When the Commerce Department released trade figures last week, they contained a pleasant surprise: exports rose in August for the fourth straight month.

For a country with a perennial trade deficit—in 2008 it was $673.3 billion—this is unalloyed good news. And one reason for the rosy export numbers is the weakening dollar. Against a basket of major currencies, the dollar is down 14 percent since its March highs and last week touched a 14-month low against the euro, its main rival.

It's the positive side to a story that is often viewed with alarm. The dollar has fallen since the Bush administration because interest rates were pushed to near zero by the Federal Reserve in its efforts to ease the financial crisis. As a result, foreign investors have been selling dollars and buying foreign currencies like the euro and Swiss franc, which has driven the value of the dollar down dramatically. Investors are concerned that huge deficits will cause inflation in the United States, which undermines the purchasing power of the currency.

For the time being, the cheaper dollar has been a boon to many domestic businesses.

“If you are an exporting business and do not rely overwhelmingly on imported materials, then a depreciating dollar helps your competitiveness,” said Nigel Gault, chief U.S. economist for IHS Global Insight, a Lexington, Massachusetts-based research firm. “It affects all business in two ways: It means you can either cut your prices in foreign currency or you can increase your price in dollars without increasing your foreign currency price. Either way, you’re more competitive.”

Although it is probably too soon to give the weakening dollar all the credit for the improved export numbers, businesses are already cheering.

“I have seen more interest from foreign customers who were sitting on the fence or waiting to pull the trigger for a sale,’” said David Ickert, chief financial officer for Air Tractor, an Olney, Texas-based manufacturer of airplanes for agriculture. The company, which had $40 million in sales last year, exports about 40 percent of its production.

“In markets where we have competition, the weakening dollar makes us more competitive,” Ickert says. “All of our inputs are U.S, dollar denominated, so basically a weak dollar is good for us in our export markets.”

The weaker dollar affects small businesses such as Air Tractor in different ways than big multinationals, says Frank Vargo, vice president for international economic affairs at the National Association of Manufacturers in Washington, D.C.

“Small businesses tend to be hurt more when the dollar is rising and tend to be helped more when the dollar is falling,” Vargo says. ‘The reason for that is larger exporters tend to have production operations around the world and a lot of their exports are intracompany trade.”