Rainfall seems to increase just after a stroke of lightning. Imagination?

Question

Or is there some meteorological phenom I don't understand?

Answer 1

It just depends on the storm.  The lightning and thunder with dark clouds, most of the time precedes the storm front.  That's why it's always so dark just as the storm hits, then lightens as the storm progresses, even if it's still violent.  It can rain before the lightning strikes, but lightning will strike several times before the rain starts falling.  So your observation is correct.

Answer 2

I've noticed the same thing. My theory is this: before the lightning strike a large electrical charge is built up in the cloud. This charge is either positive or negative but never a mixture or it would just fizzle out instead of forming a lightning bolt. As the water droplets are all charged with the same polarity they repel each other. As soon as the lightning bolt discharges most of the built up charge the water droplets are no longer repelled from each other. Free to come together they quickly form large droplets whose weight causes then to fall from the sky, in large quantities.