Jet streams are relatively narrow bands of high-speed winds in the upper atmosphere. They generally blow from west to east (in both the northern and southern hemispheres), but can swoop northward and southward. Jet stream winds get their name because they travel at high speeds and at high altitudes (from 6,000 to 12,000 meters). Developing when strong temperature differences cause great pressure differences at high altitudes, in general the twists and turns follow the boundaries between warm and cold air.
In winter, because temperature contrasts are greater, jet streams tend to blow faster with core speeds in excess of 300 kilometers per hour. In the northern hemisphere they move south as cold air pushes southward, coincidentally moving out of some restricted air zones. These are two of the reasons that most of the around-the-world ballooning attempts have launched during this time of year.
There are two strong west-to-east blowing jet streams in the Northern Hemisphere, the Polar Jet and the Subtropical Jet. The Polar Jet marks the boundary between cold, polar air and the temperate, mid-latitude air masses, or Temperate Zone. The Subtropical Jet in turn marks the boundary between the Temperate Zone and the warmer air of the Tropics.
The Temperate Zone extends roughly from 30º to 60º latitude in both hemispheres. Most of the continental United States and Europe lie in this zone. These middle latitudes are where polar cold and tropical heat battle creating storms. Cold air moving south and warm air moving north produce a wave-like pattern to the jet stream.
The figure below shows a graphical analysis of the jet stream winds over the Northern Hemisphere on 22nd of September, 2000.
Image from San Francisco State University
Notice the strong jet over North America extending from California over to the Canadian Maritime Provinces. Compare this jet to the weak flow over Europe, and the split jet stream over eastern Asia.
Balloonists live and die by the winds. The pilot can only make the balloon rise and sink, so in order to go in a particular direction the pilot has to find an altitude with the wind blowing that direction. Westerly jet stream winds are at high altitudes. If a balloon is in unfavorable wind it can sometimes go to lower altitudes to find a favorable north or south wind to move into a position to intercept a jet stream.
North of the Northern Hemisphere's temperate zone lies the Polar Vortex. In this region reduced sunlight results in cooling of the air. Cold air sinking to the ground produces a weak northerly and easterly flow in the atmosphere. These polar easterlies rotate around the globe in the opposite direction of the mid-latitude westerlies. In the game this is usually not a good place to be as the effect is to push one's ballon backwards.
As surface air moves toward the equator from the north or south, it is heated by tropical sunlight. Warmer air rises, and rising air creates calms, or doldrums. At low levels in the atmosphere, very little wind occurs at all. Even at higher altitudes, winds are much lighter than jet stream strength. In the game, if caught in the Doldrums one's balloon does not move at all.