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What Is Condensation?
To put it simply, the process of condensation occurs when gaseous water vapor turns to liquid. Condensation is the result when warm air containing tiny molecules of water vapor has been cooled. As warm air rises within the atmosphere, cool air falls, and the cooled water molecules condense. The condensed water molecules form liquid water droplets. This is why your cold glass of iced tea appears to sweat on a hot summer day. The hot air immediately around the glass is cooled on contact, and the water in its liquid state builds around the outside of the glass.
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Examples of Condensation
Water doesn't only condense on cold beverage glasses. Think about the grass beneath your feet. On warm summer mornings in humid parts of the country, the grass is wet even when no rain has fallen the night before. Dew is just another name for this particular type of water condensation. You'll also notice fog on warm mornings following an evening thunderstorm. Although fog isn't attached to a glass or a blade of grass, it is still a type of condensation. Fog occurs close to the ground when the atmosphere reaches its dew point.
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The Dew Point
The dew point is the level at which the atmosphere is completely saturated with water, or 100% relative humidity. As the name suggests, this is the point of saturation at which you see dew, fog, or other types of atmospheric condensation. Think about the clouds in the sky. The higher the elevation, the cooler the air temperature becomes. When the cool air condenses, there is less space for the gaseous water vapor molecules. The cooled water vapor molecules condense into liquid molecules high over the Earth's surface in the form of clouds.
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Condensation and the Water Cycle
The water cycle is essentially the path that water takes as it moves from the rivers, up to the clouds and back down to the Earth as rain. Water is always evaporating from streams, lakes and the ocean. As it rises into the atmosphere, it cools and condenses in the form of clouds. Condensed water molecules can travel for miles as clouds before the atmosphere becomes so saturated with water droplets that the water falls back down to the Earth as rain or snow.
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Why It Matters
Condensation and the water cycle affect our weather, the location of the Earth's deserts and the amount of rain that falls in our rivers and lakes. By default, these processes influence where we live and the type of crops we can grow. The next time you see condensation building on the side of your glass of iced tea, think about how many millions of miles that water has traveled to get there, and the process it has gone through to become condensation.
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The Physical Environment, http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/atmospheric_moisture/condensation_process.html
Meteorological (Climate) Concepts, http://wc.pima.edu/Bfiero/tucsonecology/climate/concepts.htm
Hydrologic Cycle, http://www.jhu.edu/~dwaugh1/fluid_earth/hydro.htm