Do pilots avoid jet stream?

‘Jet streams’ were first discovered during the second world war. Pilots were regularly flying between Britain and the United States of America and they noticed that it was quicker to fly to the United Kingdom and reported tailwinds of over 100 miles per hour. These winds blew in narrow ribbons and were named ‘jet streams’.

Nowadays jet streams are closely monitored and forecast. Pilots want to know where to find them as their added push will save them time and fuel, and therefore money. But jet streams are not only important to pilots. When Breitling Orbiter 3 became the first balloon to fly non-stop around the world it used knowledge of the position of the jet streams to speed up its flight.

They are relatively narrow bands of strong wind in the upper levels of the atmosphere. The winds blows from west to east in jet streams but the flow often shifts to the north and south. Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere winters.

Since the earth rotates, the axis is tilted, and there is more land mass in the northern hemisphere than in the southern hemisphere, there are three global circulations.

Hadley cell — Low latitude air movement toward the equator that with heating, rises vertically, with poleward movement in the upper atmosphere. This forms a convection cell that dominates tropical and sub-tropical climates.
Ferrel cell — A mid-latitude mean atmospheric circulation cell for weather named by Ferrel in the 19th century. In this cell the air flows poleward and eastward near the surface and equatorward and westward at higher levels.
Polar cell — Air rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the polar highs. At the surface air diverges outward from the polar highs. Surface winds in the polar cell are easterly (polar easterlies).

The earth’s rotation is also responsible for the jet stream to move from West to East. The motion of the air is not directly north and south but is affected by the momentum the air has as it moves away from the equator. The reason has to do with momentum and how fast a location on or above the Earth moves relative to the Earth’s axis.

Your speed relative to the Earth’s axis depends on your location. Someone standing on the equator is moving much faster than someone standing on a 45� latitude line. In the graphic (left) the person at the position on the equator arrives at the yellow line sooner than the other two. Someone standing on a pole is not moving at all (except that he or she would be slowly spinning). The speed of the rotation is great enough to cause you to weigh one pound less at the equator than you would at the north or south pole.

The momentum the air has as it travels around the earth is conserved, which means as the air that’s over the equator starts moving toward one of the poles, it keeps its eastward motion constant. The Earth below the air, however, moves slower as that air travels toward the poles. The result is that the air moves faster and faster in an easterly direction (relative to the Earth’s surface below) the farther it moves from the equator.

In addition, with the three-cell circulations mentioned previously, the regions around 30� N/S and 50�-60� N/S are areas where temperature changes are the greatest. As the difference in temperature between the two locations increase, the strength of the wind increases. Therefore, the regions around 30� N/S and 50�-60� N/S are also regions where the wind, in the upper atmosphere, is the strongest.

The 50�-60� N/S region is where the polar jet located with the subtropical jet located around 30�N. Jet streams vary in height of four to eight miles and can reach speeds of more than 275 mph. The actual appearence of jet streams result from the complex interaction between many variables — such as the location of high and low pressure systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else.

Jet streams also «follow the sun» in that as the sun’s elevation increases each day in the spring, the jet streams shifts north moving into Canada by Summer. As Autumn approaches and the sun’s elevation decreases, the jet stream moves south into the United States helping to bring cooler air to the country.

JET Streams for Pilots

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.

JET stream Anatomy

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.

Speeds

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.

Length

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.

Direction

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.

Flight planning

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.

Turbulence, CAT

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.

Weather maps

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.

OAT, outside air temperature

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.

Weather charts

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.