Weather effects are ambient graphics that appear in-game that simulate real weather. They have no mechanical effects except triggering spawns on certain rare battle pets . In Patch 1.10 rainfall, snowstorms and sandstorms were added to various zones in World of Warcraft.
Current Weather Effects
- Clouds - varies over time.
- Fog - apart from obscuring one's view, has no mechanical effects.
- Lightning - appears in some zones (like the Blasted Lands), but currently has no mechanical effect as a weather phenomenon.
- Snow - can be seen on the ground and on trees in some zones, and now also as precipitation since Patch 1.10. In such zones, you can see a character's breath, much like you would in real life in a cold environment.
- Rainfall - added in Patch 1.10.
- Sandstorms - added in Patch 1.10.
- Snowstorms - added in Patch 2.3.2, possibly to showcase heavy snow that may be in WotLK. This is a more intense form of snowfall with visible heavy wind blowing snowflakes in each and every direction.
Zones with weather
Kalimdor and Eastern Kingdoms
Weather currently occurs the following areas:
- Arcane Storms
- Azuremyst Isle (Arcane energy bolts)
- Blood Rain
- Arathi Highlands
- Arathi Basin
- Deadwind Pass
- Dustwallow Marsh (can be triggered in Theramore for the duration of the event)
- Eastern Plaguelands (including Stratholme)
- Elwynn Forest
- Hillsbrad Foothills
- Loch Modan
- Mulgore (including Thunder Bluff, but not at the same time)
- Redridge Mountains
- Silverpine Forest
- Stranglethorn Vale (including Zul'Gurub)
- Tirisfal Glades (including Scarlet Monastery)
- Un'Goro Crater
- Note: A sandstorm starts automatically when you engage Ossirian the Unscarred in the Ruins of Ahn'Qiraj and does not stop once the encounter is over.
- Arcane Storms
- Emerald Dragonshrine in Dragonblight
- Sholazar Basin
- Ulduar (Rain will automatically start to fall nearing phase 2 of the Freya encounter)
"Instead, each sub-zone among the 11 where weather is currently available is now linked to a set of variables that allow for the possibility of precipitation. The variables have been set to make the potential weather appropriate, meaning heavy showers are more likely in one region and drizzles in another. As Kaplan described it, the zone is practically asking itself whether there will be weather. "It's sort of randomly picking: 'I'm ready to rain but do I want to rain?' " he said. Duration will be chosen based on an automated system as well, with enough variables to make things unpredictable. "We didn't want it to be such a mathematical formula that somebody could literally sit with a stopwatch, log in, go to a zone and go, 'OK in two, one, go — it's going to rain.' "
Possible future inclusion
- Raining blood - possible that gamers who kill Hakkar, the blood god, will cause a rain of blood.
- Volcanic ash - a storm of ashes surrounding in-game volcanoes.
- Monsoon rainfall - the developers may add an intense monsoon everywhere for about a week.
- Christmas Snow - on Christmas it might snow everywhere all at once. 
- Wind - does not currently appear noticeably in-game.
Possible future "realistic" weather effects
The effects in-game do not affect gameplay, but in some games can have an effect that influences gameplay beyond aesthetics:
- Rain or snow could extinguish fire arrows or thrown bombs in flight randomly or put out cooking fires.
- Fog, sandstorms or wind could reduce the accuracy of ranged weapons.
- Lightning could cause nature damage if struck or to nearby player characters or mobs. Perhaps even doing more damage to mail or plate wearers.
- Snow could lessen the effects of fire damage or amplify the effects of frost damage.
- Characters not wearing appropriate clothing when it snows could die due to exposure.
- Kaplan has seen gamer requests for rain that isn't just cosmetic but has the effect of rusting armor and muddling travel. Those are even the types of features promised in the upcoming MMO "Dark and Light," whose Web site says winds will slow winged characters and accumulating snow will stymie dwarves. Kaplan is not into that kind of thing. "There's already an oppressive feel when the lightning comes in and the rain starts to hit," he said. "I think that's enough that we don't need to straight-up slow gameplay down." That isn't to say Blizzard isn't considering weather that has effects. "If we ever do add things that would tie directly into the weather, we'd rather do something more as a bonus," Kaplan said. "Like perhaps more herbs grow after it rains."