Jet streams are extremely fast-moving air currents in the troposphere and stratosphere, with wind speeds clocked at between 92 and 398 kilometres per hour. All the major jet streams flow from west to east; some local easterlies do exist, but have a relatively small impact on severe weather events. The direction of the major jet streams is shaped by the “slip” of the air trying to catch up with the earth’s rotation, so it does not invert in the southern hemisphere.
Four major jet streams are known: polar jets circling the arctic and antarctic which rise and dip seasonally between latitudes 30 and 60, and subtropical jets close to latitude 30. The polar jets are lower and stronger than the subtropical jets. Each of the jet streams acts as a barrier to large pools of air of differing temperatures. The polar jets keep the bitterly cold polar air from moving further toward the equator. At the same time, the subtropical jets do the reverse to warm, equatorial air.
If the major jet streams remained neatly parallel, keeping each band of air cleanly separate, the weather between them would be stable and relatively boring. This condition does sometimes happen, even for a couple of weeks at a time: but solar heating and the Coriolis force ensure that the jet streams never remain exactly parallel for long.
What happens much more frequently is that the polar jet stream develops large north-south bends, which move east along with the jet stream as a whole. These bends allow polar air to enter lower latitudes and siphon tropical air poleward: resulting in the meeting of clashing air masses. Sometimes the polar jet stream even splits in two: creating and pulling along islands of air. Jet streams are very narrow, so the temperature gradient between air masses separated by jet streams is very high.
Wherever tropical air meets dry cold air, severe weather results. The highest temperature gradients result in the most violent weather. In North America, the crucial latitudes combine with north-south geographic features which do nothing to block clashing air masses from coming together with violent outcomes. The result is Tornado Alley.
The speed and exact direction of the polar jets is influenced by interaction with very large, stable, high pressure and low pressure areas. In the northern hemisphere, the Siberian High and Icelandic Low are two of the major players.
During the north Atlantic hurricane season, the Bermuda High also comes into play. South of the subtropical jet stream, the trade winds created by the subtropical ridge flow from east to west. Thus hurricanes begin their move eastward south of the Bermuda High, pulled by the trade winds: but the strength and size of the Bermuda High determines how far east a hurricane can come before the Coriolis force pulls it north and weather systems driven by the jet stream force it west. The exact direction of the jet stream, combined with the strength of the Bermuda High, are key to determining whether a hurricane will come violently ashore or turn harmlessly out to sea.