As you can see below, I'd like to know why the saturation is decreasing with the temperature while the absolute amount of water remains constantly. The image is taken from a description of a transformer where the relative humidity in oil is decreasing with the temperature, so I'd be happy if an explanation could fit to the image.

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I asked a similar Why has warm air a higher water saturation than cold air? but meanwile I think it doesn't really explain the reason. But maybe I miss the forest for the trees.

However: Why is the relative humidity decreasing with the (oil) temperature?


Relative humidity is defined as 100 times the partial pressure of the water vapor in the moist air divided by the equilibrium vapor pressure of the water vapor at the temperature of the air mixture. The equilibrium vapor pressure always increases with temperature. So, for a fixed amount of water vapor in the air (i.e., a fixed partial pressure at constant total pressure), increasing the temperature lowers the relative humidity.

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  • $\begingroup$ Thank you. But why is the equilibrium vapor pressure increasing with the temperature? $\endgroup$ – Ben Jul 22 at 12:55
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    $\begingroup$ The development of the relationship between equilibrium vapor pressure and temperature is based on a combination of the 1st and 2nd laws of thermodynamics leading to and expression for the variation of Gibbs free energy as a function of temperature and pressure, combined with the condition that, at equilibrium, the change in Gibbs free energy between the liquid and vapor at equilibrium is zero at all combinations of temperature and pressure along the saturation curve. This all leads to the Clausius clapeyron equation. $\endgroup$ – Chet Miller Jul 22 at 13:49
  • $\begingroup$ Well, I probably have to read up some literature for that. Thank you! $\endgroup$ – Ben Jul 22 at 14:08

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