A cumulonimbus (Cb) is the towering thunderstorm cloud, growing up to 12 km high. It bundles lightning, violent updrafts and downdrafts, hail, microbursts and a leading gust front that races ahead of the cloud itself.
For drone pilots the conservative rule is simple: do not fly within roughly 10 km of a Cb. If one is approaching, land now — the dangerous wind arrives well before the rain and the cloud itself do.