High-level, narrow, fast-moving currents of air
that are typically thousands of kilometers long, hundreds of
kilometers wide, and several kilometers deep. Jet streams typically
form near the tropopause, 6–9 miles (10–15 km) above
the surface, and may reach speeds of 115–230 miles per hour
(100–200 knots). Rapidly moving cirrus clouds often reveal
the westerly jet streams moving air from west to east. Several
jet streams are common—the subtropical jet stream forms
about 8 miles (13 km) above the surface, at the pole-ward
limit of the tropical Hadley Cell, where a tropospheric gap
develops between the circulating Hadley Cells. The polar jet
stream forms at about a 6–mile (10–km) height, at the tropospheric
gap between the cold polar cell and the mid-latitude
Ferrel cell. The polar jet stream is often associated with polar
front depressions. The jet streams, especially the subtropical
jet, are fairly stable and drive many of the planet’s weather
systems. The polar jet stream tends to meander and develop
loops more than the subtropical jet. A third common jet
stream often develops as an easterly flow, especially over the
Indian subcontinent during the summer monsoon.
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