Portfolio managers taking off for the beach this August probably won't be able to leave the market behind as easily as they have in the past.

Traders know that the height of summer is often the nadir of stock market performance, with both the Dow Jones Industrial Average and S&P 500 posting their weakest monthly performances in August over the last 15 years. According to Ned Davis Research, a Venice, Fla.-based trend monitor, the Dow has fallen 1.6%, while the S&P has shed 1.4%, on average, during the last 15 years in August -- more than in any other month. The worst month on the Nasdaq Composite is October, followed by July and August.

The same is true of volume. Since 1929, NYSE trading volume in August has dropped 6.7%, on average, the biggest drop of any month.

"When volume dries up, there are fewer buyers," said Jeff Hirsch, president of the Hirsch Organization, which publishes the Stock Trader's Almanac. "As a result, there are lower prices."

But this year, August is shaping up as a watershed, with the Dow off 7.9%, the S&P down 8.4% and the Nasdaq down 7.7% after only three trading days. On Monday, IBM ( IBM) and Applied Materials ( AMAT), both market bellwethers, hit 52-week lows.

While trading volume has been moderate, it hasn't yet seen the full dissipation of prior years. Instead, with concerns mounting about another plunge into recession, traders have been forced to stick around and trade.

Indeed, if anything can keep volume high in August, it's volatility. That's exactly what the market is providing.

"We're getting great volatility at the beginning of the month," said John Bollinger, a technical analyst and head of Bollinger Capital Management. "Bottoming processes are typically volatile. We should expect it to continue, as we are in for several months of base-building."

Consider that August's selloff follows a 1,000-point rally to close July. "We have to go back down and test the July low," said Ed Nicoski, a technical analyst at U.S. Bancorp Piper Jaffray. "In the past, all major climactic lows were tested a month or two later."

The rally, of course, was preceded by one of the great selloffs of the last two decades, with 1,200 stocks hitting new 52-week lows on July 24.

"That was a momentum low," said Bollinger. "In August we will likely find out whether or not it will be sustained."

Some market analysts fear that if August is not bad, September and October could be worse. "The weakness may be of the water torture type," said Jeff deGraaf, a technical analyst at Lehman Brothers. "But the risk is August is mediocre to a fault, and you get an ugly September and October."

By then, of course, it will be too late for anyone to take a summer vacation.