Weather differs on different planets at different times. It can affect a number of things.
Empire at War[]
In Star Wars: Empire at War the weather effects in infantry's effectiveness.
Clear[]
Clear weather has no effect on units.
Sandstorm[]
The sandstorm icon from the information screen in Forces of Corruption.
Sandstorms reduce rocket accuracy.
Snowstorm[]
Snowstorms reduce Vehicles sight range.
Heavy rain[]
Heavy rain reduces laser accuracy.
Rain of ash[]
Ash reduces infantry sight range.