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Jet Streams in the UK

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An aeroplane flys over a village.
If you've travelled to the USA and back, you may have noticed that your return flight was quicker. This is not because you fell asleep, but due to a fast flow of air called a 'jet stream'.

Key Points
  • Jet streams are narrow fast flowing "rivers" of air.
  • Wind speeds can reach 300 miles per hour.
  • Jet streams play a fundament role in our weather.
Also in this Series

Jet Streams around the World

Also in BBC Weather

Air Pressure
High Pressure
Fronts


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'Jet streams' were first discovered during the Second World War. Pilots were regularly flying between United Kingdom and the United States of America and they noticed that it was quicker to fly to the UK, reporting tailwinds of over 100 miles per hour. These winds blew in narrow ribbons and were named 'jet streams'.

Jet streams are narrow fast flowing "rivers" of air.
Jet streams are narrow fast flowing "rivers" of air. They are formed by 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.

In our latitude the jet stream is generally found at around 35,000 feet and is called the Polar Front Jet Stream. The polar jet stream, as its name implies, separates the cold polar air to the north and the warm sub-tropical air to the south.

With the temperature contrast of these air masses greatest in the winter time, the jet stream is stronger at this time of the year, reaching 300 miles per hour (but have been measured at over 400 miles an hour in southwest Scotland). Jet streams are typically thousands of miles long, hundreds of miles wide and a few miles deep.

Entering and leaving a jet stream can be a turbulent time for any aircraft...
With these kinds of speeds you see why aeroplanes are so keen to use them, saving both time and fuel, and therefore money. However, to navigate in a jet stream is not as easy as you might think. Entering and leaving a jet stream can be a turbulent time for any aircraft no matter how big it is.

The strong winds along the jet stream generally blow from west to east due to the rotation of the earth. That is why, especially in winter time, flights from the USA often land early in this country as they are blown along by these very strong winds. (Incidentally it is also the reason for some "bumpy" rides with clear air turbulence). Planes never land early going the other way.

Jet streams 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.

The wind direction in the jet stream can change from the normal west to east to almost north to south. This is one of the methods that the Earth uses to transport excess heat from the equatorial regions towards the poles, and in turn bring cold polar air southwards. It also helps to steer our Atlantic weather depressions from their normal eastward movement. At times it can even block their movements altogether. Jet streams can strengthen up or even die out so.

Jet streams do play a more fundamental role in our weather.

Many years ago meteorologists thought that the rain bearing depressions that invade us from the Atlantic, formed at the sea level and "grew" up through the atmosphere. It now seems more likely that they start to form around the jet streams and percolate downwards.

The winds in the jet stream do not necessarily blow at a constant speed or in a straight line. Within this fast moving air there are accelerations and decelerations as the air speeds up, slows down or in fact changes direction. It is at these points in the atmosphere that high and low pressures starts to form, and either moves quickly in the wind flow, or develops into a bigger depression or anticyclone. These positive or negative acceleration points are very important to the weather forecaster and these occur at the entrance and exits of the jet stream.

Meteorologists used to spend a long time looking for them on the high level weather maps. Now this task is performed by a computer. By looking at a simple diagram of a jet stream it is possible to pick out the areas below which a depression or anticyclone is most likely to form.

This is the fundamental way that forecasters use jet streams to try to predict whether and where a rain-bearing depression will form, and if it forms whether it will develop into a full blown storm which may cause structural damage as it rushes in from the Atlantic, or whether it will just be a little blip in the fine weather that rushes along at 60 miles per hour.





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