This URL contains special characters, which Ruby can't handle:

>> URI.parse 'http://www.yr.no/sted/Finland/Västra_Finland/Askainen/varsel.xml'
URI::InvalidURIError: bad URI(is not URI?): http://www.yr.no/sted/Finland/Västra_Finland/Askainen/varsel.xml
    from /usr/local/lib/ruby/1.8/uri/common.rb:436:in split'
    from /usr/local/lib/ruby/1.8/uri/common.rb:485:in parse'
    from (irb):5

Your browser can probably open the URL. To fix this error encode the URL before handing it to the parse method:

URI.parse(URI.encode('http://www.yr.no/sted/Finland/Västra_Finland/Askainen/varsel.xml'))
=> #<URI::HTTP:0x18bfb40 URL:http://www.yr.no/sted/Finland/V%C3%A4stra_Finland/Askainen/varsel.xml>

You can use helpers in your model by first including them:

class Post < ActiveRecord::Base
  include ActionController::UrlWriter
  def to_uri
     post_path(self)
  end
end

Some might consider this heresy, I don't. In my opinion it's similar to the to_param method. You could of course move it to a delegate class or something similar.

def url_join(*parts):
    # This does not work with many parts:
    # import urllib.parse
    # urllib.parse.urljoin(*parts)
    return os.path.join(*parts)