Monday, 13 February 2017

Evaporative cooling

Is a major pathway for transferring heat from the earth into the atmosphere.

  1. On earth (land and sea), liquid water is converted to water vapour by absorbing latent heat of vapourization (LHoV). LHoV does not increase the water's temperature. It is a specific amount of energy required to change the state of water from liquid to gas. [There is also analogous latent heat of crystallization (required to melt ice), but no more on that right now.]
  2. The surface is warmer than the atmosphere, and water vapour (H2O, M.Wt = 18) is lighter than air. So air rises.
  3. On reaching a certain height, this water vapour is much warmer than surrounding atmosphere. So the atmosphere steals its LHoV. The water vapour changes back to liquid. It turns to cloud. The energy in LHoV has moved to the atmosphere.
  4. PS: Water vapour can also give off heat by infra red emission. In which case the atmosphere is cooled by loss of radiative heat to space.
  5. Moisture in clouds fall to earth as rain. LHoV is left in the atmosphere.

The net effect is transfer of heat into atmosphere.

This is how latent heat transfer, "24", in the diagram above works.

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