The tragedy at the Indiana State Fair this weekend, in which at least five persons were killed, was caused by a gust front. As many of us have surely noticed, the strongest winds from an advancing line of thunderstorms often arrive before the rain and lightening; caught off-guard, those killed or injured in Indiana may have been waiting for the rain before seeking shelter.
Thunderstorms generally form just ahead of an approaching cold front, drawing in the warm, moist air that lies in their path. Swept into the upper atmosphere, this cargo of moisture precipitates and plunges to the ground as heavy rain within the storm. Rain-cooled air accompanies this rainfall and, upon striking the ground, is forced outward in all directions; in advance of the storm, these high velocity winds produce a gust front, often reaching 60-70 mph, which arrives before the storm itself. A shelf cloud signals the presence of this rapidly moving, cold air, as warm, humid air is displaced above it, creating the cloud.
Since gust front winds move out in all directions, they may impact adjacent or trailing storms, depriving them of warm, moist inflow and causing them to dissipate. But it is the leading gust front that is of major concern to humans, plowing down trees, barns and, as we saw in Indiana, outdoor structures.
