Why rain disappears
- It is small, so each drop occupies almost nothing in the frame.
- It is fast, so at ordinary shutter speeds it smears into nothing rather than reading as a drop.
- Worst of all it is nearly transparent, so against a mid-toned background it has almost no contrast to be seen by.
The backlight solution
The most-voted answer described a picture that worked, and attributed it to headlights facing the camera. Two areas carried the rain: directly in front of the lights, where the drops were lit against a nearly black background, and the road surface, where falling drops were striking water already there. That is the general recipe — a light source pointing broadly towards you, with darkness behind the rain rather than in front of it.
Making the drops read
- Choose the angle so the drops reflect as much light as possible towards the camera.
- Frame so the lit drops sit against the darkest available background.
- Use flash to freeze them, or a fast shutter where there is enough light to allow it.
- Vary the shutter speed deliberately: faster renders drops as points, slower as streaks, and the two look very different.
- Narrow the angle of view. A longer lens makes each drop larger in the frame, which is the difference between visible and not.
Giving it depth
One answer added a point that is easy to miss: include objects at several distances. Rain further from the camera renders lighter and less distinctly than rain close to it, so having near and far elements in the frame turns a flat curtain of drops into something with depth. Rain that reads as weather rather than as speckles usually has that recession in it.