At the dramatic conclusion of the season's first thunderstorm, the base of a passing cloud takes on mountainous topography. Meteorologists use the term mammatus to describe these distinctive formations. Warm, moist air in the thundercloud rises in a typical convective updraft. It strikes a layer of cooler, dry air in the atmosphere above and spreads outward to produce an anvil-shaped cloud. Ice crystals fall to the bottom of the cloud where they sublimate, changing state directly from ice to water vapor. As this cool air sinks in pockets across the base of the cloud, localized, reverse convection currents are set up—the puffballs that give mammatus clouds their texture.

Thankfully, those words—"Pull over, ma'am"—are not the reason I find myself on the shoulder of Interstate 35W at 7:00a.m. Nor is my action the result of car trouble. A different voice has caused me to stop the car on this Sunday morning-quiet stretch of highway. I have heard the call of beautiful scenery.

I resisted as long as I could. I’ve been driving through a pervasive layer of ghostly ground fog, which hovers like thin smoke over farm