Definition
A high-altitude, fast-moving river of air that forms near 30 degrees latitude in both hemispheres, typically at altitudes between 35,000 and 45,000 feet. It is generally weaker, more persistent, and located further south than the polar jet stream, and is most active in winter months.
Plain English
A narrow band of strong winds blowing from west to east at high altitudes, found roughly above the subtropical regions of the Earth. Pilots flying long-haul routes often encounter it and either ride it for a tailwind or avoid it when flying westbound.
Context Anchor
Pilots may encounter this term in weather discussions, winds-aloft forecasts, turbulence forecasts, and flight planning for longer or higher-altitude flights.
Derivation
Subtropical comes from the Latin sub (under, below) and tropical, referring to the regions just outside the tropics. Jet stream describes the narrow, fast-flowing nature of the wind band, much like a jet of water from a hose. Together: a fast wind river found just outside the tropical belt.
Why Pilots Care
It can supply strong tailwinds on eastbound legs or headwinds on westbound legs, changing fuel burn and flight time; it is also a frequent source of clear-air turbulence.
Grounding Statement
Picture a long, narrow band of very fast wind high overhead, much like a moving belt of air that an aircraft may have to cross or use.
Intuition Check
Do not read “subtropical” as meaning low, warm, or calm. Here it identifies where the jet stream is usually found by latitude, not what the air feels like at the surface.
Example Sentence 1
The dispatcher routed the eastbound flight to take advantage of a 140-knot tailwind in the subtropical jet stream.
Example Sentence 2
A southward dip in the subtropical jet stream brought stronger headwinds and required an altitude change.