So I purchased the Seasons EP yesterday, started new game, set starting season to Fall (when asked), went to Options and set season length to 4 weeks (so I would have time to explore all that each sea...
There is weather controll machine, which can help you clear the sky and change seasons, once upgraded. On side note, try going out more with your sims and visit different worlds. Will change the scenary and imporove the gameplay in general.