Re: Why does the intensity of rain often seem to increase after thunder.

I've never noticed this effect in real life, although I've seen it done in movies and TV shows. I can't suggest a good reason why it should occur.

Perhaps you could do an experiment to confirm your observation. Put a small metal plate or bucket or something out in the yard, with a tape recorder running under it. The tape recorder will record the plinking sound of each drop hitting the top of the plate, as well as the sound of the thunder. If you can put a timer which beeps at regular intervals in there (like a metronome or a radio clock), that'll let you measure time accurately on the tape.

After the storm, play back the tape and count the number of plinks which occur in each, say, 5-second interval, and plot that over time, marking the location of thunderclaps as well. Measuring the frequency of plinks will be tough if the metal plate is too large! See if there's a connection between thunderclaps and measured raindrop frequency. There are various statistical tests you can do to measure this: one of the easiest is to take the average of all the raindrop counts 20 seconds before a thunderclap, the average of all measurements 15 seconds before, 10 seconds before, 5, 0, 5 seconds after, 10.. etc. Then you can construct a graph showing the average behavior of raindrop frequency before and after an average thunderclap. If there's an increase in this average frequency, and if the increase is greater than the spread of the individual measurements, then you've demonstrated a significant effect.

This experiment will require a whole lot of counting of individual raindrops, and will be very tedious, especially since you should probably do this experiment over a great many thunderstorms to get a good average. It might be possible to come up with a computerized raindrop counter, but you're unlikely to have the programming or electronics skills to make one. Note also that this experiment only measures the number of raindrops, not their size. Anyway, if you do do this experiment, I'd love to see your results!