Jetstreams form where warm air meets cold air. This occurs, for example, when warm, tropical air meets cold polar air. But how does it effect the south west's weather?

The jetstream is a narrow band of winds that usually occurs at a height of about 30000 feet. Wind speeds within the jetstream reach between 100 and 200 mph.

On very rare occasions speeds of 300 MPH have been recorded. It can sometimes be found in the skies above the West Country. Although it lies so high in the sky, it still exerts a big influence on our weather, but why should this be so?

It is beneath jetstreams that weather systems tend to form and develop. All the major Atlantic depressions that sweep across the British Isles are given an extra impetus as they pass beneath a jetstream.

Jetstreams form where warm air meets cold air. This occurs, for example, when warm, tropical air meets cold polar air.

Pressure falls faster with height in cold air than warm air and this difference in pressure causes a movement of air from the warm air to the cold air.

Moving escalators

The direction of the airflow is from west to east, not north to south. This is because the rotation of the Earth has the effect of 'turning' the air to the right. This is why jetstreams generally flow from west to east across the Atlantic.

Jet airliners use jetstreams as 'moving escalators' which speed them on their journey from North America to Europe. This is why flights from, say, San Francisco to London may take off late yet still arrive on time.

Aircrew use the forecast winds to plan their flight durations as the jetstream can shorten the west to east journey by an hour or more. It is far better, from the airlines' point of view, to depart late, arrive at Heathrow or Gatwick on time, and then land immediately.

Not taking note of the forecast strength of the jetstream would have caused that same aircraft to arrive well ahead of its scheduled time.

It is more than likely that it would then have had to spend much of the saved time in a holding pattern above the airfield, wasting fuel.

Headwinds

Jetstreams are not always of benefit to jet liners. Journeys from east to west will obviously take much longer because of the headwinds.

Jetstreams are not really like 'streams'. They often behave more like 'rapids' because air moves across the jetstream as well as along it. This can make for a good deal of clear air turbulence.

This is why certain points on the jetstream are favoured for the development (and decay) of weather systems. At some points air within the jetstream is descending and at others it is rising.

Rising air is associated with developing weather systems and it is here that depressions can form thousands of feet below the jetstream.

You cannot see jetstreams, just as you cannot see any other type of wind. But they are up there in our skies; not every day but when they are there you can be sure they are playing a big part in influencing Gloucestershire's weather.

by Richard Angwin

BBC Gloucestershire
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