Why doesn't it lightning on a cloudy day? I mean lightnings when there are rain clouds but normal clouds on a sunny day don't yeild lightning. Is it rainwater's conductivity?
Rain carries the charge between cloud and ground[b/], allowing the clouds to become charged. So yes, rain is instrumental in causing lightning.
You can have lightning without rain in some rare situations. When air is sufficiently dry and warm at low altitude, you can have rain drops that evaporate before they reach the ground. In that case, the charge is carried between two clouds, and you can see lightning between these clouds without there being any rain at ground level. But it still has to be raining up there.