Marble has a weather plugin to show weather stations on the map. 
Currently two services are implemented, a "Fake" service and another one 
for bbc.co.uk. Your task is to add another service which uses geonames.org [1].

Checkout Marble and compile and install it (see edu.kde.org/marble)
Study the code of the existing weather plugin (in src/plugins/render/weather)
Add another weather service for the geonames.org one similar to the existing 
one for BBC. Note that there are other Marble plugins that already use 
other geonames services (e.g. earthquakes). Having a look at them might be 
helpful and save you some time
You can get help via IRC in #marble in Freenode or using the 
marble-devel@kde.org mailing list (and also ask here, of course)
 

[1] http://www.geonames.org/export/JSON-webservices.html#weatherJSON