Weather was one of those features where the hard part was deciding what it should not do.
There are five kinds: clear, cloudy, rain, thunderstorm and fog. One weather per day with soft transitions, and about one day in four it turns over at midday. The phone has a Weather app with today and the next three days, and the forecast is always right, because everything is worked out from a single number per save. That also means a co-op guest can work out the host's sky on their own machine without anything being sent over the network.
The thing it does not do: change the rules. Weather in GUNS INC. is look only. No penalties for driving in the rain, no customer who stays at home because of fog. I imagined players planning their production around a forecast, and it felt like homework rather than fun.
So the weather is there for mood, and mood it does well. The sky really closes over on grey days, rain stops under a roof, the tarmac darkens when it gets wet, and since October the streets have puddles along the kerbs and in the wheel tracks.

