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The two main types of clouds that form low in the atmosphere, that is, below 6,500 feet, are cumulus and stratus. Stratus clouds develop horizontally, whereas cumulus clouds are formed vertically. Stratus clouds are flat, uniformly gray in appearance, and tend to fill the sky with layers of gloom. White, fluffy cumulus clouds, on the other hand, often float in a bright blue sky and have been called "fair weather clouds." Unlike stratus clouds, cumulus clouds are individual structures. They're flat on the bottom and more puffy toward the top. Given the right conditions in the atmosphere, cumulus clouds may also turn dark with rain, signaling the coming of a thunderstorm.
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