Image caption Wind turbines can affect local weather, the study confirms - but not further afield

Wind farms can affect weather in their immediate locality, raising night-time temperatures on the ground, researchers working in Texas have shown.

They used satellite data to show that land around newly constructed wind farms warmed more than next-door areas.

The result - published in the journal Nature Climate Change - confirms an earlier, smaller study from 2010.

The scientists believe the effect is caused by turbines bringing relatively warm air down to ground level.

They suggest that turbines in other places might not produce the same value of ground temperature change.

The study area, in west-central Texas, saw a major turbine building programme in the middle of the last decade, with the number soaring from 111 in 2003 to 2325 just six years later.

Researchers used data from the Modis instruments on Nasa's Aqua and Terra satellites to measure ground temperatures across the study region and between the beginning and end of the construction boom, defined as as the difference between the average for 2003-5 and that for 2009-11.

The entire region saw a rise, but it was more pronounced around wind farms.

The researchers looked for other factors that could have affected the results, such as changes in vegetation, but found these were too small to produce the observed change.

Limited reach

The change was not identical across all of the wind farms. Having averaged the data, the researchers say the scale of the effect they saw is equivalent to a warming of about 0.72C per decade.

Recognising that this could wrongly be interpreted as suggesting the local temperature will continue to rise, lead researcher Liming Zhou cautioned: "The estimated warming trend only applies to the study region and to the study period, and thus should not be extrapolated linearly into other regions or over longer periods.

Image caption Could the local weather changes affect farming?

"For a given wind farm, the warming effect would likely reach a limit rather than continue to increase if no new wind turbines are added."

At night, air above ground level tends to be warmer than the ground. Dr Zhou and his colleagues believe the turbine blades are simply stirring up the air, mixing warm and cold, and bringing some of the warmth down to ground level.

"The result in the paper looks pretty solid to me," commented Prof Steven Sherwood from the Climate Change Research Centre at the University of New South Wales in Australia.

"Daytime temperatures do not appear to be affected. This makes sense, (and) this same strategy is commonly used by fruit growers who fly helicopters over their orchards to combat early morning frosts."

The 2010 study, also from the US, used data from a single location and computer modelling to show that wind turbines could produce local warming.

Dr Zhou, from the State University of New York in Albany, US, now plans to look across bigger scales and to decipher the mechanisms better.

"This article is a first step in exploring the potential of using satellite data to quantify the possible impacts of big wind farms on weather and climate," he told BBC News.

"We are now expanding this approach to other wind farms, and building models to understand the physical processes and mechanisms driving the interactions of wind turbines and the atmospheric boundary layer near the surface."

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