Weather can add something different to a wargame. A sudden storm can slow a winning side down enough for the defense to recover and re-form. The only problem is that simplifying the conditions so that they are playable and not a nuisance to track is tricky.
I've settled on a system that is composed of two elements: Weather and Environmental Conditions. Weather is either Clear, Cloudy. Raining, or Storming. This can effect the environmental conditions. This also effects line of sight and missile fire. Environmental conditions are Fine, Wet, and Mud. These mostly effect movement.
The Weather can change up and down (from clear to cloudy, cloudy to raining, raining to storming and back in the other direction) at the start of a new turn. If the weather is raining or storming at the start of a turn, there is a chance that the environmental conditions will change from Fine to Wet or from Wet to Mud. As soon as it is storming, the weather changes to the next worse type immediately.
Unlike weather which can gravitate from stormy to clear, once Environmental Conditions change for the worse, they remain that way for the remainder of the scenario.
I'll fight a battle using this as a template and post some pics and commentary sometime soon and the chart I'm using to track the effects for the game.