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From a Ph.D dissertation, the writer change the land surface from nature earth to urbn and built up to compare the difference involving with this land use change.

Due to the urban heat land effect, the temperature of lower atmosphere above the surface increase, and the dew point decrease.

Dew point is a parameter represent the absolute moisture. When the temperature increase, how to analysis the dew point increase?

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There are two separate effects:

  1. The average temperature above urban land is greater than above rural land. This could be because of the cooling effect of plants evaporating water, or because of the higher albedo of vegetation than pavement and roofing.

  2. The absolute humidity decreases; that is, the total amount of water vapor in a given volume of air decreases, perhaps due to the capture and run-off of rainwater into storm drains, rather than evaporating.

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  • $\begingroup$ Thanks for your reply. This is my understanding of your second opinion: the water vapor in a given volume above the certain area has its origin and sink process. From rural land to urban and built up, the amount of water content which evaporate from the surface has decreased due to less vegetation and impermeable surfaces. So, the water above the area are not directly influenced by the surface temperature. Is it correct? $\endgroup$ – Han Zhengzu May 10 '16 at 7:39
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    $\begingroup$ Your understanding appears correct, except for "So, the water above the area are not directly influenced by the surface temperature. " One cannot assume the two effects are not related without more data. $\endgroup$ – DrMoishe Pippik May 11 '16 at 0:55

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