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Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation. Condensation is responsible for ground-level fog, for glasses fogging up when one goes from a cold room to the outdoors on a hot, humid day, for the water that drips off the outside of a glass of ice, and for the water on the inside of windows on a cold day.
The phase change that accompanies water as it moves between its vapor, liquid, and solid form is a consequence of differences in the molecular arrangement of water. Water molecules in the vapor form are arranged more randomly than in liquid water. As condensation occurs and liquid water forms from the vapor, the water molecules acquire a specific orientation or structure, which is less random than in vapor, and heat is released into the atmosphere as a result.
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Even though clouds are absent in a crystal clear blue sky, water is still present in the form of water vapor and droplets which are too small to be seen. Depending on meteorological conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which grow and develop into clouds, a form of water we can see. Cloud droplets can vary greatly in size, from 10 microns (millionths of a meter) to 1 millimeter (mm), and even as large as 5 mm. This process occurs higher in the sky where the air is cooler and more condensation occurs relative to evaporation. As water droplets combine (also known as coalescence) with each other, and grow in size, clouds not only develop, but precipitation may also occur. Precipitation is essentially water cloud in its liquid or solid form falling form the base of a cloud. This seems to happen too often during picnics or large groups of people gather at swimming pools.
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The clouds formed by condensation are an intricate and critical component of Earth's environment. Clouds regulate the flow of radiant energy into and out of Earth's climate system. They influence the Earth's climate by reflecting incoming solar radiation back to space and outgoing radiation (terrestrial) from the Earth's surface. Often at night, clouds act as a "blanket," keeping a portion of the day's heat next to the surface. Changing cloud patterns modify the Earth's energy balance, and, in turn, temperatures on the Earth's surface.
Clouds form in the atmosphere because air containing water vapor rises and cools. The key to this process is that air near the Earth's surface is warmed by solar radiation. Generally, air pressure, is the reason why the atmosphere cools above the Earth's surface. Air has mass (and, because of gravity on Earth, weight) and at sea level the weight of a column of air pressing down on one's head is about 14 1/2 pounds (32 kilograms) per square inch. The pressure (weight), called barometric pressure, that results is a consequence of the density of the air above. At higher altitudes, there is less air above, and, thus, less air pressure pressing down. The barometric pressure is lower, and lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense. Since fewer air molecules exist in a certain volume of air, there are fewer molecules colliding with each other, and as a result, there will be less heat produced. This means cooler air.
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Condensation also occurs at ground level. The difference between fog and clouds which form above the Earth's surface is that rising air is not required to form fog. Fog develops when air having a relatively high humidity content (i.e., moist) comes in contact with a colder surface, often the Earth's surface, and cools to the dew point. Additional cooling leads to condensation and the growth of low-level clouds. Fog that develops when warmer air moves over a colder surface is known as advective fog. Another form of fog, known as radiative fog, develops at night when surface temperatures cool. If the air is still, the fog layer does not readily mix with the air above it, which encourages the development in of a shallow ground fog.
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