Condensation and the Water Cycle Completed

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Condensation and the Water Cycle

The air is full of water vapor, although most of the time we can't see it. Condensation is the process by which water vapor in the air is changed into liquid water; it’s the opposite of evaporation. 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. The arrangement of water molecules changes as water changes between its gaseous (vapor), liquid, and solid forms. 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 become more organized, and heat is released into the atmosphere as a result. 
 

Common examples of condensation

You probably see condensation in your day-to-day life! If you wear glasses and go from a cold, air-conditioned room to outside on a humid day, the lenses fog up as small water droplets coat the surface via condensation. 

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People buy coasters to keep condensed water from dripping off their chilled drink glass onto their coffee tables. Condensation is responsible for ground-level fog, the water covering the inside of a window on a cold day, and for the moisture on the inside of car windows, especially after people have been exhaling moist air. All of these are examples of water leaving the vapor state in the warm air and condensing into liquid as it is cools. 

Condensation in the air

Air contains water molecules. Clouds exist in the atmosphere because of rising air. As air rises and cools, the water in it can "condense out," forming clouds. Since clouds drift over the landscape, they are one of the ways that water moves geographically around the globe in the water cycle. 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 weather conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which combine and grow, developing 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 where more condensation occurs relative to evaporation. As water droplets combine with each other (also known as coalescence), and grow, clouds not only develop, but precipitation may also occur. Precipitation is essentially water in its liquid (rain) or solid (ice) form falling from the base of a cloud. 
 

You might ask why is it colder higher up?

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 some incoming solar radiation (heat) back to space and reflecting some outgoing (terrestrial) radiation back to the Earth's surface. Often at night, clouds act as a "blanket," keeping a portion of the heat near the surface. Changing cloud patterns modify the Earth's energy balance, and, in turn, temperatures on the Earth's surface. 

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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. But do you know why the atmosphere is cooler than air on Earth’s surface? 

Generally, air pressure is the reason. Air has mass (and, because of gravity on Earth, it also has weight) and at sea level, the weight of a column of air pressing down on your head is about 14 ½ pounds (6.6 kilograms) per square inch.  

Barometric pressure (the weight of the air) changes with the density of the air. At higher altitudes, there is less air above, and, thus, less air pressure pressing down, meaning the barometric pressure is lower. Lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense. 

The total heat content of a system is directly related to the amount of matter present, so it is cooler at higher elevation because fewer air molecules exist in a certain volume of air higher up. This means cooler air. 

Even though a cloud weighs tons, it doesn't fall on you because the rising air responsible for its formation keeps the cloud floating in the air. The air below the cloud is denser than the cloud, thus the cloud floats on top of the denser air nearer the land surface. 

Contrails: Human-made clouds

Have you ever seen the cloud-like trails that high-flying airplanes leave behind? They are called contrails. The form of exhaust from the airplane contains water vapor, and if the air is very cold (which it often is at high altitudes), then the water vapor in the exhaust will condense out into what is essentially a cirrus cloud. 

Learn more about contrails. 

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Condensation near the ground

Condensation also occurs at ground level, as this picture of a cloud bank in California shows. 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 encounters 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 of shallow ground fog.