If you've travelled to America and back, you may have noticed that the flight was quicker on your return flight than it was on your way to the States. This is not because you simply fell asleep, but due to a fast flow of air called a '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.

An example of jet streams across the USA © SFSU

Jet streams are formed by the temperature differences in the upper atmosphere, between the cold polar air and the warm tropical air. This abrupt change in temperature causes a large pressure difference, which forces the air to move. During the winter there is a greater difference in the temperature, so this is when the jet streams are at their fastest.

They generally move in a west to easterly direction due to the rotation of the earth. They move north and south too, following the boundary between warmer and colder air. These boundaries are also where weather fronts generally develop, so when a front passes overhead, bringing wind and rain, it is quite likely that a jet stream is passing undetected too.

Jet streams flow approximately 5 to 10 miles above the surface of the earth, with speeds up to 310 miles per hour. With these kinds of speeds you see why the aeroplanes are so keen to use them. However, to navigate in a jet stream is not as easy as you might think, as it is only a few hundred miles wide, just a few miles thick, and constantly moving. Entering and leaving a jet stream can be a turbulent time for any aircraft no matter how big it is, and most passengers will want to leave the roller coasters in America.

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle