Mammatus clouds form on the high, overhanging portion of thunderstorm clouds. They look like they are bubbling downward. Mammatus happen in different thunderstorms at any time of the day but they are most noticeable when the sun is low in the sky because that creates shadows, and often tints the mammatus clouds yellow and orange.
This is exactly what happened on Sunday evening, August 9, 2015. A single thunderstorm, reaching 10 miles into the sky, moved from central Baldwin County, southwestward, through central and southern Mobile County, between 5:30pm and sunset. Given that the day was dry and quiet before than, many people were outside. Based on the height of the storm, it was impossible to not see it from miles away. With most adults now carrying smartphones, with cameras, the storm was documented in detail. Had this happened 3 decades ago, there would have been very few “instant” pictures. Had this happened 60 miles north, where the population is much lower, fewer people would have noticed.
Often, strong thunderstorms grow tall enough that their high clouds spread out when they can’t grow any further. They form a shape known as an anvil. Mammatus form in these high clouds as a turbulent layer of clouds. They are not directly dangerous to people on the ground but you wouldn’t want to fly through them!
Mammatus clouds are not new. The name mammatus comes from the word “mamma”, as in mammary, because that’s what they look like. Here’s more on mammatus from NOAA and a more detailed meteorological description from the University of Illinois.