Wait for it to rain..?

I jest- although that is an option if you can water proof equipment/film from under cover.

Other ways include a post-production overlay (almost always looks naff) or just wetting everything before hand and then using a hose on the actors.

Remember to light the scene for an overcast, turbulent effect.

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There's daggers in men's smiles

One of the issues with rain is that it doesn't show up on camera very well. If you want to fake rain your best bets (a) makes sure it's not a sunny day (b) wet the actors if their characters are outside (c) Wet the ground/windows/etc. (d) show a closeup of a puddle with drops hitting it and other shots to specifically relay that it is raining.

Depending upon your scene you might consider putting the characters inside near a window. Rain will show up nicely on a window and save you doctor's costs and lost production time associated with sick actors. The window could be in a house, or them sitting in a car.

RJSchwarz
San Diego, CA

Thunder is a sound effect that is done in post production. there are hundreds of places to download sounds. Before the internet (yes I'm old - I started making movies before there was an internet) I simply made a thunder sound with my mouth, very close to the mic and slowed it down. It was perfect.
Lightening is a flash of light. I bet you can figure out how to make a light flash. How about a work light just outside the window that you turn on and off really fast?
Rain is simple. A hose works great. Several hoses can spread out the rain.

When you don't have a lot of money to spend you need to get creative. But that what filmmakes do, right? Maybe you can't get a wide, night exterior in the woods with pouring rain - you might have to shoot closer. Cheat a little - get creative. You might need to enhance the visual with some great audio - a lighter spray of water from two or three hoses can seem more powerful if you use the sound of hard. pouring rain. How do you get that sound? Record it yourself - go outside and spray water from a hose on different surfaces then lay several of these audio tracks down and adjust the levels (some lower - some louder) until it sounds the way you like it.

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The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

Thanks for the suggestions friends. shall try them out.