Metro Atlanta home prices fared poorly compared to the national average from July to August, the latest Standard & Poor's Case-Shiller index of home prices shows.

Nationally, the index ticked up by .2 percent between July and August in the 20 cities tracked. But Atlanta was the chief laggard, showing a 2.4 percent drop.

Metro Atlanta prices were 6.3 percent lower than they were in August 2010, while the national average was down 3.8 percent. Only Minneapolis, Portland and Phoenix had higher year-over-year drops in value than Atlanta.

Jim Grissett, a real estate investor and professor at Emory University, said many forces move the market now, including falling prices and inventories, government refinance programs, interest rates and a large move into the home market by investors. So focusing on monthly gains or losses in prices can be misleading, he said. It is more helpful to look at the year-long movement of prices.

Among the 20 cities, Atlanta is in the lower third based on home price losses.

"That is not good news, but it is not as bad as it might appear when you look at that negative 2.4 percent [monthly] drop," he said.

There are some positive trends in metro Atlanta, Grissett said.

"If you look at supply, price and demand, I am much more interested in what is going on with demand," Grissett said.

"We know we have excess inventory," he said, but the sooner buyers soak up that inventory, the sooner prices will stabilize.

Metro Atlanta home closings rose through the summer, according to SmartNumbers, a Kennesaw real-estate tracking firm. There were 4,138 closings in September, up nearly 25 percent  from September last year and the fourth straight month that home sales increased at a double digit pace.

However, the average sales price was $162,491, 11 percent lower than the price in September of 2010, the firm said. That average price is about the same as in 1997.