On Shelf Clouds
On my way home yesterday I was treated to one of the more impressive weather sights I've seen in a while: a shelf cloud stretching across the horizon. I've seen a few of them over the years, but they never fail to catch my attention. Unfortunately I wasn't in a spot where I could safely pull over and get a picture, but the folks over at TN Valley Weather managed to capture a great shot that looked almost exactly like what I saw.
That sent me down the inevitable rabbit hole. I knew what a shelf cloud was in general, but I realized I couldn't confidently explain the difference between a shelf cloud and a wall cloud. As it turns out, they're almost opposites despite being confused with one another all the time.
According to the National Weather Service, a shelf cloud forms along the leading edge of a thunderstorm where cool outflow from the storm forces warm, moist air upward. They tend to be long, wedge-shaped, and are usually a sign that strong winds and heavy rain are just minutes away. Wall clouds, on the other hand, form beneath the rain-free base of a supercell in the storm's inflow region. Rather than signaling outflow, they mark an area of strong updraft and can sometimes precede tornado formation. One of the easiest ways to tell them apart is that shelf clouds move out ahead of the rain, while wall clouds generally remain attached to the storm's updraft region.
If you'd like to learn more, the National Weather Service has an excellent comparison of the two cloud types: Shelf Cloud versus a Wall Cloud .
One of my favorite things about nature is that there's always something new to learn. Sometimes all it takes is looking up on the drive home.
