Thursday night I was up at the Mount Lemmon SkyCenter doing some work aligning a telescope with a colleague and we noticed this beautiful thunderhead in the western sky at sunset. It looked remarkably like a nuclear blast and the colors in the sky painted it beautifully.
Technically, I believe this is what is commonly referred to as an anvil cloud or in meteorological terms a single-cell cumulonimbus incus. (Incus is Latin for anvil). These are very impressive thunderheads that can reach very high into the stratosphere, even to altitudes of 60,000 feet!
The image was taken with my Canon T2i and a 24-105mm f/4L lens operating at 50mm. The exposure was handheld at 1/60 second, ISO 400, f/4.