Hawzah News Agency – Ground that is impure becomes pure if rain falls on it; and if rainwater begins to flow on the ground and while it is still raining it comes into contact with an impure area under a roof, it purifies that area as well.

If impure soil is completely soaked by rainwater, it becomes pure on condition that it is not known whether or not the water has turned into mixed water by means of it coming into contact with soil.

Whenever rainwater collects in a place – even if its quantity is less than kurr (Kurr water is an amount of water that fills a container which has dimensions [i.e. length, breadth, and depth] totaling thirty-six hand spans, and this is equivalent to approximately 384 liters) – in the event that an impure object is washed in it while it is raining and the water does not acquire the smell, color, or taste of the impurity, the impure object becomes pure.

Source: https://www.sistani.org/