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Wind Direction U and V http://www.weathergraphics.com/forum/viewtopic.php?f=7&t=1248 
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Author:  Ian [ Thu Apr 12, 2007 8:18 pm ] 
Post subject:  Wind Direction U and V 
Hi Jeff, I am using DEGRIB to convert grib2 files into a XML format, but i need to calulate wind direction and wind speed. I believe this is stored in a grib message as UGRD VGRD both in M/S I used the formla atan2(y,x) Where y is the V and x is U is this correct? and is the output in DEG. I am getting a little lost here, and wondered if you could help me out. Ian 
Author:  jkrob [ Fri Apr 13, 2007 2:16 am ] 
Post subject:  
Ian, Well, you are half right...but not quite. Yes, UGRD & VGRD are the way wind is 'packed' into GRIB messages. However, the numbers are not straight forward. Here are the formulas to get wind speed (M/S) & direction (grid relative) from the U & V DIRECTION=57.29578*(arctangent(UGRD,VGRD))+180. SPEED=SQRT(UGRD*UGRD+VGRD*VGRD) Clear as mud...right? In essence, the UGRD is the North/South wind component (North is + & South is ) and VGRD is the East/West wind component (East is + West is  [I think...]). Remember...you asked for it Thanks, Jeff 
Author:  Ian [ Fri Apr 13, 2007 7:23 am ] 
Post subject:  
Hi Jeff, Thanks for that. However (DIRECTION=57.29578*(arctangent(UGRD,VGRD))+180) Does this formula work in what ever quadrant? Or does the +180 change to suit V and U values? i.e 180 if V>=0 0 if U<0 and V<0>0 and V<0 Ian 
Author:  Ian [ Fri Apr 13, 2007 8:13 am ] 
Post subject:  
Hi again, Tried the DIRECTION=57.29578*(arctangent(UGRD,VGRD))+180 Could not get it to work. Entered DIRECTION=57.29578*(ATAN2(UGRD,VGRD))+180 Works OK  where U = 6 and V = 5, result dir= 220 deg Speed = 8 m/s Could you check it please. Thanks Ian 
Author:  jkrob [ Fri Apr 13, 2007 7:01 pm ] 
Post subject:  
Ian, Yes, those numbers look good. Sorry for the confusion; arctangent & ATAN2 were the same thing. I did not know if you knew Fortran functions so I wrote the formula out longhand. Jeff 
Author:  Ian [ Fri Apr 13, 2007 9:09 pm ] 
Post subject:  
Jeff, No probs mate for the confusion, just glad you could help me out. While Iam picking your brain for the maths, perhaps you would be able to help me out again. I have two more calcs to do 1. PRATE, [kg/(m^2 s) to mm/h 2. Weather strings. Now i was going to use the DAVIS weather software which came with my weather station to give a description of the weather based on the output of the GRIB model, but this is only based on pressure trends. Is there a WMO standard for simple weather strings. I have looked on google, but have not found anything as of yet which made much sence to me. Ian. P.S Wingridds is getting better with every relaese  keep up the good work. 
Author:  jkrob [ Sat Apr 14, 2007 2:15 am ] 
Post subject:  
Ian, Sorry...I don't really follow what it is you are trying to accomplish. Can you be more 'verbose' in your explaination? Thanks, Jeff 
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