After the cold, the heat. High pressure spreading across the UK from Siberia last spring brought record cold temperatures. Now more high pressure, this time from the tropical Atlantic, is bringing a sweltering heatwaveMovie Camera. These high-pressure zones are blocking the jet stream which usually brings the country's normal changeable weather. "Blocking highs" are an increasing theme of North American weather reports too, bringing concern of a long-term shift. New Scientist looks at the issues.

One of a series of recent blazes in south-east England(Image: Julien Behal/Press Association)

What's going on?

Blocking highs are a regular feature of the weather in mid-latitudes. They cut out the normal rain-bearing weather systems that are dragged from west to east by the jet stream, a near-continuous high-speed wind flow in the upper atmosphere. The result of blocking highs is episodes of stable and often extreme weather. That is why, with temperatures repeatedly exceeding 30°C in the last week, the UK has been under a "level three" heatwave alert – one level short of a national emergency.

How unusual is the sweltering British weather?

It's very unusual. This is the first significant heatwave in the UK for seven years. Older British readers may remember the long, hot summer of 1976, which saw five days with temperatures above 35°C, which is still a record. That year also brought water shortages, standpipes in the streets and widespread forest and heathland fires. But the current heatwave is not far behind. The UK Met Office issued wildfire alertson Friday after a series of small blazes in south-east England.

Is it part of a trend?

It's hard to be sure. There is no firm statistical evidence of more or longer-lasting blocking highs. But we have had some nasty ones recently, widely blamed for the Russian forest fires during the extreme heatwave of 2010, some extreme hot and cold in the US, and even floods in Pakistan, also in 2010.

More of the story, click image