The whale’s mouth forms as a result of very strong ‘downdraughts’ of cool air, which are associated with such severe storm systems.
On reaching the ground, this cool air spreads out like a tide beneath the advancing storm. As it does so, it lifts the warmer, lighter air at ground-level, which then cools enough for its moisture to condense into cloud.
This cloud is the roof of the whale’s gaping mouth, while the blubbery-looking rim along the front of the spreading air is the whale’s top lip*.
You’ll know pretty quickly why this thunderstorm formation is called a “whale’s mouth.” Take a look at this mesmerizing timelapse of a whale’s mouth formation in Selman, Oklahoma.