Increasing snowfall,mist, fog or rainfall in the air.

When its raining, foggy or the snow is falling, but it isn’t as much as you’d like, you can use flash to exaggerate the amount of precipitation in the sky.

By firing the flash when you take the shot you blast every water particle or snowflake with light, greatly increasing the number of visible particles.  It’s the same effect as a driver putting on his full-beam headlights in fog; suddenly the fog looks twice as thick.