Yahoo weather API at https://developer.yahoo.com/weather/#get-started has a link under 'Get your API Keys' there is a button named 'Creat an App' that points to https://developer.yahoo.com/apps/create/.
Both in Chrome and IE11 when I click on it I get "An internal error occurred (eid 3030034)."
Yahoo forum requests that user us SO with tag #yahoo-weather-api
Thanks!