An online open and crowdsourced weather service. People set up automatic weather stations (which are fairly cheap) and contribute measurements that are aggregated into forecasts. Has an API so you can pull data out of it as well as contribute it: http://openweathermap.org/appid Free accounts are, of course, limited in several ways. You can also get weather maps of various kinds from the service to visualize the forecast data. Forecast data is in XML, JSON, and HTML formats.
There is also an air pollution API: https://openweathermap.org/api/air-pollution
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