A plan to summon rain out of a cloudless sky by painting the ground black could be tested this year in Israel's Negev desert.

Scientists hope the technique will banish drought and turn deserts green.

But it is unlikely to be of help in south-east England, now experiencing its worst water shortages since 1933.

Large swathes of English countryside would have to be covered in a black thermal material similar to lightweight tarmac.

In a full-scale system, panels of the special radiating material would be laid over several square kilometres of land.

Computer simulations have shown that heated air rising from the panels will cause water vapour to condense into clouds and fall as rain.

Researchers behind the Geshem Project, named after the Hebrew word for rain, believe manipulating air currents in this way can overcome droughts in sub-tropical regions which dry up in the spring and summer.

Project leader Professor Leon Brenig, from the University of Brussels, said: "It will make a huge difference. In a region where there is 150mm [of rain] a year it would go up to 600-700mm a year."

The scientists estimate that crop yields for a given area would be boosted by 40 per cent.

Parts of the world that could benefit most include northeastern Brazil, north Africa, the Kalahari and Sahara deserts and south-eastern Spain.

A first test of the technique, costing around £1.3 million, is planned on a 3,000 square metre (9,300 square feet) area of Israel's Negev desert, making use of water vapour and breezes from the nearby Mediterranean sea.

If sufficient funding can be found, the experiment could take place within the year.

Israeli authorities have already indicated that they will approve the required land area.

"The air above the black surface could be raised by 40-50C above the surrounding temperature, creating a 'chimney' of rising air currents. The artificial thermal will boost water vapour to around 3,000 metres where it can condense into water droplets that create clouds," said Prof Brenig.

The black material absorbs energy from the sun and then radiates it back into the atmosphere.

Covering between five to nine square kilometres with the material is expected to make it rain on an area of 40 to 100 square kilometres (25 to 62 square miles) downwind.

Clouds will form in the afternoon along a strip as wide as the black surface and up to 30 kilometres (18.6 miles) long.

The technique can be applied to any subtropical dry region within 150 kilometres (93 miles) of an ocean, sea or large lake.

Setting up a full-size black surface would cost about 80 million euro (£54 million), about the same as building a desalination plant.

But the Israeli scientists point out it would be environ-mentally friendly and cost little to run.

The idea of using a large solar-heated surface to make rain has been around since the 1960s. However at that time the only suitable black material was asphalt, and the computing power needed to test the theory was still decades away.

"It is hard to simulate because meteorological predictions are often not very good," said Prof Brenig. "We need very accurate predictions, such as how much rain will be produced, to evaluate the efficiency of the system."


* Sub-tropical regions are vulnerable to summer drought due to an atmospheric phenomenon known as the Hadley Cells

* First described by the 18th-century meteorologist George Hadley, they are convection cycles of air affecting the weather in tropical and sub-tropical climate zones

* Hot equatorial sun heats air in the lower atmosphere, causing it to rise and flow away from the equator to the north and south

* The air thermals carry water vapour into the atmosphere, forming clouds that produce rain. However, as the air currents reach sub-tropical regions, they cool and descend, preventing condensation high enough to generate clouds

* The object of the scheme, according to Prof Brenig, is to locally fight against that descent