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I am processing a big meteorological data set, which is a global forcings dataset derived by other datasets and observations, and calculating relative humidity from qair,temperature, and pressure according to these formulas How do I convert specific humidity to relative humidity?

I am noticing that I have a lot of occurrences where RH is slightly higher than 100% and I am wondering if this makes sense or not considering this is air temp from 2m, I would expect that near surface RH over 100% is some errors in the data set.

*I am not a climatologist nor a meteorologist, I use this data to study plants, so I hope for a not too technical answer, thank you.

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    $\begingroup$ Is it higher like 100.4% or higher like 400%? Also, please provide more specific information about the dataset. $\endgroup$ – milancurcic Aug 16 '16 at 17:11
  • $\begingroup$ @milancurcic no they are just a few 1-2% higher but it shows the bias of the dataset compared to other data set where RH is never over 100% $\endgroup$ – Herman Toothrot Aug 16 '16 at 17:41
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    $\begingroup$ @user4050 please do edit that additional information into the question $\endgroup$ – EnergyNumbers Aug 16 '16 at 18:56
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It is reasonable to assume that air (atmosphere) acting as as a solution could become supersaturated with water vapor. Supersaturation condition is where the air literally contains more water vapor than is needed to cause saturation. How common these conditions might be, I do not know.

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The relative humidity is the ratio of the partial vapour pressure of air, to the equilibrium vapour pressure over pure water, at the same pressure - yes, it's slightly pressure dependent.But I would put an emphasis upon 'Equilibrium'. In both physics and chemistry one can get all kinds of bizarre results under non-equilibrium conditions, and >100% RH is one of them. It is just a transient feature caused by rapid changes in temperature, pressure, or absolute vapour pressure, and as such it is not really a genuine meteorological observation.

Also, I advise caution when interpreting RH results. There is often a strong diurnal variation in RH, so if there is a met site taking only one reading a day, it could be unrepresentative of true average conditions. This is not so much a problem in advanced countries, where the measurements are (usually) reliably taken at set times. But it is often wildly misleading in developing countries where the observer may take a reading when/if he/she gets around to it, or maybe fakes the data altogether!

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