<blockquote>
<p>The specific humidity of ERA5 data is kg/kg, but some articles work with this variable in g/kg. How can I convert this?</p>
</blockquote>
<p>Conversion from kg/kg to g/kg is as simple as multiplying by 1000.</p>
<blockquote>
<p>Or to compare with precipitation, evaporation, and other variables, how to convert to mm/day?</p>
</blockquote>
<p>I would say that this conversion is not possible because the units are not &quot;equivalent&quot;.  Relative humidity is a description of the <em>content</em> of moisture in the air whereas <code>mm/day</code> is a <em>rate</em>, which has a temporal component.</p>
<p>Consider using the UDUNITS package, which has various implementations in python and R:</p>
<ul>
<li><a href="https://ncas-cms.github.io/cfunits/" rel="noreferrer">cfunits</a> (python)</li>
<li><a href="https://cran.r-project.org/web/packages/udunits2/index.html" rel="noreferrer">udunits2</a> (R)</li>
</ul>

CC BY-SA 4.0
2022-09-13T15:51:59.363Z
Yes, a dimensional analysis shows that relative humidity is dimensionless, while the dimension of precipitation is LT$^{-1}$.
Jean-Marie Prival
CC BY-SA 4.0
2022-09-14T06:31:19.370Z
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
JeopardyTempest
CC BY-SA 4.0
2022-09-13T14:43:55.183Z
2022-09-13T14:43:55.183Z
alex_danielssen
<p>The specific humidity of ERA5 data is kg/kg, but some articles work with this variable in g/kg.
How can I convert this?</p>
<p>Or to compare with precipitation, evaporation, and other variables, how to convert to mm/day?</p>
<p>It is possible?</p>
<p>Thank you!</p>

<meteorology><humidity><data-formats><era>
How to convert specific humidity ERA5 in kg/kg to g/kg or mm/day?
smi
# How to convert specific humidity ERA5 in kg/kg to g/kg or mm/day?

(Asked by: username_0 on 2022-09-13)

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!

**Tags:** meteorology, humidity, data-formats, era


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## 1 answers

### Answer by username_1 on 2022-09-13. Score: 5 | Accepted answer

> 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:

* [cfunits](https://ncas-cms.github.io/cfunits/) (python)
* [udunits2](https://cran.r-project.org/web/packages/udunits2/index.html) (R)

> **Comments:**
> **username_2**: Yes, a dimensional analysis shows that relative humidity is dimensionless, while the dimension of precipitation is LT$^{-1}$.  
> **username_3**: 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  

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