Why does it get so windy before summer thunderstorms?
—Margaret Robertson, Chicago
It’s a phenomenon that we have all experienced: The cooling winds immediately ahead of the onset of heavy rain in a summer thunderstorm. Raindrops vary in size, but the largest drops are no larger than about 0.2 inch in diameter (greater than that, the raindrops shatter into smaller drops because of air resistance.) In a t-storm, the largest raindrops usually fall at the onset of the storm, and falling raindrops (especially large ones) drag down some of the air through which they are falling. Raindrops hit the ground and stop, but air coming down with the rain cannot do this and so the air pushes out ahead of the rain, producing the gusty “outflow” that we experience at the storm’s outset.