No subject

  *w = RH * w_s(T)*

You will need a function to compute the saturation mixing ration w_s(T).  It
is common to first compute the saturation vapor pressure, say, using
function *satvap() *from the libbjt in the opengrads extensions:* *

        http://opengrads.org/doc/udxt/libbjt/

and from there compute the saturation mixing ratio *w_s *from the formula

  w_s = 0.622 * e_s / ( p - e_s )

where *p* is the air pressure and *e_s* the saturation vapor pressure just
computed. The last step is to convert mixing ration to specific humidity.
The formula is

   *q = w / ( 1 + w **)*

The most convenient way to get to the opengrads extensions is to install the
latest opengrads bundle. More information here:

http://opengrads.org/wiki/index.php?title=Installing_the_OpenGrADS_Bundle#Important

  Good Luck,

    Arlindo



--
Arlindo da Silva
dasilva at alum.mit.edu

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On Fri, Jul 24, 2009 at 10:21 PM, itesh dash <span dir=3D"ltr">&lt;<a href=
=3D"mailto:itesh.dash at gmail.com">itesh.dash at gmail.com</a>&gt;</span> wrote:=
<br><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote" style=3D"m=
argin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div>Hi,</div>
<div>=A0</div>
<div>Can anybody suggest how to calculate specific humidity=A0using air=A0t=
emperature=A0 (deg K) and relative humidity. </div>
<div>=A0</div></blockquote><div>=A0</div><blockquote class=3D"gmail_quote" =
style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><d=
iv></div>
<div>The available GrASDS Script <strong><font color=3D"#ff0000">tvrh2q</fo=
nt></strong> calculates specific humidity based on virtual temperature and =
relative humidity.</div>
<div>=A0</div></blockquote><div><br></div><div>To a good approximation</div=
><div><br></div><div>=A0=A0RH =3D e / e_s((T) ~ w / w_s(T)</div><div><br></=
div><div>where=A0</div><div><br></div><div>=A0=A0 e =3D vapor pressure</div=
><div>=A0=A0 e_s =3D saturation vapor pressure</div>
<div>=A0=A0 w =3D mixing ratio</div><div>=A0=A0 =A0w_s =3D saturation mixin=
g ration</div><div><br></div><div>From=A0this=A0formula=A0you=A0can=A0compu=
te</div><div><br></div><div>=A0=A0<font class=3D"Apple-style-span" color=3D=
"#CC0000"><b>w =3D RH * w_s(T)</b></font></div>
<div><br></div><div>You=A0will=A0need=A0a=A0function=A0to=A0compute=A0the=
=A0saturation=A0mixing=A0ration=A0w_s(T). =A0It is common to first compute =
the saturation vapor pressure, say, using function <b>satvap() </b>from the=
 libbjt in the opengrads extensions:<b>=A0</b>=A0</div>
<div>=A0</div><div>=A0=A0 =A0 =A0 =A0<a href=3D"http://opengrads.org/doc/ud=
xt/libbjt/">http://opengrads.org/doc/udxt/libbjt/</a></div><div><br></div><=
div>and from there compute the saturation mixing ratio <b>w_s=A0</b>from th=
e formula</div>
<div><br></div><div>=A0=A0w_s =3D 0.622 * e_s / ( p - e_s )</div><div><br><=
/div><div>where <b>p</b> is the air pressure and <b>e_s</b> the saturation =
vapor pressure just computed. The last step is to convert mixing ration to =
specific humidity. The formula is</div>
<div><br></div><div>=A0=A0 <b><font class=3D"Apple-style-span" color=3D"#CC=
0000">q =3D w / ( 1 + w </font></b><b><font class=3D"Apple-style-span" colo=
r=3D"#CC0000">)</font></b><span style=3D""></span></div><div><br></div><div=
>The most convenient way to get to the opengrads extensions is to install t=
he latest opengrads bundle. More information here:</div>
<div><br></div><div><a href=3D"http://opengrads.org/wiki/index.php?title=3D=
Installing_the_OpenGrADS_Bundle#Important">http://opengrads.org/wiki/index.=
php?title=3DInstalling_the_OpenGrADS_Bundle#Important</a></div><div><br></d=
iv>
<div>=A0=A0Good Luck,</div><div><br></div><div>=A0=A0 =A0Arlindo</div><div>=
<br></div><div><br></div></div><br>-- <br>Arlindo da Silva<br><a href=3D"ma=
ilto:dasilva at alum.mit.edu">dasilva at alum.mit.edu</a><br>

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