From giant puffs billowing toward the sun to thin wisps slinking past the moon, clouds come in countless shapes and sizes. This makes them wonderful fodder for the imagination—at some point, hasn’t everyone looked heavenward and seen a fire-breathing dragon floating by? Aside from sparking creativity, clouds serve several important scientific purposes, from helping meteorologists predict the weather to facilitating life on Earth.

Clouds generally form within the troposphere, or the layer of atmosphere closest to the earth. As they rise and fall, they may appear in infinite variations. To create some order scientists have established three broad categories into which most clouds can be grouped.

At the upper reaches of the troposphere you’ll find high clouds, which, depending on geographic