Data from Personal Weather Stations called senseBoxes.
openSenseMap API is a free Weather API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.

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Yes. The openSenseMap API is listed in the Weather category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The openSenseMap API requires no authentication, so you can start making requests without signing up.
Yes. The openSenseMap API supports CORS, so front-end JavaScript can call it directly.
Yes. The openSenseMap API is served over HTTPS, so it's safe to use in production.