From Philip E. Taylor:

Re: Claim of alien cells in rain may fit historical accounts: study (Jan. 22): “Red rain” events are clearly caused by fungal spores, of the Puccinia species (rust). Puccinia outbreaks are common in the autumn, especially on fields of grass. Strong winds (updrafts) can lead to very high atmospheric counts for these wind-borne spores. Water impacts or condenses onto these spores in the atmosphere. The red carotonoids in the spores give the distinct coloration to the rain droplets. I suspect that atmospheric changes that occur during a thunderstorm enhance this process.

Dr Philip E. Taylor
Australia