Once a concern mainly for farmers, the weather has now become a crucial factor for businesses because of increased numbers of hurricanes and storms and the rigors of stock delivery that must arrive exactly on time to meet orders.

"We're talking trillions of dollars that are directly or indirectly affected by weather," said Walter Lyons, the outgoing president of the American Meteorological Society, which holds its 86th annual convention in Atlanta this week.

Lyons cited statistics showing nearly a third of the $10-trillion-a-year US economy is dependent to some degree on the weather.

That impact is not lost on the world's largest businesses.

Wal-Mart Stores Inc., the world's biggest corporation, maintains a weather and emergency response center 24 hours a day to alert its stores, executives and overseas procurement facilities on weather developments ranging from thunderstorms to broader climatic trends.

After Hurricane Katrina devastated the city of New Orleans last August, Wal-Mart's 4,000-square foot (372-sq metre) emergency center buzzed with 200 employees dealing with the aftermath of the hurricane, a company official said.

Daily weather updates are available to individual store managers while longer term forecasts are used by regional managers to determine inventory mix and energy purchases.

"Weather is the single greatest (variable) that we have," Jason Jackson, Wal-Mart's head of emergency management, said at the American Meteorological Society meeting.

Home Depot Inc., the world's largest home improvement retailer, uses a similar if less elaborate weather tracking system which has been honed over the past two years of heightened Atlantic hurricane activity.

Utility companies and energy producers rely on a constant stream of data from government and private weather forecasters to determine whether to power up costlier generating plants or buy and sell fuel on a computerized world market susceptible to exaggeration and vulnerable to forecasting errors.

Patrick Walshe, who manages short-term power supply planning for the Tennessee Valley Authority and its 8.3 million customers, said that increasing the accuracy of short-term forecasts can save millions in costs.

A three-degree Fahrenheit difference between forecasted and actual temperature, for example, could result in a 1,350-megawatt difference in demand.

On hot days, that demand must be met by the use of older, more expensive power plants, which if used unnecessarily boost supply costs by $600,000 a day, Walshe said. "You can see how that begins to add up very quickly."