Provide weather information, earthquake information, and climate data.
Hong Kong Obervatory API is a free Weather API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Observatory’s data sets released on Government Open Data Portal. An Open Data Application Programming Interface (API) and the related documentation are available to facilitate retrieval of required data.
Details from hko.gov.hk.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Portuguese weather and climate data
Radar-detected hail history for any US address from NOAA NEXRAD Level-III hail detections, by year
Multi-model weather: GFS, ECMWF AIFS/IFS and a 31-member GEFS ensemble, keyless point forecasts
Yes. The Hong Kong Obervatory 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 Hong Kong Obervatory API requires no authentication, so you can start making requests without signing up.
CORS support for the Hong Kong Obervatory API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Hong Kong Obervatory API is served over HTTPS, so it's safe to use in production.