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EDIT: I just want to understand why, intuitively, higher atmospheric pressure results in a higher dew point? (Rather than how to calculate dew point - I realise a clever person than I can probably get from the calculation to they why, but I'm a bit slow for that)

I understand that

  1. dew point is the temperature below which condensation starts to form;

  2. dew point is higher the more water vapour there is in the air and the higher the atmospheric pressure.

I think I understand why more water vapour means a higher dew point: because the more water as vapour there is the more (heat) energy you need to keep it as vapour.

I don't understand why higher pressure means the dew point is higher. Is it because at higher pressure more of the available heat energy is taken up by other gases? Or because at higher pressure you get less heat energy for a given temperature? Something else?

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  • $\begingroup$ I am prepared to give you an intuitive explanation, but you first need to tell me the source of the wording of your statement number 2 because dew point discussions generally revolve around 1 atm pressure. Can you cite the source for me? $\endgroup$
    – Bob D
    Mar 12, 2019 at 21:35
  • $\begingroup$ @Bob D 3."What is the effect of pressure on dew point?" at vaisala.com/sites/default/files/documents/… $\endgroup$ Aug 29, 2021 at 9:18

2 Answers 2

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The dew point temperature is the temperature at which the partial pressure of water vapor in the air is equal to its vapor pressure. This means that the dew point temperature is not a function of atmospheric pressure, nor is it a function of the amount of water vapor that is currently in the air. To calculate the dew point temperature, do the following:

1) Note the current temperature and relative humidity. The relative humidity is the percent saturation of water vapor in the air at the current temperature.

2) Calculate the partial pressure of water at 100% relative humidity and at the current temperature with the Antoine equation. For more info on this equation, see https://en.wikipedia.org/wiki/Antoine_equation.

3) Multiply the partial pressure of water at the current temperature, by the relative humidity. This is the actual partial pressure of water in the atmosphere for the current relative humidity.

4) Re-solve the Antoine equation at this partial pressure, by manipulating the temperature term in that equation. This new temperature is the point at which the water vapor in the air will be at 100% humidity, which is also known as the dew point temperature.

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  • $\begingroup$ Hi there, thanks for replying. I'm trying to gain an intuitive understanding of what's happening so I struggle with the explanation by equation. That said, you have temp in your step two so presumably it is a function of temp (among other things) no? (And does atmospheric pressure not come in at 2. also?) $\endgroup$ Mar 12, 2019 at 19:55
  • $\begingroup$ @DonaldSmith, the Antoine equation relates the vapor pressure of a liquid to its temperature. The dew point occurs when the partial pressure of water vapor in air equals the pressure calculated by the Antoine equation for the given temperature. Also, the pressure term in the Antoine equation is the vapor pressure of water, NOT atmospheric pressure. For more info, see en.wikipedia.org/wiki/Dew_point $\endgroup$ Mar 12, 2019 at 20:05
  • $\begingroup$ @DavidWhite, naive question: if I have a graph of the vapor pressure of water at each temperature and also have the actual present partial pressure of water in the air, can't I find the dew point by finding the temp. at which my graph shows this value? This can be done in replacement of step 4, yes ? $\endgroup$
    – Sal_99
    Aug 27, 2020 at 14:40
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As pressure increases, vapor particles are pressed more closely together, thus increasing the chance they will form into droplets/dew. Since this happens easier, the temperature does not need to be as cool, thus increasing the dew point.

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