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Last Updated: Friday, 16 July, 2004, 00:31 GMT 01:31 UK
Jet stream to blame for poor summer

By Michael Fish
BBC weather forecaster

Most people cannot have failed to notice that the weather has been generally overcast and dull lately. But before everyone writes off summer and mumbles something about global warming, let me explain why.

A beach attendant puts out deck chairs at Bournemouth on Thursday
Temperatures in Bournemouth on Thursday did not get above 20C

High in the sky above the United Kingdom, in a layer of the Earth's atmosphere called the troposphere, wind speeds can reach 200mph.

This ribbon of high altitude high speed wind is known as the polar jet stream, and is responsible for steering weather in our direction.

To bring us the summer we are longing for, the jet stream needs to pass to the north of the UK.

This allows the Azores High, an area of high pressure situated in the mid-Atlantic, to bring us warm and settled weather.

Think back far enough and you may recall that most of the UK had beautifully settled weather for much of May and the first half of June
Michael Fish

This type of weather set-up allowed the temperature to reach 33 degrees Celsius (91 degrees Fahrenheit) in Cardiff on this day last year, and brought us the prolonged period of hot weather in August 2003.

Lately, however, the jet stream has not been playing ball.

It is currently passing straight over the top of the UK, and is steering low pressure systems our way that are responsible for the damp, windy weather we've had to put up with recently.

This week in Cardiff we expected the mercury to only reach about 19C (66F). But as depressing as it sounds, 19C is just one degree below the average daily maximum for Cardiff in July.

Gloom

The polar jet stream naturally wavers around in the upper atmosphere and so it is not uncommon for it to be passing over our latitudes.

We are therefore not in a "freak" weather pattern, it's just average.

Our cause is not helped by the fact that we Brits appear to have short memories.

Think back far enough and you may recall that most of the UK had beautifully settled weather for much of May and the first half of June.

Since then summer has taken a holiday.

Northern Europe is suffering the gloom the same as us.

So when will it be back? Well, there is no immediate return to summer as this weekend promises more rain or showers for most.

The Met Office seasonal forecast perhaps can offer a little glimmer of hope however - "through the middle of the month as the Azores high extends gradually towards the UK, the weather will become more settled, with temperatures gradually rising."

So don't pack away the sun lotion and beach towels just yet.

1 Jetstream passing north of the UK bringing settled, dry and warm weather
2 Jetstream passing further south, causing weather that is unsettled and cool for the time of year




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