A Jet stream is a fast flowing narrow band of air located at the breaks at the top of the tropopause between the boundary of air masses. There are several Jet streams around the planet. The polar Jet is generally located at around 30 °N and there is one located at around 60 °N called the tropical Jet. The same phenomenon occurs in the southern hemisphere at 30 °S and 60 °S.
The direction of flow is west to east and the speed varies usually between 50 and 150 kts. Jet aircraft flying to the east can take advantage of this and reduce their flight time and save fuel too. Flying to the west you will try to avoid this high speed wind area.
Although most experimental aircraft pilots will not encounter Jet streams within their flying career, these streams do influence weather at lower levels so some knowledge of Jet streams can be helpful in understanding the effects of the frontal systems and their movement through your area.
Both streams have different origins, the polar Jet forms due to temperature differences in air mass between cold polar air and warm moist subtropical coming from the mid-latitudes in the Ferrel cell. During the winter season the Jet will usually attain the greatest wind speeds.
The equatorial or subtropical Jet finds its origin in tropical air, not coupled with frontal systems and its location is rather well defined in contrary to the polar Jet which can be quite variable. The exact location of the Jets is influenced by phenomena like El Niño and La Niña in the Pacific Ocean.
Typical wind speeds in a Jet stream are around 100 kts, but during the winter season (when the temperature differences are large) speeds can reach up to 200 kts in extreme cases.
The length varies but is usual some several thousand nautical miles. The width is a few hundred miles and they are a couple of thousand feet thick. When flying westbound a crew can decide to climb out of the Jet by ascending or descending several thousand feet, with ATC clearance and traffic permitting of course.
Likewise with its length, the direction of the Jet may not always be west-east. Sometimes they break and reverse for a couple of days. North-South orientated Jets also occur when they flow around the frontal systems.
When flying to the east, air transport aircraft can take advantage of the extra speed to reduce their flight time and fuel consumption. Flying to the west airline crews will most likely try to avoid the Jet.
As the Jet consists of a narrow band of high speed air, the adjacent air has relatively lower speed and in this boundary layer wind shear will exists and possibly causing some turbulence when passing this layer. It is commonly known as clear air turbulence, or CAT. More on that in our turbulent weather article.
The stability of the air, or lack of it within the Jet, will have an certain amount of influence on the turbulence. Depending on the amount of moisture this turbulence might be visible by the formation of cirrus clouds. Which are purely made of ice crystals due to the low temperatures at these high altitudes.
High level significant weather charts usually show the altitude, location, direction and speeds of the Jet stream. Speeds are indicated with special chart symbols where each triangle is 50 kts and each wind barb 10 kts (half a wind barb is 5 kts). This makes it easy to calculate the estimated speed and the flight levels (FL) where you will find it.
When flying near the tropopause it pays to keep an eye on the outside air temperature indicator (OAT). Sudden changes indicate a change in air mass, typically a front, and a Jet stream may be nearby. Changing your route to cross the Jet as soon as possible might be an option at that point.
You will have to study the most current and past weather charts to see how fast and in which direction these Jets are moving. Fast moving systems will make location prediction difficult without some experience. For low level pilots these maps are a good indicator where weather is coming from and with what speed it will approach their intended route of flight or area of interest.