Weather effects

Weather effects are ambient graphics that appear in-game that simulate real weather. In Patch 1.10 rainfall, snowstorms and sandstorms were added to various zones in World of Warcraft. For several years after being added, weather had no gameplay effect. With the introduction of the Pet Battle System in Mists of Pandaria, some capturable battle pets now only appear during specific weather.

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 1.10.0. 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 1.10.0.
 * Sandstorms - added in Patch 1.10.0.
 * 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.

Kalimdor and Eastern Kingdoms
Weather currently occurs the following areas:


 * Arcane Storms
 * Azuremyst Isle (Arcane energy bolts)


 * Blood Rain
 * Scarlet Enclave


 * Duststorms
 * Badlands


 * Rain
 * Ashenvale
 * Arathi Highlands
 * Arathi Basin
 * Azshara
 * Darkshore
 * Darnassus
 * Desolace
 * Deadwind Pass
 * Durotar
 * Dustwallow Marsh (can be triggered in Theramore for the duration of the Take Down Tethyr! event)
 * Duskwood
 * Eastern Plaguelands (including Stratholme)
 * Elwynn Forest
 * Feralas
 * Hillsbrad Foothills
 * Hinterlands
 * Loch Modan
 * Mulgore (including Thunder Bluff, but not at the same time)
 * Redridge Mountains
 * Silverpine Forest
 * Stranglethorn Vale (including Zul'Gurub)
 * Teldrassil
 * Tirisfal Glades (including Scarlet Monastery)
 * Tol Barad
 * Un'Goro Crater
 * Westfall
 * Wetlands


 * Sandstorms
 * Silithus (including Ahn'Qiraj)
 * 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.


 * Tanaris
 * Uldum


 * Snow
 * Alterac Mountains
 * Dun Morogh
 * Winterspring

Outland

 * Arcane Storms
 * Netherstorm


 * Rain
 * Zangarmarsh

Northrend

 * Rain
 * Emerald Dragonshrine in Dragonblight
 * Sholazar Basin
 * Ulduar (Rain will automatically start to fall nearing phase 2 of the Freya encounter)


 * Snow
 * Dragonblight
 * Storm Peaks

Pandaria

 * Rain
 * Isle of Thunder
 * The Jade Forest
 * Valley of the Four Winds


 * Snow
 * Mount Neverest in Kun-Lai Summit

Draenor
Rain


 * Gorgrond
 * Shadowmoon Valley
 * Talador (Zangarra)
 * Tanaan Jungle

Blood Rain


 * Spires of Arak (Sethekk Hollow, Lost Veil Anzu)

Snow


 * Frostfire Ridge

Fog


 * Shadowmoon Valley (Umbrafen)

Ash


 * Tanaan Jungle

Broken Isles (and Argus)
Rain


 * Azsuna
 * Eye of Azshara
 * Highmountain
 * Stormheim

Arcane Sparkles


 * Suramar City

Snow


 * Highmountain (Mountains)

BfA Zones (Kul Tiras, Zandalar, Nazjatar, Island Expeditions, etc)
Rain


 * Nazmir
 * Un'gol Ruins
 * Zuldazar

Blood Rain


 * Nazmir

Snow


 * Dread Chain

Sandstorm


 * Un'gol Ruins
 * Vol'dun

Fog


 * Skittering Hollow
 * Zuldazar

Ash


 * Molten Cay

Coding details
"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

 * 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.