How to calculate specific humidity with relative humidity, temperature, and pressure

Question

I know there is this question already How do I convert Relative Humidity into specific humidity

but I don't have ρws = density of water vapor (kg/m3) and ρ = density of the moist or humid air (kg/m3).

Is there another way to calculate it? Also I have air pressure but it's in the range of 95000-96000.

I have used the rh2qair function from the PEcAn data.atmosphere R package but I get values such as 1.5463 converts relative humidity to specific humidity

    @title RH to SH 
    @param rh relative humidity (proportion, not %) 
    @param T absolute temperature (Kelvin) 
    @export 
    @author Mike Dietze @aliases rh2rv 
    rh2qair <- function(rh, T) {   
        qair <- rh * 2.541e6 * exp(-5415.0 / T) * 18/29   
        return(qair) 
    }

What input are you providing (rh, T) that gives you the quoted answer? — casey, Jun 28 '15 at 01:16
Looks like you are passing rh in % rather than proportion, I've updated my answer to reflect this. — casey, Jun 28 '15 at 02:01
note that we've recently updated this function (to take pressure as an argument), per discussions in this thread on github. And make it more consistent with the qair2rh function discussed in a separate question http://earthscience.stackexchange.com/a/2385/174 — David LeBauer, Jul 02 '15 at 15:10

casey · Accepted Answer · 2015-06-28T01:55:15.570

It looks like your program is using an approximation based on $q \approx w = w_s*RH$ with an approximation of Clausius-Clapeyron to find $w_s$. Looking at a few values of RH,T and P, your approximation is pretty close (+/- 5%) to an analytic answer. Based on the output you quoted it looks like you are providing incorrect values of RH. Note in the comments to your routine it says:

@param rh relative humidity (proportion, not %)

This means you need to provide the RH proportion, not the percentage. E.g. divide by 100 -- RH=1 for 100%, RH=0.5 for 50%, etc.

If you adjust your input data you should be able to use your code as-is. If you wish to compare it to something, you can reference the solution below.

If you are given $RH$ (in the range [0,1]), $T$ (K) and $p$ (Pa) you can proceed as follows.

Knowing that $$RH = \dfrac{e}{e_s},$$ $$w = \dfrac{e\ R_d}{R_v(p-e)},$$ and $$q = \dfrac{w}{w+1}$$

Then we can solve for specific humidity $q$.

Rather than combining this into a single formula and solving, it is more straightforward to present this incrementally.

First, find $e_s(T)$ where $$e_s(T) = e_{s0}\exp\left[\left(\dfrac{L_v(T)}{R_v}\right)\left(\dfrac{1}{T_0}-\dfrac{1}{T}\right)\right]$$ and then find $e$ from the first formula ($e = RH*e_s$). Then plug $e$ into the formula for $w$ and then that result into the formula for $q$.

Variables used:
_{$q$ specific humidity or the mass mixing ratio of water vapor to total air (dimensionless)

$w$ mass mixing ratio of water vapor to dry air (dimensionless)

$e_s(T)$ saturation vapor pressure (Pa)

$e_{s0}$ saturation vapor pressure at $T_0$ (Pa)

$R_d$ specific gas constant for dry air (J kg$^{-1}$ K$^{-1}$)

$R_v$ specific gas constant for water vapor (J kg$^{-1}$ K$^{-1}$)

$p$ pressure (Pa)

$L_v(T)$ specific enthalpy of vaporization (J kg$^{-1}$)

$T$ temperature (K)

$T_0$ reference temperature (typically 273.16 K) (K)}

How is $L_v(T)$, the specific enthalpy of vaporization, calculated? Is it a function of temperature? — Christo, Oct 20 '16 at 09:11

How to calculate specific humidity with relative humidity, temperature, and pressure

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