The idea is simple (and simple to do I think ...).
several weathers :
_ Sunny (as now)
_ cloudy (as now but with less light)
_ rainy (cloudy + a rain picture on the screen)
_ snowy (cloudy + a snow picture on the screen, only if about 50% at least is snow tiles).
and could be added :
_ wind (will make rain and snow pictures scroll from left to right of the screen or Right to left)
_ stormy (rainy or cloudy + flash)
I think it would make the game much less monotonous.
Related topic: The Equatorial Wind (http://forum.simutrans.com/index.php?topic=984.0).
Not much related. This is about weather visual effects, while that is about weather effect on the game world.
some clouds could be faked as moving ground objects. this could be a way to go, IMO
I did some tests with clouds for pak64 (clouds for 0.100 (http://archive.forum.simutrans.com/topic/08091.0/index.html)) and the results were pretty good.
The connection that I see is that if ST shows weather conditions, they should match the climate of the region that is shown, that is, different weather conditions should exist on the map at the same time.
Climate zones (as implemented in ST) are currently attached to height levels, but that need not stay this way.