A free, open, and documented weather forecast API, built as a compatible alternative to the Dark Sky API.
Weather forecasts are primarily determined using models run by government agencies, but the outputs aren't easy to use or in formats built for applications. To try to address this, I've put together a service (built on AWS Lambda) that reads public weather forecasts and serves it following the Dark Sky API style. It is not a reverse engineering of the API, since their implementation relies on radar forecasts for minutely results, as well as a few additional features. The API aims to return data using the same json structure as what Dark Sky uses.
Free API keys are capped at 20,000 API calls per month (once every 15 minutes).
Open-Meteo offers free weather forecast APIs for open-source developers and non-commercial use. No API key is required. Pulls its data from the weather services of multiple countries. 7 day forecasts updated every three hours with a maximum resolution of 2km. All data is licensed CC-By-NC v4.0.