A cloud is water drops or ice crystals floating in the sky.
The sky is full of water vapor. Water is always present in the air. The surface of the Earth is more than 70 percent water. When the Sun heats the Earth, water evaporates and turns into water vapor, and floats up in the sky. The heat from the Sun evaporates trillions of gallons of water into the air every day.
Water vapor is invisible water. Water vapor is a gas, not a liquid (water) or a solid (ice). Warm vapor rises with warm air. Also floating in the air are tiny particles such as dust, soot, or tiny bits of sea salt. These are called aerosols.
As warm water vapor rises, it bumps into aerosols. Warm water vapor cools as it rises because the air is increasingly cool the higher it is in the atmosphere. As water vapor rises and cools it sticks to aerosols floating in the air. Attached to the aerosols the water vapor turns back into water droplets. Water droplets attached to aerosols increase in size as more water vapor condenses and clings to aerosols and other water droplets.
When water droplets stick together, they eventually form clouds. This happens when the water vapor cools so much that it turns from a gas back into a liquid. (You can actually see this process if you watch hot water vapor escaping from a boiling tea kettle; a small “cloud” forms in your kitchen above the tea kettle.)
Condensed droplets of water float through the sky attached to aerosols and as they collect and grow larger and larger forming bigger water droplets, they eventually become so heavy that they fall back to the Earth as rain. If the droplets are lifted high enough into the air, where the temperature is below freezing, the moisture forms ice crystals and when ice crystals fall into lower warm air, they become snow crystals and join together to form snowflakes. When rain or snow falls back to Earth, the process of evaporation, condensation, and precipitation repeats. This is called the water cycle.
Some clouds are high up in the sky. Clouds high in the sky are called Cirrus clouds; they look like floating feathers. Some clouds are in the middle of the sky. Mid-sky clouds are called Cumulus clouds; they look like giant cotton balls. Some clouds are low in the sky; they are called Stratus clouds. They look like bedsheets over the earth. Some clouds touch the ground. These clouds are called fog.
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