Clouds: One way of classifying types of clouds is by the way in which they are formed. Two categories are cumulus and stratus. Cumulus clouds are formed when areas of moist air are warmed and rise quickly; cumulus clouds are puffy or billowy and resemble piles of cotton. Stratus clouds are formed when warm moist air rises over cold air; stratus clouds resemble a blanket spread out across the sky. Which of the following is the main idea of the paragraph?