This is the leading edge, or "gust front" of a squall line rural Kansas, May, 2006. A squall line is a strong line of thunderstorms; they're somewhat common on the plains of America during the spring. I am not sure how the rain ended up purple in this shot; I am guessing it had to do with a mixture of the warm light of the setting sun and the typical green cast that clouds take on when they're producing hail. The flickers of lightning may have had something to do with it, too.

In the Storm Chaser photo essay.