Special Report

The Rarest Types of Weather on Earth

Source: Eazydp / Wikimedia Commons

21. Tubular clouds

Tubular clouds, a type of arcus cloud also known as roll cloud, form low and horizontally in the sky. They tend to form along the edges or in the downdrafts of thunderstorms. While relatively rare and ominous-looking, these clouds are not usually a sign of dangerous weather.