"Michele C." wrote:
>
> Sono alla ricerca di una formula che calcoli l' umidita relativa conoscendo
> la T_ambiente e la T_bulboumido.
Tatte dal sito
http://www.best.com/~williams/avform.htm#RH.
Ciao, Franco
Relative humidity, dewpoint, frostpoint etc.
The relative humidity, f (as a fraction) is related to the
temperature, T and dewpoint Td by:
f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))
and to the frostpoint temperature Tf by:
f= exp(21.87(Tf/(Tf+265.5)-T/(T+265.5)))
Temperatures are in Celsius. Multiply f by 100 if you want a
percentage. The above are based on an empirical fit to the saturation
vapor pressure of water due to O. Tetens in Zeitschrift fur Geophysik,
Vol VI (1930), quoted in "Principles of Meteorological
Analysis" by W. J. Saucier (Dover NY 1983).
This fit is:
e_s=6.11 * exp(bT/(T+a)) for the saturation vapor pressure e_s in
mbar
over water a=237.3, b=17.27
over ice a=265.5, b=21.87
An alternative slightly more accurate fit (over water) is:
e_s = 6.10779 + T * (4.43652e-1 + T * (1.42894e-2 + T * (2.65064e-4 +
T *
(3.03124e-6 + T * (2.03408e-8 + (6.13682e-11 * T))))))
(from Lowe, JAM (1977), 103)
The latest and greatest fit based on "Thermodynamic properties of Dry
Air, Moist Air and Water, and SI Psychrometric charts" by
Arnold Wexler and Richard Hyland, National Bureau of Standards;
Richard Stewart, University of Idaho.
Tables of Relative Humidity and Dewpoint vs Temperature and Wet Bulb
Temperature can be found in "Introduction to
Meteorology" by Franklyn Cole (Wiley NY 1975).
Inverting this to find dewpoint in terms of temp and RH:
Dewpoint Td=237.3/(1/(ln(f)/17.27+T/(T+237.3))-1)
Frostpoint Tf=265.5/(1/(ln(f)/21.87+T/(T+265.5))-1)
Given the wet bulb temperature Tw (C), the dry bulb temperature T (C),
and the pressure, p in mbar one gets the (approximate)
relative humidity and dewpoint by the following:
ed= 6.11*exp(17.27*T/(T+237.3)) /* SVP at dry-bulb temp
ew= 6.11*exp(17.27*Tw/(Tw+237.3)) /* SVP at wet-bulb temp
wd=0.62197*ed/(p-ed) /* saturation mixing ratio at T
ww=0.62197*ew/(p-ew) /* saturation mixing ratio at Tw
w=(2500.0*ww-1.0046*(T-Tw))/(2500.0+1.81*(T-Tw)) /* mixing ratio
f= w/wd /* relative humidity as a
fraction
e= p*w/(0.62197+w) /* vapor pressure (mb)
Td=(237.3*log10(e)-186.527)/(8.286-log10(e)) /* the dewpoint (C)
This uses the Tetens fit for the saturated vapor pressure and treat
water vapor as an ideal gas, both of which are pretty good
approximations. If you want better refer to the Smithsonian
Meteorological Tables ( Smithsonian Institute 1963 )
Received on Thu Apr 06 2000 - 00:00:00 CEST