About 35 million Americans suffer from stuffy noses and other symptoms associated with allergies, and about 20 million of us will experience an allergy-trigged asthma attack at some point. In general, rain tends to wipe the air clean of pollen, reducing allergy symptoms. But not always.

Researchers have found that asthma attacks may actually increase after the passage of some thunderstorms. This occurs especially when the thunderstorms are preceded by what meteorologists call a gust front - that's the rush of cool wind that you sometimes feel minutes before the rain arrives. Here's why the gust front makes a difference.

When it rains, grass pollen grains tend to swell with water and then rupture, releasing their allergens. Typically, these would be washed out by the rain, but if there's a gust front, some of them can blow ahead of the rain, where high concentrations can develop and produce allergy-triggered asthma attacks. It's in cases like these that rain might not always mean relief.