Cloud Coverage

Endlessly variable, continually in motion, covering and uncovering the window into space that is our atmosphere, clouds may have occupied human observation and myth as much as the stars and the sun. We watch them to forecast the weather, we entertain ourselves by interpreting their shapes, we marvel at their ghostliness and beauty. But as much as we think of them in other ways, clouds are simply water either in the form of vapor or ice crystals.

Clouds form as air cools and reaches its saturation point. At the right temperature, the water held as vapor condenses out of the air as tiny droplets of water. What we see as clouds are opaque fields of water droplets. Given that these droplets are very small, the force of wind and air currents exceed the pull of gravity and keep them aloft. Rain and snow happen when these droplets begin coalescing, eventually becoming too heavy to remain suspended in the air.

Clouds are classified by how they form. Those that form as air rises and subsequently cools are called cumulus. These are the puffy clouds that rise like piles of cotton. "Thunderheads" are large cumulus clouds that demonstrate this process dramatically. Those clouds that form as layers of air cool without rising are known as stratus. These clouds are sheet-like and lack the sharp definition that cumulus have. They are the true rain clouds that bring extended periods of precipitation. Cirrus clouds are high altitude masses of ice crystals that appear as wispy brush strokes against the blue background of sky. Cirrus are often referred to as "mares tails." These categories are further divided and modifiers added such as "nimbus", meaning rain cloud, and "alto" meaning high. So, a thunderhead would be called a cumulonimbus cloud and an altostratus refers to high stratus clouds. This picture features a few different types of clouds including altocumulus, cumulonimbus and a little bit of stratus. Can you pick them out?

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