<p>Essentially I would like to know how current direction "DIR C" and wind direction "WDIR" for gribs is computed.</p>

<p>I'm trying to compute some angles from u and v vectors in grib files. My understanding from <a href="https://www.eol.ucar.edu/content/wind-direction-quick-reference" rel="nofollow noreferrer">this link</a> is that:</p>

<p>Positive U component represents wind blowing to (not from) the East (90 degrees). And positive V component is wind to (not from) the North (0 degrees). Is this correct?</p>

<p>In grib files values UGRD &amp; VGRD are stored this way. How does this apply to currents like UOGRD and VOGRD? Is the same convention used as for UGRD and VGRD?</p>

<p>For example, <em>assuming the vectors are defined the same</em> (UGRD and UOGRD) I'm calculating current headings like this from the GRIB data:</p>

<pre><code>$angle = round(atan2($u, $v) * 180 / pi());
if ($angle &lt; 0) {
    $angle = 360 + $angle;
}
</code></pre>

<p>And wind like this:</p>

<pre><code>$angle = round(atan2(-1*$u, -1*$v) * 180 / pi());
if ($angle &lt; 0) {
    $angle = 360 + $angle;
}
</code></pre>

<p>The current output looks like this (using u &amp; v values shown):</p>

<pre><code>u=1  v=1 45° NE
u=-1 v=1 315° NW
u=1 v=-1 135° SE
u=-1 v=-1 225° SW
</code></pre>

<p>And wind looks like with the vector being "from" not to:</p>

<pre><code>u=1 v=1 225° SW
u=-1 v=1 135° SE
u=1 v=-1 315° NW
u=-1 v=-1 45° NE
</code></pre>

<p>Does this look correct?</p>

<p>Note:  This atan2(y,x) function calculates the arc tangent of the two variables x and y. It is similar to calculating the arc tangent of y / x, except that the signs of both arguments are used to determine the quadrant of the result. </p>

CC BY-SA 3.0
2017-04-08T04:44:30.620Z
@gansub But my question also is about how u and v are defined for the current layers.
user6972
CC BY-SA 3.0
2017-04-11T08:44:07.317Z
I worked upon a very simple coupled ocean-atmosphere model code in a graduate course once, and don't remember there being any difference in definition.  Convinced you have the idea right for u and v in the atmosphere (u = 1, v = 1 is wind from the SW).  You can always compare current results to https://earth.nullschool.net/ which gives both wind and current animations.
JeopardyTempest
<meteorology><wind><weather-forecasting><ocean-currents>
Do U & V vectors differ depending on GRIB layers (wind vs current)?
user6972
# Do U & V vectors differ depending on GRIB layers (wind vs current)?

(Asked by: username_0 on 2017-04-07)

Essentially I would like to know how current direction "DIR C" and wind direction "WDIR" for gribs is computed.

I'm trying to compute some angles from u and v vectors in grib files. My understanding from [this link](https://www.eol.ucar.edu/content/wind-direction-quick-reference) is that:

Positive U component represents wind blowing to (not from) the East (90 degrees). And positive V component is wind to (not from) the North (0 degrees). Is this correct?

In grib files values UGRD & VGRD are stored this way. How does this apply to currents like UOGRD and VOGRD? Is the same convention used as for UGRD and VGRD?

For example, *assuming the vectors are defined the same* (UGRD and UOGRD) I'm calculating current headings like this from the GRIB data:

```
$angle = round(atan2($u, $v) * 180 / pi());
if ($angle < 0) {
    $angle = 360 + $angle;
}

```

And wind like this:

```
$angle = round(atan2(-1*$u, -1*$v) * 180 / pi());
if ($angle < 0) {
    $angle = 360 + $angle;
}

```

The current output looks like this (using u & v values shown):

```
u=1  v=1 45° NE
u=-1 v=1 315° NW
u=1 v=-1 135° SE
u=-1 v=-1 225° SW

```

And wind looks like with the vector being "from" not to:

```
u=1 v=1 225° SW
u=-1 v=1 135° SE
u=1 v=-1 315° NW
u=-1 v=-1 45° NE

```

Does this look correct?

Note: This atan2(y,x) function calculates the arc tangent of the two variables x and y. It is similar to calculating the arc tangent of y / x, except that the signs of both arguments are used to determine the quadrant of the result.

**Tags:** meteorology, wind, weather-forecasting, ocean-currents

> **Comments:**
> **username_0**: @gansub But my question also is about how u and v are defined for the current layers.  
> **username_1**: I worked upon a very simple coupled ocean-atmosphere model code in a graduate course once, and don't remember there being any difference in definition.  Convinced you have the idea right for u and v in the atmosphere (u = 1, v = 1 is wind from the SW).  You can always compare current results to https://earth.nullschool.net/ which gives both wind and current animations.  

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## 0 answers

