Abigail is right ... you sould allways check the result of a call to open. I suspect that either your script isn't being run in the same working directory as your weather.js file, or your script isn't being run by a UID that has permission to open that file -- especially on that open to write call.

PS: You realize you have a "+" in the URL you are trying to fetch -- you probably dont' want that there, that URL doesn't seem to work unless you delete it.