Cumulonimbus clouds are thunderstorm clouds. They develop from convection — the transport of heat and moisture upward into the atmosphere. But, whereas other clouds form when air currents rise several thousand feet and then condense where those currents stop, the convective air currents that create cumulonimbus are so powerful, their air rises tens of thousands of feet, condensing rapidly, and often while still journeying upward. The result is a cloud tower with bulging upper portions (that look something like cauliflower).
If you see a cumulonimbus, you can be sure there’s a nearby threat of severe weather, including bursts of rainfall, hail, and possibly even tornadoes. Generally, the taller the cumulonimbus cloud, the more severe the storm will be.
Source: Means, Tiffany. “Clouds That Spell Severe Weather.” ThoughtCo, Aug. 1, 2021, thoughtco.com/clouds-that-spell-severe-weather-4089934.