Embattled farmers in the Sahel countries of West Africa can take heart from a new study that should boost the accuracy of rainfall prediction in one of the world's most fragile regions.

Sharp differences in moisture in small patches of land can trigger precious rain, says the paper, published online on Sunday in the journal Nature Geoscience.

Researchers from Britain, France and Australia looked at satellite data that located nearly 4,000 rainstorms which occurred in the Sahel between 2006-2010.

Between 80 and 90 percent of rainfall in the Sahel comes from this kind of storm, which can brew suddenly when moisture-laden air lifts from heated earth.

The scientists then overlaid this data with satellite information on soil moisture.

They found that an area where there are large differences in soil moisture plays a big role in making rain.

A moist area just 10 to 40 kilometres (six to 25 miles) across can trigger rain provided it is next to a far drier patch.

This small-is-beautiful finding contrasts with conventional weather models.

These tend to calculate the probability of rainfall on the basis of huge swathes of moist land and on the presence of rain-making features like mountain ranges.

"Rainfall is difficult to predict, particularly in regions such as the Sahel where huge storms can grow from nothing in a matter of hours," said lead author Chris Taylor of Britain's Centre for Ecology and Hydrology.

"We found that areas with contrasting soil moisture can play an important role in the creation of new storms, a factor not accounted for in current climate models.

"(...) This effect is important for typically one in eight storms, in a region particularly prone to droughts and associated crop failures."

In areas where there are these sharp differences in soil moisture, rainstorms are twice as likely compared to regions where the moisture level is uniform.

The rain often falls around 10 kilometres (six miles) upwind of the moist patch, not downwind, Taylor explained to AFP.

This occurs when the moist air is driven against weak prevailing winds by a strong local breeze coming from the opposite direction.

The study could help fine-tune knowledge about how climate change could affect the Sahel, he said.

Semi-arid, tropical west Africa is one of the most demanding regions in the world for agriculture, given its fragile soils and a short growing season that is crucially dependent on when and where rain will land.

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