A CHANGE in the jet stream has turned Britain into a divided nation, with the North drenched in heavy storms while the South suffers from a desperate shortage of water.

Parts of the northwest have had rainfall equivalent to 140 per cent of what is usual between November and May, leading to flooding. Meanwhile, many southern areas have had seven successive months with below-average rain, the lowest recorded since the drought of 1975-76. This summer might see a repeat of the record heatwave of August 2003.

Meteorologists have blamed changes in the jet stream — a river of wind that flows close to the stratosphere and helps to drag depressions across the Atlantic — for the unusual divide.

In winter the jet stream tears across the Atlantic usually at furious speeds along a fairly direct track from west to east, helping to dump heavy rain on the UK and much of Western Europe. But in the past winter and spring a large block of high pressure has sat in the Atlantic instead of over Europe, and, like a boulder in a river, sent the jet stream on a detour north to sweep over northern Britain. This delivered waves of Atlantic depressions loaded with rain to deluge Scotland and northern England. Scotland had its fifth wettest November-to-May period on record.

As the jet stream eventually forced the depressions further south, their rains became exhausted, leaving the south unusually dry. As the jet stream dived even further south, the depressions were so dry they created the worst drought on record in Spain and Portugal.

Water shortages are now so acute in much of the Iberian Peninsula that water rationing has begun in some areas, and is likely to escalate through the summer.

Nobody knows why the high pressure developed in the Atlantic. It might be a host of different factors — the unusually cold and snowy conditions in the western US, storms over the Arctic, possibly even conditions in the Pacific. But these are all mysteries that meteorology cannot answer.

Terry Marsh, hydrologist at the Centre for Ecology and Hydrology at Wallingford, Oxfordshire, sees plenty of other dry winters in history. “In olden days in the 1800s we had clusters of dry winters, but this doesn’t happen these days, otherwise it would be a serious problem.” But what is clear is that as climate change takes hold, the winters are growing wetter and the summers drier, although this winter bucked the seasonal trend. Climate change is bringing heavier and more violent downpours of rain, and last weekend’s thunderstorms fitted that pattern.

Although flash floods like those at Helmsley are nothing new, we can expect more of these disasters as the climate warms up.

“The general pattern of rainfall is for heavier rains,” said David Viner of the Climate Research Unit at the University of East Anglia. “With a warmer climate, thunderstorms have more energy, so they are becoming more vigorous, with heavier rain.”

What will the rest of the summer bring? Long-range forecasts point to more of the same — drier than usual in the South, more rain in the North.

But something much more extreme could be on the cards for southern England, and a crucial warning sign lies in France, Spain and Portugal.

Temperatures there are roasting and the ground is drying out rapidly — a key ingredient for cooking up severe heatwaves because dry land heats up more than wet ground.

Add plenty of hot sunshine under a long bout of high pressure, and the ground could turn into a furnace and send temperatures soaring.

If any of that hot air from Europe drifts over the English Channel, it will reheat on the dry ground in southern England, and we might possibly see a repeat of the record-breaking heatwave of August 2003.

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