Scientific communities can use Open Source projects to create standardized, tested, and free tools for their use, but such projects are only as good as the community around them. Here we explore the evolution of MetPy from a repository of graduate school code in 2008 to a National Science Foundation funded project with a strong community of users and growing community of contributors. Essential factors in this community-building include scoping, providing training materials, creating transition paths from the use of legacy tools, and creating a welcoming environment for new contributors. These principles of community building are broadly applicable across any scientific software project and are crucial to adoption and sustainability.

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