Clouds are essentially large masses of crystallized water molecules and tiny water droplets. When water is heated, it evaporates into the air as vapor. If the air cools down fast enough to its saturation point, that water vapor can condense into droplets or crystals and appear as a cloud.
The condensation of water vapor into droplets and the formation of clouds are influenced by many different conditions, including temperature, humidity and altitude. Water vapor can also condense around particles called nucleators. These particles, like dust or soot from a wildfire or even certain types of plant bacteria, can all act as a focal point for condensation and the formation of clouds. Given the proper weather conditions, clouds can carry dust for very long distances – even across oceans. Dust picked up from storms in Africa can be carried across the Atlantic and deposited in North America. And the West Coast has been known to receive dust from as far as China [source: Discover]. So next time you look up and see a cloud, there might be more than just a bit of rain waiting inside.
Clouds affect the Earth's temperature in two ways. During the day, clouds reflect about 20 percent of the sun's heat back into space [source: Tarbuck]. They also absorb, along with water vapor and other atmospheric gases, another 20 percent of solar radiation. A great amount of heat is reflected by low-level clouds. This is why cloudy days bring cooler temperatures. At night, though, a cloudy sky usually indicates slightly warmer temperatures because high-level clouds can create a blanketing effect. These clouds absorb some of the heat released by the ground as it cools in the evening and reflect it back toward the Earth's surface.
Since water vapor traps heat rather efficiently, scientists are studying the effects of climate change on clouds. Higher surface temperatures could equate to more water vapor in the atmosphere, but our understanding of how clouds regulate and react to temperatures is still under development [source: ScienceDaily].
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