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The specific humidity of ERA5 data is kg/kg, but some articles work with this variable in g/kg. How can I convert this?

Or to compare with precipitation, evaporation, and other variables, how to convert to mm/day?

It is possible?

Thank you!

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1 Answer 1

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The specific humidity of ERA5 data is kg/kg, but some articles work with this variable in g/kg. How can I convert this?

Conversion from kg/kg to g/kg is as simple as multiplying by 1000.

Or to compare with precipitation, evaporation, and other variables, how to convert to mm/day?

I would say that this conversion is not possible because the units are not "equivalent". Relative humidity is a description of the content of moisture in the air whereas mm/day is a rate, which has a temporal component.

Consider using the UDUNITS package, which has various implementations in python and R:

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  • $\begingroup$ Yes, a dimensional analysis shows that relative humidity is dimensionless, while the dimension of precipitation is LT$^{-1}$. $\endgroup$ Commented Sep 13, 2022 at 15:51
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    $\begingroup$ Though you can get the temporal component by noting the change in a day. And can multiply by the volume of air considered to get H2O volume. But even then (and even if specific humidity is the column average, because it changes with height)... local changes in specific humidity don't typically show rainfall, but instead airmass changes (fronts/wind direction). In reality changes in specific humidity locally are ≈ advection + the change from evaporation/condensation + the change from precipitation... if you have precip and evap, maybe you could theoretically estimate advection from your values $\endgroup$ Commented Sep 14, 2022 at 6:31

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