How Do Clouds Form

Ever wonder how do clouds form? It is fun to look for familiar shapes in the passing clouds, but why do they have those shapes? How did they get there?

Water in the Sky
Even though clouds come in a variety of shapes and sizes, they all have one thing in common: water. Without water, there would be no clouds. Clouds form when water evaporates from rivers, streams and oceans. Heat from the sun evaporates surface water. This evaporated water is lighter than the air we breathe, so the water vapor starts to rise into the atmosphere.

Our upper atmosphere is much cooler than the surface of our planet. As the water vapor rises, it begins to cool and condense. The condensation process transforms the evaporated water from a vapor into a liquid. Droplets of cooled water vapor begin to group together into clumps.

Thermal energy that's released when the droplets clump keep them from turning into drops of water that would fall as rain. Instead, the water vapor molecules group together in a very loose structure that we know as clouds. Fog has the same basic structure as a cloud. If you've ever walked through a fog bank, you know that it feels damp but it doesn't really get you wet.

Types of Clouds
There are four basic cloud types. Each cloud varies depending on its height, movement of air around it and how much water vapor it contains.

  • Stratus clouds are low to the ground and layered. They are the clouds we associate with rain
  • Fog, as noted, is a cloud that lies very close to the ground.
  • Cirrus clouds are high and wispy. They contain ice crystals.
  • Cumulus clouds are big and puffy. They are a sign of clear weather.

There are even man-made clouds. The long, thin streaks of white trailing after airplanes are called contrails. They are clouds formed by the condensation of water vapor and fuel used in jet engines. The word contrail is short for condensation trails. Like cirrus clouds, contrails are filled with ice crystals. They disappear rapidly unless there's a high level of water vapor in the atmosphere.

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