Weather
The weather on your world is determined by the current season (according to the game-date) and the values you use in the 'Landscape' weather settings. There is one setting for each season.. so, for instance, you can make it always snowy in winter and always sunny in summer.
These are the values that can be used for each of the 4 seasonal weather settings..
0 - Summer, no rain
1 - Spring/Autumn Cycle - bit of rain, some clear days
2 - Summer cycle 2 - mostly sun, one day of rain in 10
3 - Winter Cycle 1 - Clouds and snow
4 - Spring Cycle 2 - light cloud and showers
5 - Permanent rain
6 - Permanent snow
7 - Permanently black sky
8 - Permanently foggy
9 - Long-days summer cycle 3 - never rains
10 - Long-days summer cycle 4 - rains 1 day in 15
11 - Long days winter cycle 2 - bright and often snowy
*advancedate [days] command) then set the appropriate weather value in the Winter Weather slot.