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Q & A: vapor pressure and dew point

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Most recent answer: 09/17/2020
Q:
Why the vapour pressure is same to the dewpoint vapour pressure? if i decrese the tempereture of a confined room from 25 to say 18 degree celcius and then to its dewpoint is there any change of vapour pressure?
- mohaiminl (age 21)
Dhaka
A:

The dew point is defined to be the tempearture at which the equilibrium vapor pressure equals the actual partial pressure of the water in the atmosphere. As the room is cooled there's little change in the actual partial pressure of the water vapor until the dew point is reached. Below that temperature the partial pressure drops rapidly as water condenses out as liquid.

Mike W.


(published on 07/30/2013)

Follow-Up #1: dew point and partial pressure

Q:
Why is the vapor pressure at dew poit equals to the partial vapor pressure at room temperature?
- samantha kumara (age 32)
kalawana
A:

We've mostly answered that above, but there's one part we haven't completely filled in. Why doesn't the partial pressure change much as the temperature is lowered to the dew point? The total air pressure typically changes very little, since it's set by the braod outside pressure. The fraction of molecules tha are water doesn't change unti they start to condense at the dew point. The ater partial pressure is almost exaclty the total pressure times the fraction of molecules that are water. So as the air cools to the dew point, the partial water pressure doesn't change.

Mike W.


(published on 09/17/2020)

Follow-up on this answer.