Hi. You need to know the saturation vapor pressure which depends on temperature. Then transform specific humidity to absolute humidity and you should be close to the answer. Try to look at the following document which explains different conversions. http://www.vaisala.com/Vaisala%20Documents/Application%20notes/Humidity_Conversion_Formulas_B210973EN-F.pdf

http://www.vaisala.com/Vaisala%20Documents/Application%20notes/Humidity_Conversion_Formulas_B210973EN-F.pdf

@Thomas Flury, thanks a lot, I already have that document you suggested but I may have to go through it again following your suggestion. Thanks once again.

 Your problem is very simple

use the equation

e = p*w/(w+eps)

where e is the vapor pressure (in hPa), p the pressure (in hPa), w the specific humidty in kg/kg and eps=0.622

(ref.Atmospheric Science. An Introductory Survey. Wallace and Hobbs. Academic Press. 1977 p 71. Problem 2.7)

Thomas has given the right answer. It is not hard, as the saturate water vapor pressure depends on temperature. You can easily get water vapor pressure when you have relative humidity and saturate vapor pressure.

@All, I am really very grateful to all, this is the beauty of one world. But it's like Robichaud suggestion is very simple and direct, I must check out his reference!

Oladiran,

I have to add up that since usually w is much smaller than eps,

then the equation given boils down to

e  = pw/eps

note that eps=0.622 is the ratio of specific gas constants or the ratio of MM(H2O)/MM(dry air)

see https://en.wikipedia.org/wiki/Humidity (look under specific humidity)

@Robichaud, thanks a lot. I got the 2006 2nd Ed. of the reference you gave and I got the solution at section 3.5.1 on page 80. Thanks once again!

To obtain an accurate estimate of the saturation vapor pressure, you can consult http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281993%29032%3C1294%3AAOTAAC%3E2.0.CO%3B2