Encyclopedic Entry

Dew collects on a red rose.

Photograph by James L. Stanfield, National Geographic

Dewy Eyes
The water droplets that form on eyeglasses in hot, humid atmospheres is dew. However, the term "condensation," which applies to the process as well as the moisture itself, is usually used.

Dew is the moisture that forms as a result of condensation. Condensation is  the process a material undergoes as it changes from a gas to a liquid. Dew is the result of water changing from a vapor to a liquid.
 
Dew forms as temperatures drop and objects cool down. If the object becomes cool enough, the air around the object will also cool. Colder air is less able to hold water vapor than warm air. This forces water vapor in the air around cooling objects to condense. When condensation happens, small water droplets form—dew. 
 
The temperature at which dew forms is called the dew point. The dew point varies widely, depending on location, weather, and time of day.
 
Humid locations, such as the warm, coastal tropics, are more likely to experience dew than arid areas. Humidity measures the amount of water vapor in the air. Warm, humid air is full of moisture that can condense during calm, cool nights.
 
Weather conditions can also influence an area's dew point. Strong winds, for instance, mix different layers of air, containing different amounts of water vapor. This reduces the atmosphere's ability to form dew.
 
Cold weather can also prevent the formation of dew. As temperatures drop below freezing (0°Celsius, 32°Fahrenheit), a region may reach its frost point. At a frost point, water vapor does not condense. It sublimates, or changes directly from a gas to a solid. Moisture changes from water vapor to ice.
 
Dew is most likely to form at night, as temperatures drop and objects cool. However, dew can form whenever a dew point is reached. 
 
Although warm, humid areas commonly experience heavy dew, dew does not form in amounts people could to collect as a water source.
 
Still, several organizations are working to develop technology that will allow them to harvest dew as a freshwater resource. They hope that foil condensers will help meet the water needs of arid areas. The most successful of these condensers have been put to work in Kutch, India, where hundreds of liters of water can be harvested every night at certain times of the year.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

arid

Adjective

dry.

condensation

Noun

process by which water vapor becomes liquid.

Encyclopedic Entry: condensation

condense

Verb

to turn from gas to liquid.

condenser

Noun

tool to turn gas to liquid.

dew

Noun

water droplets condensed from the atmosphere onto cool surfaces near the ground.

Encyclopedic Entry: dew

dew point

Noun

temperature at which water in the air condenses to form water droplets on objects near the ground.

freshwater

Noun

water that is not salty.

frost

Noun

thin coat of ice covering objects when the dew point is below freezing.

Encyclopedic Entry: frost

gas

Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

harvest

Noun

the gathering and collection of crops, including both plants and animals.

humid

Adjective

air containing a large amount of water vapor.

liquid

Noun

state of matter with no fixed shape and molecules that remain loosely bound with each other.

moisture

Noun

wetness.

sublimate

Verb

to change from a solid state to a gas state, without becoming liquid.

technology

Noun

the science of using tools and complex machines to make human life easier or more profitable.

temperature

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

Encyclopedic Entry: temperature

tropics

Noun

region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

Encyclopedic Entry: tropics

vapor

Noun

visible liquid suspended in the air, such as fog.

weather

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Encyclopedic Entry: weather

Credits

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Editor

Jeannie Evers

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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