Separation of electric charges in falling rain, caused by the breaking up of the water droplets, the drops becoming positively charged and the air negatively charged.

More details:

Lenard(1915)found that when water is atomized (e.g. on impact of a water droplet), negative and positive charges are SEPARATED.

Molecules which are torn from the surface of the water bear a NEGATIVE charge(small negative ions) whereas large drops of the entire mass of water are POSITIVE.

This provided an unexpected explanation for the refreshing,invigorating effect of residences does to a waterfall or spring, or even after rain.