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That is the same as my first formula 57.2957*ATAN2(u,v)+180 That, as well as yours, should determing the quadrant in the sum. Ian

My thoughts exactly. John - The replys I get from NOAA are very hard to understand, they do not make things clear to me. I think that because they are all hold Phd's they are unable to explain in simple terms to the likes of which i can understand. While the Met Office forecast did make to clear, but i believe they got it wrong. I reply i got from the UKMO was infact relayed though their customer enquires, so perhaps thats where the mistake was made. Mind you they did get the 1987 storm wrong - perhaps if they new which way the wind was blowing, they would of got it right LOL. Ian

Hello John, Why do you need u and v components? I am procducing my own forecasts based on the GFS 0.05deg NWP model. Unlike others who use this data I use a program that converts GRIB data into its raw state. and does not do the caculations for me. The reason for this is: I can then add my own caculations based on the topography of the area i am forecasting. Plus I can change the data, if some of the values are missing - quite comman with data set as large as the GFS. This should allow me to produce a more realistic forecast - Well thats the plan away Now wind is packed in a GRIB message as U m/s and V m/s at the required height Example: A wind @10 meters would return as 10 HGTL V=2.3 m/s U=2.6 m/s Now to caculate the direction you need for find the angle V = North to South if V<0 V= South to North if V>0 U= West to East if U>0 U= East to West if U<0 Next you need to find the Quadrent, and this is where my trouble lies. Now N 360 | | W 270-----------90 E | | S 180 Now based on the example above U=2.6 and V=2.3, this to me would put it in the quadrent SW (between S and W) As U>0 and V>0. I caculated the angle and ask if a met office forecaster would check the anwser and my formula, before i placed it in to my math model. The relpy was that it should be in the SE quadrant, but i can't see how. I contacted NOAA who gave me the formula in my earlier message, with should caculate the Quadrent for me, as well as the angle. I just need some further advice weather this formula is correct, so it can be placed in my model, and carry on with my work. Ian

Hello, I am not to sure weather i have posted this in the right section of this forum. I need help caculating wind direction from the U and V components of wind. I can caculate the angle and degree, but having trouble detrerming the quadrent. After spending many weeks on this i think i have come up with a function that returns the inverse tangent of (y/x) in the range -π ≤ return_val ≤ π, using the signs of both arguments to determine the quadrant of the return value The formula: r2d = 45.0/atan(1.0) (radians to degrees) or 57.2957795 dir = atan2(u, v) * r2d + 180 If some one here is able to check this formula, or knows another way please post. I would be most greatful. Ian Gooch

Current Conditions For Ipswich UK Temp : 6.8 °C Wind :NW at 2.0 mph Rainfall : 0.6 mm Barometer : 1006.9 mb Today's Extremes Temp Hi : 10.8 °C Lo : 4.7 °C Wind Gust : 20.0 mph Rain Rate : 0.2 mm/hr Barometer Hi : 1012.9 mb Lo : 1006.8 mb Forecast: Mostly cloudy and cooler. Precipitation possible within 12 hours, possibly heavy at times. Windy. Sunrise: 7:18 Sunset: 16:12 Moon phase: Full Moon Hi all , not to sure if this is the right place to post this, you will have to tell me.