Joe Bollinger sent in this question via the WeatherBlog: "What is the prevailing wind direction for this region, and from which direction does it blow the least?" Interesting question. I managed to track down a "wind rose" valid for Baltimore during the "ozone season" - from April through October. It shows the wind blowing most frequently (and hardest) from the west-northwest, and least often from the north. But on days when ozone levels violate federal standards, they most often come from the west or southwest.