Release Date 25 July 2007
From the Walker Institute for Climate System Research and the Department of Meteorology, University of Reading.
A washout is the term that comes to mind when we think of the summer of 2007. But why has this summer been so wet and is climate change to blame?
Perhaps we were lulled into a false sense of security by the balmy sunshine of April, but we've certainly had our come-uppence since! During May and June we've seen rainfall well above normal.
June was particularly wet. Parts of north east of England had 4 times the normal amount of rainfall – much of it falling during the 24th/25th with devastating floods the result. We've seen yet more flooding during July, this time mostly over the midlands and south of the UK, with the deluge of rain we received last Friday (20th July).
So just what has happened to the summer of 2007?
Whether the UK experiences a dry or wet summer depends on the course of low pressure systems which track across the Atlantic. High up in the atmosphere is a ribbon of fast moving air – known as the jet stream. It is the jet stream that steers the weather systems which bring much of our rainfall. Since early June it has been further south than normal and this means it has been steering rainfall systems straight over the UK. What's more, once these systems have reached the UK, they've tended to "park", dumping rainfall over us for hour on hour.
Associated with the displaced jet stream, there has been a persistent low-pressure system to the west of the UK and a high over southern Europe. This high has given many countries such as Greece very hot sunny weather, leading to forest fires. The low pressure has given us our wet summer, including the exceptional rain on the 20th July.
Last Friday (20th July) very moist, warm air was transported over us that a few days earlier had been over the subtropical Atlantic. This warm, moist air met colder air and so there was lots of energy and moisture available to produce very heavy rainfall. We saw rainfall rates of more than 50mm an hour - we would normally think of 10mm an hour as heavy.
The sort of torrential downpours we experienced on Friday are more typical of the rain we get from summer thunderstorms which are fairly localised and short lived. But on Friday, the rain was organised by an area of low pressure and so affected a wide area of the country and persisted for many hours.
But why, you might ask, has the jet stream been further south than normal and why does it seem to be stuck in this position? Well this is rather more difficult to answer! Many different things affect our weather and the real answer at present is that we just don't know. Cool temperatures in the tropical Pacific and wet conditions over India are factors that we might speculate as having an influence.
Is this a sign of climate change?
Predictions of future climate change suggest that during winter we could experience more frequent heavy rainfall and so an increased risk of flooding over central and northern Europe (including the UK). During summer, while most models suggest average rainfall could decrease, there could still be more intense rainfall events. The fact that warmer air can hold a lot more moisture than cooler air means that if in weather systems that bring moisture over the UK, there is the potential for higher peak rainfall rates in warmer world.
Many climate models also suggest that the average tracks of storms that bring rainfall to the UK and Europe could move northward, pushing wet conditions over northern Europe. Exactly what happens in the UK will depend on the precise details of how the storm tracks change and these details are beyond what current climate models can tell us.
Predicting rainfall changes, particularly when we start to zoom in to areas the size of the UK remain a challenge for climate models. However, it is hoped that more research, with better models, will lay the foundations for improved predictions of the changing risks from extreme events.
Notes to editors:
(1) For more information and to arrange interviews with relevant scientists please contact:
Kathy Maskell on 0118 378 7380.
(2) The Walker Institute for Climate System Research at the University of Reading is concerned with understanding our climate, in order to deliver better knowledge of future climate and its impacts for the benefit of society. It is composed of groups from a number of departments across the University. See www.walker-institute.ac.uk.
(3) The Department of Meteorology at the University of Reading is one of the largest and most renowned in the world. With 20 academic staff and over 100 research scientists and students, the department is able to pursue many areas of atmospheric, oceanic and climate research and teaching. See
Facts and Figures:
Typical June rainfall in the lowland areas of the UK is around 50 - 60mm for the whole month with a similar amount expected to fall during July. Amounts vary greatly from year to year and from place to place in any given month.
During June 2007 some regions in NE England and Shropshire the rainfall for June was more than 4 times the average amount, with large areas from Norfolk to Durham, and in the Midlands, having over three times the normal amount. This followed a wet May when a large area of England from the SW to Norfolk received more than twice the normal rainfall
Features of the rainfall on Friday 20th July
Maidenhead. Berkshire was one of the first places to flood on the morning of Friday 20th July. Very heavy rain fell during the morning with more than 50mm falling in an hour. With this amount of rain falling so quickly the drains simply couldn't cope and flash floods occurred in parts of the town, well away from the River Thames. The floodwaters had cleared by late afternoon.
The rainfall was not unprecedented, even for the Thames Valley. The highest hourly rainfall total on record in the British Isles occurred on 12 July 1901 when 92mm fell in Maidenhead. (Data courtesy of the Met Office)
On Friday 20th July, 142.6mm of rain fell in Pershore, Worcestershire, while places in Gloucestershire and Oxfordshire also recorded in excess of 100mm of rain in 24 hours. In these cases the flooding was caused by rivers bursting their banks - the rivers being swelled as a result of heavy rain over a large area.
The top two daily rainfall figures from Friday 20th July 2007:
1) Pershore, College of Agriculture, Worcestershire, 142.6mm
2) Brize Norton, Oxfordshire, 126.2mm, this being the largest daily rainfall recorded at Brize Norton since records began in 1968. The previous daily maximum was 70.9mm recorded in July 1968. (Data courtesy of the Met Office)
The highest daily rainfall ever recorded was 279mm in Martinstown, near Dorchester, Dorset, 18 July 1955. (Data courtesy of the Met Office)