below 60 kt, the jet has a break. A jet can also split into two or more branches, join up again, or turn to a new direction. A jet core must blow at 60 kt or more.

Memory hook: 60 kt minimum. Vertical shear about 5 m/s per km. Streaks over ridges.

Takeaway: A jet is a narrow river of fast wind high in the troposphere: at least 60 kt (30 m/s) at its core, thousands of km long, hundreds of km wide and only a few km deep (often under 5 km). The axis is the line of fastest wind; the core is the tube around it. Jet streaks, patches of even faster wind, sit over the ridges and move slower than the wind. Where the speed drops below 60 kt, the jet has a break.