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All About Clouds Clouds • For a cloud to form, there must be water vapor in the air. • humidity- the measure of the amount of water vapor in the air. • relative humidity- the percentage of the amount of water vapor in the air compared to the maximum amount of water the air can hold. Measured with a psychrometer. precipitation Forms of precipitation: a) rain- drops of water that are at least .5 mm in diameter (mist or drizzle if smaller) b) sleet- drops of water that freeze as they fall from the sky; sleet-ice particles that are less than 5mm in diameter c) freezing rain- don’t freeze as they fall but freeze as they touch something cold such as tree branches or power lines. more forms of precipitation d) hail- round pellets of ice larger than 5 mm in diameter that form in cumulonimbus clouds; sent up and down in the cloud, freezing a new layer of ice around the hailstone. e) snow- water vapor in a cloud is converted directly into ice crystals What is a cloud? • A cloud is a large collection of very tiny droplets of water or ice crystals. • The droplets are so small and light that they can float in the air. Why do clouds look white? • Clouds reflect sunlight, making them white. • Clouds reflect all the colors of the rainbow the same amount, making them appear a white color. Why do some clouds appear gray? • When the clouds get really thick or really high in the sky, all the light can’t travel through the cloud. This makes the cloud look gray instead of white. • The shadows of other clouds can make some clouds appear gray also. How are clouds formed? Clouds form when: 1. warm air rises, expands and cools. 2. some of the vapor condenses onto tiny pieces of dust because cool air can’t hold as much water as warm air. 3. A tiny droplet forms around each dust particle. 4. They become a visible cloud when billions of these droplets come together. Various Cloud Types • Stratus clouds- low level clouds that are thick enough to block the sun • Cumulus clouds- low to midlevel clouds that are puffy and white • Cirrus clouds- thin, whispy clouds that are high in the troposphere • Nimbus clouds- bring rain; dark colored Click here to see a video about cloud formation • Click to view animation of precipitation over mountains: • http://bcs.whfreeman.com/thelifewire/conte nt/chp56/5602001.html Click here to see a video detailing the various cloud types!