# Calculating wind direction from CARRA u, v grids

**URL:** <https://forum.ecmwf.int/t/calculating-wind-direction-from-carra-u-v-grids/1869>\
**Category:** Datasets and Usage\
**Tags:** carra\
**Created:** [15 February 2024 12:42 UTC](https://forum.ecmwf.int/t/calculating-wind-direction-from-carra-u-v-grids/1869 "2024-02-15T12:42:34Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Talfan\_Barnie](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/t/f9ae1b/32.png) [@Talfan\_Barnie](https://forum.ecmwf.int/u/Talfan_Barnie)\
**Post date:** [15 February 2024 12:42 UTC](https://forum.ecmwf.int/t/calculating-wind-direction-from-carra-u-v-grids/1869/1 "2024-02-15T12:42:34Z")

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Hi everyone,

I’m trying to calculate wind direction from u and v wind components following the instructions in section 5.3 here:

[https://confluence.ecmwf.int/display/CKB/Copernicus+Arctic+Regional+Reanalysis+(CARRA)%3A+Data+User+Guide#CopernicusArcticRegionalReanalysis(CARRA):DataUserGuide-Variablesat10-metreheight](https://confluence.ecmwf.int/display/CKB/Copernicus+Arctic+Regional+Reanalysis+%28CARRA%29%3A+Data+User+Guide#CopernicusArcticRegionalReanalysis(CARRA):DataUserGuide-Variablesat10-metreheight)

I can follow the steps and run the python code to get the grid of alpha values, but I’m not sure how to interpret equation (4). Is the variable U (capital u) the wind speed from equation 3? And I’m confused as to the meaning of V (capital v) ?

Many thanks for your help! And apologies if I’m missing something obvious,

Best wishes,

Talfan

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**Author:** ![Talfan\_Barnie](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/t/f9ae1b/32.png) [@Talfan\_Barnie](https://forum.ecmwf.int/u/Talfan_Barnie)\
**Post date:** [19 February 2024 14:46 UTC](https://forum.ecmwf.int/t/calculating-wind-direction-from-carra-u-v-grids/1869/2 "2024-02-19T14:46:11Z")

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I see the equation has been updated and makes more sense now, many thanks for looking into it!

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**Author:** ![Andras\_Horanyi](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/a/ecd19e/32.png) [@Andras\_Horanyi](https://forum.ecmwf.int/u/Andras_Horanyi)\
**Post date:** [20 February 2024 10:29 UTC](https://forum.ecmwf.int/t/calculating-wind-direction-from-carra-u-v-grids/1869/3 "2024-02-20T10:29:54Z")

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Dear Talfan,

Indeed, we had a discussion on this issue behind the scenes and we realised that the original details were not correct (in the original equation it should have been U instead of V and the link to the code for computing alpha was not correct neither).

For any case as you have spotted yesterday, we updated the Data User Guide, see at [Copernicus Arctic Regional Reanalysis (CARRA): Data User Guide - Copernicus Knowledge Base - ECMWF Confluence Wiki](https://confluence.ecmwf.int/display/CKB/Copernicus+Arctic+Regional+Reanalysis+%28CARRA%29%3A+Data+User+Guide#CopernicusArcticRegionalReanalysis(CARRA):DataUserGuide-Variablesat10-metreheight).

We hope this is indeed clearer and precise.

Thank you very much for spotting this issue.

Best regards

Andras Horanyi