Compute dewpoint temperature from temperature and relative humidity

The computation of dewpoint temperature is done using the formula $$\frac{b\left(ln(RH) + \left(\frac{aT}{(b+T)}\right)\right)}{\left(a-\left(ln(RH) + \left(\frac{aT}{(b+T)}\right)\right)\right)}$$ And the computation for relative humidity is done following $$RH=\frac{e}{e_s}$$ where $$e=\frac{QP}{0.378Q+0.622}$$ $$e_s=6.112e^{\frac{aT}{b+T}}$$ In the above formulae:
$T$ is the temperature in $\degree C$
$RH$ is the relative humidity expressed as a decimal between 0 and 1
$Q$ is the specific humidity in $kg.kg^{-1}$
$P$ is the atmospheric pressure in $hPa$
$a$ and $b$ are tuning parameters

The default values for $a$ and $b$ are 17.67 and 243.5 respectively as recommended by NOAA.

Usage

rh_to_td(rh, t, a = 17.67, b = 243.5)

q_to_rh(q, t, p = 1013.25, a = 17.67, b = 243.5)

Arguments

rh: A vector of relative humidities expressed as decimal fractions between 0 and 1. If the maximum is greater than 5, it is assumed that all relative humidities are in $\%$ and will be divided by 100.
t: A vector of the same length as rh temperatures in $\degree C$. If the maximum is greater than 200, it is assumed that all temperatures are in $Kelvin$ and they will be converted to to $\degree C$.
a: A tuning parameter. The default value is $17.67$.
b: A tuning parameter. The default value is $243.5$.
q: The specific humidity in $kg.kg^{-1}$
p: The air pressure in $hPa$

Value

A numeric vector of the same length as t.

Details

Other tunings for $a$ and $b$ are possible:

$a=17.27$, $b=237.3$ is valid for $0\lt T \lt 60$

Alternatively:

$a=17.62$, $b=243.12$
$a=17.625$, $b=243.04$

Finally A.L. Buck (1981) recommends different tunings based on temperature:

$a=17.368$, $b=238.88$ for $T \gt 0$
$a=17.966$, $b=247.15$ for $T \lt 0$

Note that the formulation used for dewpoint temperature does not take atmospheric pressure into account, so may not be correct at higher altitudes.

Examples

rh_to_td(0.8, 20)
#> [1] 16.44765

# In Kelvin and %
rh_to_td(80, 293.15)
#> Warning: rh assumed to be in %. Coverting to 0-1.
#> Warning: t assumed to be in K. Converting to degrees C.
#> [1] 16.44765
#
# Relative humidity is returned as a decimal fraction between 0 and 1
q_to_rh(0.013, 20)
#> [1] 0.8990899