Hello fellow modders, I'm creating a new world type at the moment of just a desert and like we all know there isn't much downfall in hot arid deserts but what there is are Sand Storm and thought it might be cool to add that as a custom downfall rather than have rain or snow. where would I start to create a new downfall?
Was just going to have like heavy fog, with a color to it for now

Well there isn't anything really that special about a weather system. You just code what you want to happen and when. So in this case I assume you'd have some randomness for the starting and stopping of the sandstorm so you'd use some sort of world or server tick method and on the server side you'd check to see if it should start or stop the storm. Then you would need to notify all the clients using a packet about the state of the weather. Then on the client you'd either use an overlay render event or the fog density event and add your fog effect.

 

Basically the state of the weather would be determined on the server, probably with some randomess, it would tell all the clients and the clients would use that information to control the fog or other effects.

I'd say look at the vanilla implementation of rain/snow etc and how its applied to biomes. Try and find some other peoples code that does similar things (theres a couple mods in FTB that have custom weather, don't know which ones though)