It's absolute rubish!!!! Light cannot by any means trigger rain!!
However,there are various devices out there which can!!They are patented by various people and governments and it is even against geniva convention to do so....yet in countries they still do eg.places that snow all the time.
These devices use either electro magnetics or chemicals to do so.H.A.A.R.P. is one of them despite the denials.It is proven and there are about or more than a dousen of these things.I cant sit and look for them now however if you wish to read up a bit on weather modification devices give us a shout and I will look them up for you.
Light CANNOT trigger rain unless you running it at around 50hZ which is imposible by normal standards!!!