Stratus clouds are one of the two types of low-altitude clouds. The name is derived from "strato," meaning layers. Stratus clouds develop horizontally, appear flat and gray from below and produce drizzle or light rain, according to the National Weather Service. A subtype are stratocumulus clouds, which most commonly move in before or after a frontal system that brings a storm. Nimbostratus clouds are the last type and are fat, dense clouds that deliver ongoing rain and snowstorms.
Cumulus -- from "cumulo," meaning heap -- clouds also appear flat from below, but they're tall clouds that start low in altitude and build upward. The tallest of these are cumulus congestus, also knows as towering cumulus. If the air gets unsteady, these clouds transform into cumulonimbus clouds, which deliver thunderstorms and heavy, ongoing snow and rain.
Low-altitude clouds also play a role in keeping the Earth cool, according to Amy Clement, a climate scientist at the University of Miami. Low clouds reflect so much of the Earth's heat out into the atmosphere that Clement says they're "like the mirrors of the climate system." In a study Clement published in "Science" magazine, she found that warmer oceans dissipate low-altitude cloud cover which, in turn, starts a vicious cycle of warming because fewer low-altitude clouds increases the Earth's temperature even more.