Poison rain
"Poison rain" was a term used by the Novans, descendants of the Earth colony ship , to describe fallout. Specifically, rain contaminated with radiation that originated from a radioactive debris cloud. 

Following the rediscovery of the Terra Nova colony by the crew of the  in 2151, one of the colony's survivors Nadet blamed their present state of being on that the poison rain came when "the Humans dropped the poison, burned our skin, gutted the grown ones. There was no place to go but here." Following her description, Jonathan Archer explained that thought he knew what she was talking about, and upon further investigation, he and T'Pol determined that it was caused by an large asteroid impact around 2081, and the beresium ore that was present at the location of the impact crater, caused thermo-shock, which created a radioactive cloud that covered most of the northern hemisphere for over a year. 

By the 2150s, the region's water sources were becoming contaminated with the radiation and it was no longer safe for them to live there, further explaining that "when the asteroid hit, the fallout contained certain poisons. Humans under the age of four or five can usually build up an immunity to them. That's why they survived." Nadet, who was one of those surviving children, herself, later suffered from lung cancer. 

Archer later informed the Novans that they located a region to the south where "the poison rain never fell"  and the people would be safe to live. ()

Meteorology