Puffy white clouds that drift across the sky on a bright summer day, like those on the right, are called cumulus. When they are small and scattered, they are a sign of fine weather. Cumulus means "heaped," and the clouds form in columns of rising air above ground that is being heated strongly by the sun or in air that is made to rise rapidly. As the air rises it cools, and its water vapor condenses onto particles of matter called condensation nuclei. The latent heat of condensation warms the air around the droplets, so the air rises further and more water vapor condenses. Air around the clouds descends to replace the rising air. As this air descends, it warms, and water droplets on the fringes of the clouds may evaporate into it. This limits the horizontal growth of the clouds.
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