water vapor

a dispersion, in air, of molecules of water, especially as produced by evaporation at ambient temperatures rather than by boiling. Compare steam ( def 2 ).


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water vapor  
Water in its gaseous state, especially in the atmosphere and at a temperature below the boiling point. Water vapor in the atmosphere serves as the raw material for cloud and rain formation. It also helps regulate the Earth's temperature by reflecting and scattering radiation from the Sun and by absorbing the Earth's infrared radiation. See also vapor.
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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
The warming of the seas and lakes, moreover, means more water vapor in the air.
The brilliant clouds likely formed from the hundreds of tons of water vapor
  emitted from the shuttle's engine during takeoff.
As of yet, there is not even agreement as to whether water vapor plays a
  positive effect or a negative one.
Climatologists are only beginning to model the role that water vapor plays in
  atmospheric circulation.
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