By Martin Cassidy BBC Northern Ireland rural affairs correspondent

Graham Hewitt stands proudly in front of his wind machine

The six-cylinder diesel engine guns into life, shattering the silence across Graham Hewitt's orchard near Stonebridge.

It is a wind machine he first saw in California seven years ago which he thought might just be able to protect his apple crop back home from frost.

As he travelled around the fruit farms of California, it became clear that most growers were relying on the 11 metre-tall galvanised towers and rotor blades for the air movement needed to prevent frost from forming.

"When we went out there, we were shocked to see the extensive use of these machines in orange groves and vineyards," said Graham.

The investment though was going to be substantial. A wind machine would cost as much as �20,000.

But with frost claiming most of his apple crops in three out of the past five years, Graham was convinced that the Californian farmers had the answer.

Thanks to a rural development grant under the European-funded Leader programme and technical assistance from the Department of Agriculture, he set about installing a wind machine in his most vulnerable orchard.

And barely had the 150-horsepower engine and five-metre blade been installed than the frost alarm sounded.

"On the night of 21 May we had quite a heavy frost in Armagh and in this orchard we got down to almost minus two degrees," said Graham.

It was three o'clock in the morning when Graham fired up the engine, and as the frost glistened on the young fruit, the giant rotor began to turn.

The next few minutes would be critical.

In less than five minutes, the leaves would stiffen on the trees and the apples would crystalise and be ruined.

It was make or break for Graham and his Californian dream.

A layer of warmer air is drawn down to replace the cold air beneath

The wind machine though was gathering pace.

Soon the rotor was turning at more than 2,400rpm.

The tree tops began to sway in the generated breeze, and then the moment Graham had been hoping for arrived - warm air wafting down through the lines of bramley trees.

It is a phenomenon which weather experts call an inversion.

It happens when the layer of warmer air is drawn down to replace the cold air beneath, explains Donald Campbell of the Meterological Office at Aldergrove, near Belfast.

"This machine is capable of drawing huge volumes of air down," says Donald.

"It's bringing warmer air down which can be anything up to ten degrees higher than the ground air temperature."

The wind machine helped to save the apple crop

And on that fateful May morning, the wind machine earned its keep.

Graham smiled as he recalled the temperature probes first stabilising and then beginning to rise.

The apple crop had been saved and all thanks to a machine developed thousands of miles away in the United States.

The fruit hangs heavy now on the apple trees and Graham is looking forward to a bumper crop.

He says the wind machine will only run on a few frosty nights each year when the blossom and young fruit are most vulnerable

But that ability to protect the apple crops may well mean that more Armagh farmers will think about investing in the Californian experience.