Cumulus humilis: the fair-weather cloud with a limit

Cumulus humilis is the small, shallow member of the cumulus family. Look for detached heaps, roughly level bases, and clouds that are wider than they are tall. The World Meteorological Organization describes their vertical extent as ranging from tens to hundreds of metres. 1

They form through convection: sun-warmed air rises, expands, cools, and condenses into droplets. Humilis often accompanies fair weather, but the useful clue is whether it stays shallow. If the towers become as tall as they are wide, or keep building upward, the cloud is developing into another cumulus species and showers become more plausible. 2

A good field habit is to check three things in order: the base, the height-to-width ratio, and whether the cloud elements remain detached or merge into a layer.