XPath will be a big help when parsing XML. Looks like hpricot has support for it, so it's incredibly easy.
The XPath expression to extract the data attribute inside a city element is as follows:
/forecast_information/city/@data
The expression says, find the attribute named data (that's what the @ sign means) inside the element named city, which is in turn inside the element named forecast_information.
Now, the XML you linked on google.ru is more complicated than the example you posted here. To extract the same information from it, use this expression:
//city/@data
This expression says, find the attribute named data inside the element named city, no matter where city is in the source XML.