Clouds form when water evaporates from rivers, ponds, oceans, and lakes. The air containing this evaporated water vapor rises and expands at higher altitudes where the air pressure is lower. The expanding air cools, and as this cooling occurs, the water vapor condenses (changes) from a vapor to a liquid. Many tiny water droplets form and a cloud is born.
Conditions have to be just right for clouds to form. Nature needs tiny droplets of water, something for the droplets to cling to (particles like dust, salt, or smoke) and the right temperature.
A variety of cloud types form depending upon their height, air movement, and the amount of water vapor and condensation particles.
The basic cloud types include the following:
* Stratus (spread out or layered). These clouds are low to the ground and layered.
* Fog is very close to the ground.
* Stratus clouds may predict rain.
* Cirrus (curly, wispy). These clouds are the highest and are made of ice crystals.
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