# Why does the dewpoint go down with increasing temperatures?

## Main Question or Discussion Point

You can http://www.wunderground.com/history/airport/VECC/2009/4/21/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA" ) (scroll down to see the hourly observations) that the dewpoint goes down as the temperature increases and goes back up again as the temperature goes down. There seems to be an error in the data for every half hour, but I think that the data is generally correct (for other days you see the same pattern).

The dewpoint is the temperature at which you have 100% humidity, so you would expect that the dewpoint would actually increase when the temperature increases because you have more evaporation, so the air would contain more moisture. But the data seems to contradict this.

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Very interesting. Plants photosynthesize best in the morning while the sun is out and before the plant leaves reach about 35 C. Photosynthesis requires water and CO2 in equal amounts to create plant matter (e.g., saccharides). Photosynthesis (in Calvin cycle) plants shuts down above 40 C because the respiration of water and CO2 equals (or exceeds?) intake, so the major photosynthesis activity is in the morning.
[EDIT] In further investigation of the dew point for various calender days (in viewing the Table), there seems to be no real correlation of the dew point with time of day. It may have some correlation with wind direction and speed.

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russ_watters
Mentor
As a general principle, there is no thermodynamic reason why that would be true: dry bulb temperature and dew point are completely independent of each other. And in that graph of yours, the variation in dew point temperature is quite small and you have only one cycle shown, so I don't know that you can draw a general conclusion from that. Looking at the data for my location for the last three days, shows a vague pattern in the opposite direction (dew point rising with temperature). Over the course of a single day, weather patterns have a much bigger impact than thermodynamics.

A couple of possible influences, though:

-When the temperature drops enough to get fog and dew, water condenses out of the air, lowering the dew point.
-When it is hotter outside, water will evaporate from the ground faster, raising the dew point.
-When it is hotter outside, there is more wind/convection and the mixing of low altitude and high altitude air lowers the dew point.

D H
Staff Emeritus
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Andy Resnick
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The dewpoint is the temperature at which you have 100% humidity, so you would expect that the dewpoint would actually increase when the temperature increases because you have more evaporation, so the air would contain more moisture. But the data seems to contradict this.
Yes, but as the temperature increases, the vapor pressure of water also increases, meaning the *relative* humidity will decrease (unless there is a source of water vapor via evaporation)

http://www.elmhurst.edu/~chm/vchembook/174temppres.html [Broken]

The dewpoint is a measure of the saturation point of water in air, and since the relative humidity decreases as the temperature increases, the dewpoint must decrease.

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The dewpoint is a measure of the saturation point of water in air, and since the relative humidity decreases as the temperature increases, the dewpoint must decrease.
Or dewpoint could stay the same, correct? Dew forms at a given temp(air is saturated).

If the air temp increases, it's ability to hold water vapor increases, relative humidity goes down. But NOT, actual amount of water in the air.

All other things, ambient pressure, total amount of water vapor in the air, etc, remaining the same.............if possible.

That dewpoint(temp) could remain unchanged, all we need to do is lower the temp again to that already defined dewpoint.

My point is dewpoint(temperature) is not dependant on ambient temperature, it IS dependant or defined by ambient pressure and actual "amount" of water in the air.

Correct?

Thanks, John

Andy Resnick