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Seasons were originally worked on in (I believe) 1999 (or there abouts) by a UO Dev named Pandemonium, however only the Spring version was completed and released as a test client (also the same test clients that added in the "big window" (800x600 game window) and blackspace, colored lighting, and dark nights).
The system never made it into the game, however the Spring season DID make it in as the terrain style of Trammel.
The second quasi-attempt was the snow cover in 2006.
In KR, for a time there was rain and cloud effects for weather, but those have been taken out (I'd personally like to see them put back in though).
I've long been a proponent of small (small meaning that they're not really active ingame content, just background stuff) things in a game that help add to the "life" of the game. Some game dev teams understand these little things, others tend not to.
Honestly with UO, I think they try to do so many other things that are considered either more important or more "active" that environmental effects continually take a back seat.
Sucks that that seems to be how it is because once they were done, it'd be a HUGE plus to UO to have a coherent world with seasonal effects rather than the zone-based independant sectors like you see in games like WoW where the climate changes inconsistently between one area and the next.
Granted in UO terms (specifically the Britannian facets (Trammel/Felucca)), the world only consists of the Northern Hemisphere (based on climate distribution seen ingame and disregarding the map-wrap physics weirdness), but even just that is more coherent than what you see in some other games.
