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Cumulus clouds are the stereotypical little white fluffy clouds which appear in children's drawings the world over. They are characterized by being extremely dense, with flat bases and puffy tops, along with a very clearly defined outline. These clouds can form in response to a wide variety of weather conditions, being associated with both fair weather and storms, and they are sometimes precursors to other types of clouds.
These clouds demonstrate a trait known as vertical development. Cumulus clouds form when a column of air rises up into the atmosphere and reaches an area cool enough for the water in the air to condense, forming a cloud. This is why the base of cumulus clouds tends to be flat, because the water vapor won't condense below a certain height, and why the tops are so fluffy, because they are formed as the water vapor drifts up into the atmosphere.
Often, cumulus clouds are associated with fair weather, appearing like cottonballs in the sky on an otherwise clear day. In other cases, they are linked with thunder and rainstorms. For example, cumulus can develop into cumulonimbus clouds, which are closely associated with heavy weather. Many people enjoy looking at the shapes of these distinctive clouds, as it is easy to imagine fanciful images appearing in the clouds as they drift across the sky.
There are a number of different types of cumulus clouds. Some notable examples include cumulus castellanus, which forms huge projecting towers of cloud, and cumulus fractus, which appears as ragged strands strung across the sky. Cumulus pileus, another type of cumulus, appears in the form of a small cap of cloud which floats over a larger cumulus cloud, while cumulis humilis tends to be extremely wide, being associated with generally good weather.
Cumulus clouds often appear quite low in the sky, but they can potentially stretch quite high. They may appear with other clouds, depending on weather conditions. Stratus clouds are also known for forming low in the atmosphere, while higher up it is possible to see cirrus, nimbostratus, and altostratus clouds, among others. Learning to identify the different cloud types can be interesting, and also useful, as some clouds can be warning signs of impending poor weather.