angle - How to calculate wind direction from U and V wind components in R -

i have u , v wind component data , calculate wind direction these values in r.

i end wind direction data on scale of 0-360 degrees, 0° or 360° indicating wind blowing north, 90° indicating wind blowing east, 180° indicating wind blowing south , 270° indicating wind blowing west.

below example data:

> dput(wind) structure(list(u_ms = c(-3.711, -2.2417, -1.8188, -1.6164, -1.3941,  -1.0682, -0.57611, -1.5698, -1.4976, -1.3537, -1.0901, -0.60403,  -0.70812, -0.49045, -0.39849, 0.17875, 0.48356, 1.5082, 1.4219,  2.5881), v_ms = c(-1.471, -1.6118, -1.6613, -1.7037, -1.7388,  -1.8748, -1.8359, -1.6766, -1.6994, -1.7505, -1.4947, -0.96283,  -1.1194, -0.6849, -0.7847, -0.80349, -0.19352, -0.97815, -1.0835,  -0.81666), u_rad = c(-0.064769155, -0.039125038, -0.031744042,  -0.028211496, -0.02433163, -0.018643603, -0.010055014, -0.027398173,  -0.026138045, -0.023626517, -0.01902583, -0.01054231, -0.012359023,  -0.008559966, -0.006954961, 0.003119775, 0.008439712, 0.02632305,  0.024816831, 0.045170857), v_rad = c(-0.025673788, -0.028131211,  -0.028995149, -0.029735168, -0.030347779, -0.032721426, -0.032042493,  -0.029262184, -0.029660119, -0.030551982, -0.026087431, -0.01680455,  -0.019537212, -0.011953758, -0.013695596, -0.014023543, -0.00337756,  -0.017071935, -0.018910639, -0.014253403)), .names = c("u_ms",  "v_ms", "u_rad", "v_rad"), class = "data.frame", row.names = c(na,  -20l)) 

i have used following code try , obtain wind direction (column td), not convinced returned angles want (i.e. 0°/360° indicating wind blowing north, 90° indicating wind blowing east etc…).

u = wind$u_rad # u component in radians v = wind$v_rad # v component in radians  d = (180/pi)*(atan2(u,v)) td = as.matrix(d + 180) df = cbind(wind, d, td)  > df        u_ms     v_ms        u_rad       v_rad         d        td 1  -3.71100 -1.47100 -0.064769155 -0.02567379 -111.6228  68.37716 2  -2.24170 -1.61180 -0.039125038 -0.02813121 -125.7164  54.28357 3  -1.81880 -1.66130 -0.031744042 -0.02899515 -132.4087  47.59129 4  -1.61640 -1.70370 -0.028211496 -0.02973517 -136.5062  43.49379 5  -1.39410 -1.73880 -0.024331630 -0.03034778 -141.2788  38.72124 6  -1.06820 -1.87480 -0.018643603 -0.03272143 -150.3269  29.67308 7  -0.57611 -1.83590 -0.010055014 -0.03204249 -162.5780  17.42199 8  -1.56980 -1.67660 -0.027398173 -0.02926218 -136.8842  43.11576 9  -1.49760 -1.69940 -0.026138045 -0.02966012 -138.6118  41.38819 10 -1.35370 -1.75050 -0.023626517 -0.03055198 -142.2844  37.71557 11 -1.09010 -1.49470 -0.019025830 -0.02608743 -143.8963  36.10365 12 -0.60403 -0.96283 -0.010542310 -0.01680455 -147.8980  32.10204 13 -0.70812 -1.11940 -0.012359023 -0.01953721 -147.6830  32.31699 14 -0.49045 -0.68490 -0.008559966 -0.01195376 -144.3939  35.60607 15 -0.39849 -0.78470 -0.006954961 -0.01369560 -153.0774  26.92258 16  0.17875 -0.80349  0.003119775 -0.01402354  167.4578 347.45783 17  0.48356 -0.19352  0.008439712 -0.00337756  111.8112 291.81121 18  1.50820 -0.97815  0.026323050 -0.01707193  122.9656 302.96561 19  1.42190 -1.08350  0.024816831 -0.01891064  127.3077 307.30771 20  2.58810 -0.81666  0.045170857 -0.01425340  107.5128 287.51279 

i appreciate advice on whether method correct, , if not how correctly obtain desired wind direction values. while calculating wind direction u , v components of wind using lapply or ifelse helpful, code did work data, , sure there easier away obtain wind direction. many thanks!

there 3 problems this:

1. you cannot convert m/s radians. in order input wind components atan2, must normalize them, don't multiplying m/s pi/180 (which did u_rad , v_rad). should make column of absolute windspeed (sqrt(u_ms^2 + v_ms^2)) , take atan2(u_ms/wind_abs, v_ms/wind_abs). (also note atan2 takes y component first - make sure that's want)
2. atan2 give answer in unit circle coordinates, increase counterclockwise , have 0 on x-axis. want answer in cardinal coordinates increase clockwise , have 0 on y-axis. convert unit circle cardinal coordinates, must subtract unit circle angle 90.
3. you must know whether wind info refers direction wind coming (standard cardinal coordinates) or direction wind blowing (standard trig/vector operations)

if given u_ms = = -3.711 , v_ms = -1.471 (on unit circle blowing down , left, coming northeast), then:

wind_abs = sqrt(u_ms^2 + v_ms^2) wind_dir_trig_to = atan2(u_ms/wind_abs, v_ms/wind_abs)  wind_dir_trig_to_degrees = wind_dir_trig_to * 180/pi ## -111.6 degrees 

then must convert wind vector meteorological convention of direction wind coming from:

wind_dir_trig_from_degrees = wind_dir_trig_to_degrees + 180 ## 68.38 degrees 

then must convert angle "trig" coordinates cardinal coordinates:

wind_dir_cardinal = 90 - wind_dir_trig_from_degrees [1] 21.62284 #from northeast.