Creating a Rain effect

As in a photo of rain or more graphical?

Evaluate the Properties of What You Want to Draw

Rain is heavier on the bottom of the drop, and thinner on the top -- usually rain doesn't fall straight down, but rather at an angle depending on the wind levels. i.e. it's on a slight angle in low wind and horizontal in a hurricane.

When there is light rain, the drops are far apart, when there is heavy rain, the drops are closer together. In both situations it can be small drops or big wet drops.

Okay, now that we know the properties of rain, lets look at the properties of water and reflection -- in general water is curved because of how the molecules hang together (it's because the water drops are falling that they get non-spherical when it's raining), and the shadows and highlights emphasize this roundness.

So! How to Make It!

Make a sphere that looks like a sphere, with the light shadow and all the highlights.

Make it transparent.

Stretch it so that it is thinner at the top and thicker on the bottom.

Turn it so it is at an angle.

Resize it so it's the right size, duplicate or make it an object, library item, a tube, an object for the image sprayer or whatever works for your graphics application.

Make many instances randomly placed in your image.

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Rain on a Window

If you want rain on a window, go get a water sprayer/mister (like for plants or ironing) or a hose and spray a window and observe the patterns of light and shadow in the drops and the tracks on the glass.

Have fun!

You know you rain!!!!