Cumulus

A puffy, white, fair-weather cloud with a flat base and rounded tops, formed by convective updrafts on warm days. Cumulus clouds indicate rising air and instability in the lower atmosphere. While small cumulus (cumulus humilis) are generally harmless and common during pleasant flying conditions, towering cumulus (TCU) indicate significant vertical development and can rapidly grow into cumulonimbus thunderstorm clouds if conditions are favorable.