The signal is so weak, anyway, that's it's practically impossible to decide cause and affect. I think it's mentioned in that article that they start with an are

The signal is so weak, anyway, that's it's practically impossible to decide cause and affect. I think it's mentioned in that article that they start with an area that's already got some rain or snow clouds and try to boost their output by a few points and hope that a trend emerges over time. Also, they do these on a small local scale, like spraying this morning to boost the rain this afternoon.