# ERA5 netcdf encoding since the new CDSAPI release

**URL:** <https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146>\
**Category:** Feedback and requirements\
**Tags:** api, cds, era5\
**Created:** [2 December 2024 06:11 UTC](https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146 "2024-12-02T06:11:03Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jerome\_LEFEVRE](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/j/b19c9b/32.png) [@jerome\_LEFEVRE](https://forum.ecmwf.int/u/jerome_LEFEVRE)\
**Post date:** [2 December 2024 06:11 UTC](https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146/1 "2024-12-02T06:11:03Z")

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Hi,  
I noticed that the result of requests in netCDF format for ERA5 product (reanalysis-era5-single-levels) are now declared in double precision. Before, the results were in short, with add\_offset and scale\_factor to convert values (done smoothy by a lot of softs, ferret ncview, xarray etc)  
In my mind, that change have 2 inconvenients :

- increasing the downloading time,
- increasing disk usage on both side, ECMWF, and on my side  
I understand that the new cdsapi extract data from a pool in grib format. But why to drop out the previous encoding in netCDF format ?  
Thank you

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**Author:** ![Anton\_Driesse](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/a/ac91a4/32.png) [@Anton\_Driesse](https://forum.ecmwf.int/u/Anton_Driesse)\
**Post date:** [15 March 2025 09:59 UTC](https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146/2 "2025-03-15T09:59:33Z")

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At first glance, there are now a lot of extra insignificant digits in the data, which are essentially noise, and do not compress well.

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**Author:** ![Theo\_Mathurin](https://forum.ecmwf.int/user_avatar/forum.ecmwf.int/theo_mathurin/32/1228_2.png) [@Theo\_Mathurin](https://forum.ecmwf.int/u/Theo_Mathurin)\
**Post date:** [6 May 2025 15:24 UTC](https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146/3 "2025-05-06T15:24:25Z")

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Another issue is the poor reading performance that results from the chunk sizes chosen.

I point to these two posts raising this issue: [Specifying the chunk size for netCDF files](https://forum.ecmwf.int/t/specifying-the-chunk-size-for-netcdf-files/8305) and [Huge performance drop when reading new ERA5 netcdf4 format](https://forum.ecmwf.int/t/huge-performance-drop-when-reading-new-era5-netcdf4-format/12831)

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**Author:** ![Xiaofeng\_Li](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/x/8491ac/32.png) [@Xiaofeng\_Li](https://forum.ecmwf.int/u/Xiaofeng_Li)\
**Post date:** [11 November 2025 08:48 UTC](https://forum.ecmwf.int/t/era5-netcdf-encoding-since-the-new-cdsapi-release/8146/4 "2025-11-11T08:48:41Z")

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I totally agree with you. Especially when I try to update these nc data into the newest years, I surprised found that there data metadata formats are not the same. Why EAR5 change their nc data formats into current one? It definitely not covenant than before. Please change back.