JET Streams
A Jet stream is a fast flowing narrow band of air located at 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 located at 30°NB and there is one located at 60°NB called the tropical Jet. The same occurs in the southern hemisphere.
The direction of flow is west to east and the speed is around 100 kts. Jet aircraft flying to the east can take advantage of this and reduce their flight time and save fuel.
Although most experimental aircraft pilots will not encounter Jet streams within their flying career, they do influence weather at lower levels so some knowledge of Jet streams can be helpful in understanding frontal systems and their movement.
JET stream Anatomy
Both Jet 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. During the winter season the Jet will have the greatest 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.
Speeds
Typical speeds in a Jet stream are around 100 kts, but during the winter season (when the temperature difference is large) speeds can reach up to 200 kts.
Length
The length of a Jet stream varies but usual some several thousand miles. The width is a few hundred miles and they are a couple thousand feet high. When flying westbound a crew can decide to climb out of the Jet by ascending or descending several thousand feet, traffic permitting.
Direction
Likewise with the length, its direction 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 frontal systems.
Flight planning
When flying to the east aircraft can take advantage of the extra speed to reduce their flight time and fuel consumption. Flying to the west 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 wind shear will exists and possibly causing turbulence. It is known as clear air turbulence, or CAT. 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 can be visible by cirrus cloud formation.
Weather maps
High level significant weather charts usually show the location, direction and speeds of the Jet stream. Where each triangle is 50 kts and each wind barb 10 kts. Easy to calculate the estimated speed of the Jet and the shown flight levels (FL).
OAT
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 maybe nearby.
Study the most current weather charts to see how fast they are moving. Fast moving systems will make Jet location prediction difficult. 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.
