calculate relative humidity and dew point with wet bulb and dry bulb

[tariik]

Hello,
to calculate relative humidity and dew point with wet bulb and dry bulb Basic formula (Sprung), adopted by WMO:
RH = 100.[es(tw) - A.P(td -tw)]/es(td)

Where:
es(tw) is the saturation vapour pressure of the wet bulb;
es(td) is the saturation vapour pressure of the dry bulb;
p is the pressure of the air;
td is the temperature of the dry bulb;
tw is the temperature of the wet bulb;
A is the psychrometer coefficient A = 0.000662 .

Psychrometer coefficient depends on air flow speed (>2.2m/s),
thermodynamic properties of water and vapor pressure and
geometry of wet bulb thermometer and is dificult to determine precisely.

References

WMO GUIDE TO METEOROLOGICAL INSTRUMENTS AND METHODS OF OBSERVATION WMO-No. 8 (2008 edition, Updated in 2010) : PART 4
https://www.wmo.int/pages/prog/www/IMOP/CIMO-Guide.html
RH = (saturation_vapor_pressure(Twb) - 0.000662 P ( Tdb -Twb )) / saturation_vapor_pressure(Tdb)

[tariik]

i tray to add this code to thermo.py
`@exporter.export
def relative_humidity(temperature, wet_temperature, pressure):
r'''Calculates relative humidity with wet bulb and dry bulb.

Parameters
----------
temperature : array_like
    Dry bulb temperature [degC]
temperature_h : array_like
    Wet bulb temperature [degC]
pressure : array_like
    The atmospheric pressure

Returns
-------
array_like
    relative humidity............................

See Also
--------
saturation_vapor_pressure

Notes
-----
Basic formula (Sprung), adopted by WMO:
RH = 100.[es(tw) - A.P(td -tw)]/es(td)

Where:
es(tw) is the saturation vapour pressure of the wet bulb;
es(td) is the saturation vapour pressure of the dry bulb;
p is the pressure of the air;
td is the temperature of the dry bulb;
tw is the temperature of the wet bulb;
A is the psychrometer coefficient A = 0.000662 kelvin ^-1.

Psychrometer coefficient depends on air flow speed (>2.2m/s), 
thermodynamic properties of water and vapor pressure and 
geometry of wet bulb thermometer and is dificult to determine precisely.

References
----------
.. WMO GUIDE TO METEOROLOGICAL INSTRUMENTS AND METHODS OF OBSERVATION WMO-No.8 
(2008 edition, Updated in 2010) : PART 4
https://www.wmo.int/pages/prog/www/IMOP/CIMO-Guide.html[3]

'''

A   = 0.000662  / units.kelvin  
return  (saturation_vapor_pressure(wet_temperature) - A * pressure * (temperature.to('kelvin') - wet_temperature.to('kelvin'))) / saturation_vapor_pressure(temperature)

`

[tariik]

and my test is
`import metpy.calc as mcalc
from metpy.units import units

P = 1013.25 * units.mbar
print ' Pressure = ',P

Tdb = 25 * units.degC
print ' temperature = ',Tdb

Twb = 21 * units.degC
print ' Wet temperature = ',Twb

rh = mcalc.relative_humidity(Tdb, Twb,P)

print ' relative humiditye = ',rh`

its work 👍
Pressure = 1013.25 millibar
temperature = 25 degC
Wet temperature = 21 degC
relative humiditye = 0.70008048528 dimensionless

[dopplershift]

Done in #345 .