If you feel like having more weather effects like snow, fog... try these triggers... if you dont, you can just leave out the ones you dont want:
Rain
Events
Player - Player 1 (Red) types a chat message containing rain as An exact match
Conditions
Actions
Environment - Turn TheEffect Off
Environment - Create at (Playable map area) the weather effect Ashenvale Rain (Heavy)
Set TheEffect = (Last created weather effect)
Environment - Turn (Last created weather effect) On
snow
Events
Player - Player 1 (Red) types a chat message containing snow as An exact match
Conditions
Actions
Environment - Turn TheEffect Off
Environment - Create at (Playable map area) the weather effect Northrend Snow (Heavy)
Set TheEffect = (Last created weather effect)
Environment - Turn (Last created weather effect) On
Sun
Events
Player - Player 1 (Red) types a chat message containing sun as An exact match
Conditions
Actions
Environment - Turn TheEffect Off
You probably already know how, but just in case... you need to create a weather effect variable to run these triggers. You do this in the trigger editor. Its under variables in the "edit" drop-down menu. All you need to do is create a new variable of type weather effect. In the triggers from above i named mine TheEffect, but you can set any name you want.