Frequently Asked Questions

What is the development status of the salem library?

As of today (August 2021), salem is used by several people (number unknown) and is used by at least one downstream larger project (OGGM). I plan to continue to maintain salem in the future, but cannot spend much time and energy in new, larger features that the community might need. These larger features (mostly: improved support for more datasets and improved plotting) should be carried out by better and more adopted projects (mostly: geoxarray and cartopy).

Salem is small but well tested for the cases I encounter in my daily work. I don’t think that salem will become a major library (there are so many great projects out there!), but I think it will be a useful complement for a few. For more information on my motivations to develop salem, see Why developing Salem? and this github discussion.

How should I cite salem?

If you are using salem and would like to cite it in academic publication, we would certainly appreciate it. We recommend to use the zenodo DOI for this purpose:

An example BibTeX entry:

  author       = {Fabien Maussion and
                  TimoRoth and
                  Johannes Landmann and
                  Matthias Dusch and
                  Ray Bell and
  title        = {fmaussion/salem: v0.3.4},
  month        = mar,
  year         = 2021,
  publisher    = {Zenodo},
  version      = {v0.3.4},
  doi          = {10.5281/zenodo.4635291},
  url          = {}

What others tools should I know about?

The python atmospheric sciences community is a bit spread between iris and xarray for N-Dimensional data handling. I find that xarray is very intuitive to learn thanks to its strong interaction with pandas.

Here are some tools that share functionalities with Salem:

  • cartopy is the reference tool for plotting on maps. Salem provides a way to plot with cartopy in addition to Salem’s homegrowm graphics. (see Graphics)

  • wrf-python provides much more WRF functionalities than salem. It is the recommended package to do computations with WRF output. Salem’s syntax is nicer than that of wrf-python, though.

  • Salem provides useful reprojection tools (see Map transformations). The transformation routines are quite fast (we use pyproj for the map transformations and scipy for the interpolation) but they are all done on memory (i.e. not adapted for large datasets). For large reprojection workflows you might want to have a look at cartopy and pyresample.

  • regionmask provides similar tools as salem’s region-of-interest functionalities if you are woking with shapefiles. regionmask seems a bit more general than Salem, but I’m not sure if it works with any map projection as Salem does.

  • In the future, I hope that pangeo-data will overtake most of the functionalities I need. But this is not going to happen tomorrow…

Several libraries are available to meteorologists and climatologists, but I don’t think they share much functionality with Salem: for example MetPy, windspharm, xgcm, aospy, and all the ones I forgot to mention.

Why developing Salem?

As an atmospheric scientist, I hate to have to take care about projections and maps. Salem was created to hide all these concerns. By the time I started, it seemed a good idea to provide map transformation tools without depending on GDAL (thanks to conda-forge GDAL is now much easier to install). It is still possible to do reprojection work in Salem using scipy and pyproj alone.

Furthermore, I use the atmospheric model WRF quite often in my work. Its output files are absolutely not compliant with the CF conventions. wrf-python is a great tool to manipulate WRF data with Python, but it also have several drawbacks (mostly: its syntax). Salem will be further developed with the WRF model in mind.

Why is my dataset not supported? (dataset Grid not understood error)

As salem gained in visibility, we started to get requests to support new dataset formats (see the corresponding github issue). While I am generally in favor of supporting new datasets, it will be impossible to support all of them in an automated manner. Here Ill try to explain why.

Salem works in the cartesian, map projection space. This means, it needs to understand the data’s map projection and the name of the eastings, northings coordinates in that projection. Most datasets (especially from older models) use their own (bad) naming convention for things, and these names and conventions have to hard-coded in salem. To my knowledge there is no automated parser of geospatial information in python: geoxarray is a currently staled attempt to do so.

Salem doesn’t make use of the 2D lon/lat coordinates on the globe (when it does it’s just for testing). Working in the projected space has several advantages, mostly for performance and precision reasons.

Note that some people don’t agree with this view, and don’t care about the projection of their data as long as they have access to the 2D lon/lat coordinates. xarray (with cartopy) can plot data based on their 2D coordinates, and xesmf performs regridding on the globe without worrying about map projections. These tools are maybe the right tools for you!

What’s this “.salem_cache” directory in my home folder?

At the first import, Salem will create a hidden directory called .salem_cache in your home folder. It will be used to download Salem’s demo files and standard shapefiles. This directory is also used by joblib to store the result of slow operations such as reading and transforming shapefiles, or downloading google maps from the internet. The cache should not become too large, but if it does: simply delete it.