<p>Yes, the unit is kilogram per kilogram, but it means kilogram of water (moisture) per kilogram of air.</p>

CC BY-SA 3.0
2015-06-23T08:33:18.053Z
2015-06-23T08:33:18.053Z
Fred
<p>The basic idea is that by mentioning $\mathrm{kg}\, \mathrm{kg}^{-1}$ you make it clear that you measure the weight. You can think of it as</p>

<p>$$ 0.01\, \mathrm{kg}\, \mathrm{kg}^{-1} = 1\, \mathrm{wt.}\%. $$</p>

<p>This is to avoid confusion with $\mathrm{vol.}\,\%$.</p>

CC BY-SA 3.0
2015-06-23T20:53:55.307Z
*kg* is mass, *N* would be weight, right?
Thomas Weller
CC BY-SA 3.0
2015-06-23T11:17:45.000Z
2015-06-23T11:17:45.000Z
yo'
<p>I am using a data set from Justin Sheffield at Princeton that includes specific humidity. The description is as follows:</p>

<blockquote>
  <p>"Reanalysis 6-hourly specific humidity interpolated to 1.0deg with account for elevation changes and scaled to maintain consistency with reanalysis relative humidity and CRU TS3.0 monthly air temperature";<br>
  The units are = "kg kg-1"</p>
</blockquote>

<p>What does this mean? Does it mean kg/kg so essentially dimensionless? Because I have seen around specific humidity measured also in g/kg, maybe that was wrong? 
These two pages give the same definition of specific and absolute humidity, I thought they were different things: 
<a href="http://www.nzifst.org.nz/unitoperations/drying3.htm">http://www.nzifst.org.nz/unitoperations/drying3.htm</a>
<a href="http://tigge.ecmwf.int/tigge/d/show_object/table=parameters/name=specific_humidity/levtype=pl/">http://tigge.ecmwf.int/tigge/d/show_object/table=parameters/name=specific_humidity/levtype=pl/</a></p>

CC BY-SA 3.0
2015-06-23T08:32:30.490Z
Humidity is generally expressed as a ratio, which is dimensionless. E.g $kg/kg$ as in your example.
user2821
CC BY-SA 3.0
2015-06-23T10:13:13.740Z
$1\,{\rm g}/{\rm kg} = 0.001$. Yes, that's technically a valid (dimensionless) unit.
Ilmari Karonen
<meteorology><atmosphere><humidity>
What does the notation "kg kg-1" for units mean for specific humidity
Herman Toothrot
# What does the notation "kg kg-1" for units mean for specific humidity

(Asked by: username_0 on 2015-06-23)

I am using a data set from Justin Sheffield at Princeton that includes specific humidity. The description is as follows:

> "Reanalysis 6-hourly specific humidity interpolated to 1.0deg with account for elevation changes and scaled to maintain consistency with reanalysis relative humidity and CRU TS3.0 monthly air temperature";  
> The units are = "kg kg-1"

What does this mean? Does it mean kg/kg so essentially dimensionless? Because I have seen around specific humidity measured also in g/kg, maybe that was wrong?
These two pages give the same definition of specific and absolute humidity, I thought they were different things:
<http://www.nzifst.org.nz/unitoperations/drying3.htm>
<http://tigge.ecmwf.int/tigge/d/show_object/table=parameters/name=specific_humidity/levtype=pl/>

**Tags:** meteorology, atmosphere, humidity

> **Comments:**
> **username_1**: Humidity is generally expressed as a ratio, which is dimensionless. E.g $kg/kg$ as in your example.  
> **username_2**: $1\,{\rm g}/{\rm kg} = 0.001$. Yes, that's technically a valid (dimensionless) unit.  

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## 2 answers

### Answer by username_3 on 2015-06-23. Score: 12

Yes, the unit is kilogram per kilogram, but it means kilogram of water (moisture) per kilogram of air.


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### Answer by username_4 on 2015-06-23. Score: 5

The basic idea is that by mentioning $\mathrm{kg}\, \mathrm{kg}^{-1}$ you make it clear that you measure the weight. You can think of it as

$$ 0.01\, \mathrm{kg}\, \mathrm{kg}^{-1} = 1\, \mathrm{wt.}\%. $$

This is to avoid confusion with $\mathrm{vol.}\,\%$.

> **Comments:**
> **username_5**: *kg* is mass, *N* would be weight, right?  

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