Update – 27 July 2016 – the alternative URL stopped working in mid-April, however you can get weather data via a simple plain, unsigned YQL GET request. See my example.
As indicated in this Reddit thread as well as numerous blogs this past week, Yahoo! have changed their XML weather feed (which provides get current observations and a forecast for the city of your choice) from an open service that responds to simple HTTP GET requests to a locked-down API using OAuth.
Unfortunately – though entirely predictably, given it’s widespread use – this has broken things all over the place. It doesn’t help that the Yahoo Developer blog hasn’t been updated since July 2013.
Fortunately there’s a quick workaround by changing the URL if you don’t have time (or the skills, or the suitable web-hosting) to setup OAuth.
Old broken URL for London:
New working URL, no OAuth required: Now doesn’t work either
Caveat: Clearly there’s no guarantee whether [sic] this will continue to work.
Other options: OpenWeatherMap looks promising.
Opinion: A public weather feed shouldn’t require authentication. I mean, it’s the weather. Caching? Yes. Rate limiting? Perhaps. API keys? No (or at least, not a legacy service with a single endpoint.)
Also, if you’re providing a service for free, great, but it’s probably worth also creating a blog or mailing list for announcements and encouraging everyone to sign up to it.
Further light reading (if a little tangential): Adactio on Digital Preservation (2012)