Condensation

Condensation is the process in which a gas or vapor is changes into a liquid (the opposite of evaporation). It commonly occurs when a vapor is cooled and/or compressed to its saturation limit (the molecular density in the gas reaches its maximal threshold). Vapor cooling and compressing equipment that collects condensed liquids is called "condenser".

The condensation of water is responsible for the formation of fog or clouds, and the water drops that form on the outside of a glass of iced tea or on the inside of your home windows on a cold day.

Dynamics
Both condensation and evaporation are happening all the time; atmospheric conditions determine which of them dominates. When the vapor is condensing, the particles on which the vapor condenses will grow bigger, in the same way when vapor evaporates from a particle, it will shrink.

Clouds form in the atmosphere because air containing water vapor rises and cools. This happens because the air near the Earth's surface is warmed by solar radiation due to higher air pressure. Air pressure is due to the fact that air has mass and therefore weight. At sea level the weight of the air is about 14.5 pounds (6.6 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 and the air is cooler.

Water Cycle

 * Main Article: Water Cycle

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.

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 (condense) and become the liquid form of water that 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 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.