Creating a rainstorm is fairly easy and not terribly expensive in its simplest incarnation. To create a scene where it appears to be raining a garden hose can be connected to a length of PVC pipe into which have been drilled a series of small holes, about 1/16" in diameter and roughly one inch apart. Don't be too precise with the hole placement to create a more realistic look. While the holes should be in a line along the length and tiny bit of variation from side to side give the rain more depth. Secure the pipe above the audiences view and turn on the hose. The rain is very convincing. Despite that rain is falling only in a single line, it isn't easy to tell that there is no depth to the rain. However, simply adding a second pipe 6-12" back from the first improves the realism. The addition of a couple flashes of light and a thunderclap complete the effect. This effect can be viewed outdoors or set up outside a window to be viewed from the indoors. If you do the indoor effect, try to get a tiny bit of the water directed onto the window to improve the realism.
Alternative uses besides a rain storm can include a slow dripping effect to simulate a dank, dripping cave or dungeon, or at full pressure, add a strobe light for a visually interesting effect. Use of colored lighting also changes the mood. For the slow drip effect, a soaker hose may work well, or instead of holes in PVC, drip irrigation with very slow emitters may be desirable. Small containers for the water to drip into will add a great dripping sound effect to the scene.
Where we live, water conservation rules are in effect and so we use a utility pump to recirculate about 25 gallons of water. We set the pump in a five gallon bucket and built a simple recovery trough out of 2x4's and a dark colored plastic tarp to funnel the water back to the pump. Even if you don't have to worry about conserving water, this system may be desirable to avoid excessive amounts of water draining across your yard, or scene stage. Our pump also increases the water pressure over the household water and yields a very believable intense storm. A valve installed in-line can reduce the pressure and apparent intensity of the storm.
In selecting a pump, keep in mind that pumps have a limitation on how much water they can deliver per hour and also how many feet up they can pump the water. The higher the water is pumped the fewer gallons that will be delivered. Make sure to select a pump that can lift the water as high as you need it to go and at an adequate water pressure.
How We Built Our Rain Effect
In our rig we used a submersible utility pump such as the type used to drain a pool or flooded basement. We placed the pump in a 25 gallon galvanized bucket and placed the bucket at the end of the where the rain effect was to be positioned.
In order to recirculate the water, we built a trough longer and wider than the overhead rain pipes to be sure to catch all the rain and splashes. We built it out of 3/4" PVC using elbow an "tee" fittings to create a rectangle with legs. We used three sets of legs along the length but experienced excessive sagging under the weight of the water and so recommend using five sets of legs and using cross-bracing at each set to prevent the sides from bowing. Use of 2x4 lumber would have been sturdier, but we like to go with the lightest and/or cheapest materials. To improve flow, we made each set of legs shorter than the last to create a slope toward the pump.
Because the tarp sagged and filled up with water instead of draining it to the pump, we used 1/16" thick luann plywood (sometimes sold as "door skins"), cut a little wider than the base (to allow for bowing) and placed it on top of the trough. We then drilled a series of holes and secured the material to the PVC pipe with zip-ties. We then draped the tarp over the trough and let it hang down to the ground.
From the pump we connected a 1" PVC fitting to a length of pipe with a "tee" on the other end. In order to make sure the pressure was equal in each of our two pipes, we ran the water to the "tee" which splits the water evenly rather connecting to one pipe and then to a second pipe which would have lower pressure on the second pipe, especially at the end. From the "tee" we used reducer fittings to go to 3/4" pipe and the on either side used 90 degree elbows to our rain pipes. To further improve even water pressure, we connected the other end of the pipes with elbows and a short length of pipe.
For the rain pipe itself, as described above, we drilled 1/16" holes along the length and spaced them roughly every inch. We varied them a small amount left and right to add depth to the rain effect. If the holes are too perfectly arranged, the rain is not as realistic. Also, if the holes vary too much, either in spacing along the length or in left-right alignment, gaps appear in the rain or water sprays at an odd angle and ruins the effect.
Finally, we used the rain effect in our walk-through maze and so we suspended it from about nine feet up from EMT conduit used to build the maze hallways.