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

Question

I have U and V wind component data and I would like to calculate wind direction from these values in R. I would like to end up with wind direction data on a scale of 0-360 degrees, with 0° or 360° indicating a wind blowing to the north, 90° indicating a wind blowing to the east, 180° indicating a wind blowing to the south and 270° indicating a wind blowing to the west. Below is some example data: ``` wind <- read.table(header = TRUE, text = " u_ms v_ms u_rad v_rad -3.711 -1.471 -0.064769155 -0.025673788 -2.2417 -1.6118 -0.039125038 -0.028131211 -1.8188 -1.6613 -0.031744042 -0.028995149 -1.6164 -1.7037 -0.028211496 -0.029735168 -1.3941 -1.7388 -0.02433163 -0.030347779 -1.0682 -1.8748 -0.018643603 -0.032721426 -0.57611 -1.8359 -0.010055014 -0.032042493 -1.5698 -1.6766 -0.027398173 -0.029262184 -1.4976 -1.6994 -0.026138045 -0.029660119 -1.3537 -1.7505 -0.023626517 -0.030551982 -1.0901 -1.4947 -0.01902583 -0.026087431 -0.60403 -0.96283 -0.01054231 -0.01680455 -0.70812 -1.1194 -0.012359023…

Accepted Answer

There are three problems with this:

- You cannot convert m/s to radians. In order to input wind components into `atan2`, you must normalize them, but you don't do this by multiplying m/s by `pi/180` (which you did to get `u_rad` and `v_rad`). You should make a column of absolute windspeed (`sqrt(u_ms^2 + v_ms^2)`) and take `atan2(u_ms/wind_abs, v_ms/wind_abs)`. (also note that atan2 takes y component first - make sure that's what you want)

- `atan2` will give you an answer in the unit circle coordinates, which increase counterclockwise and have a zero on the x-axis. You want an answer in cardinal coordinates which increase clockwise and have a zero on the y-axis. To convert unit circle to cardinal coordinates, you must subtract the unit circle angle from 90.

- You must know whether the wind info refers to the direction the wind is coming from (standard for cardinal coordinates) or the direction the wind is blowing (standard for trig/vector operations)

If you are given `u_ms = = -3.711` and `v_ms = -1.471` (on the unit circle it is blowing down and slightly to the left, so it is coming from the 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 you must convert this wind vector to the meteorological convention of the direction the wind is coming from:

```
wind_dir_trig_from_degrees = wind_dir_trig_to_degrees + 180 ## 68.38 degrees
```

Then you must convert that angle from "trig" coordinates to cardinal coordinates:

```
wind_dir_cardinal = 90 - wind_dir_trig_from_degrees
[1] 21.62284 #From the northeast.
```

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

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