Jet Stream

A narrow band of fast air at the tropopause, a few miles deep and a few hundred miles wide, running around the world. Its boundary with slower air does not show on a windscreen, and turbulence concentrates there.

Regional environment Aerial (air)

Temperate Polar

Atmospheric

Crossing it

Navigate: DC 10
Forage: Nothing to find
Notice: DC 10

Downwind, the crossing takes half the time. Upwind, a vessel covers a quarter of its normal day's distance.

Exposure

Extreme Cold

Temperatures below about zero degrees Fahrenheit, or any water near freezing, made worse by wind.

CON save, DC 10, rising by 1 for each hour after the first. End of each hour. One level of exhaustion.
- Cold-weather gear
  
  Proper layers, a sealed suit or a heated shelter removes the save. Wet layers give disadvantage on it instead.
- Immersion
  
  A creature in near-freezing water makes the save each minute instead of each hour, and is Incapacitated at three levels of exhaustion.
- Gear
  
  Unheated power cells lose half their charge, and any check made with bare hands after an hour of exposure has disadvantage.
- Adapted
  
  A creature native to a cold world, or with resistance to cold damage, ignores the save.

In play

With the stream

A vessel riding the stream halves its travel time. A vessel flying against it covers a quarter of the normal day's distance.
Shear

A pilot crossing the boundary makes a DC 14 Dexterity save. On a failure the vessel is thrown 1d6 x 100 feet and every creature aboard falls Prone.
Hidden boundary

The stream and its edges show no mark a pilot can read. Without instruments, a pilot spots the boundary with a DC 15 Wisdom (Perception) check as the vessel approaches. On a failure, the vessel enters the shear unwarned and the pilot makes the Shear save with disadvantage.

Variations

Polar jet

Stronger, lower and further from the equator in winter.
Subtropical jet

Higher and steadier. This is the jet used as a route.

Worlds it occurs on

Ionic A rocky world inside a field strong enough to hold a permanent ion storm across its surface and atmosphere.
Terran (Verdant) A rocky world with liquid water, breathable air and a magnetic field to protect them, rare and the cause of most wars.
Zephyr A gas world with no surface, crossed by fast bands of wind that run at different speeds.