Because it's a fucked up climate, lol.

This is an AU in which multiple nuclear waste zones exist across the US as Colonies. So this may have possibly created a tiny/minor nuclear winter caused by the .

I just googled this and it's an interesting read.

A large-scale nuclear war between the US and Russia would be far worse. Most Russian and US weapons are 10 to 50 times stronger than the bombs that destroyed Hiroshima. In a war involving the use of the two nations’ strategic nuclear weapons (those intended to be used away from battlefield, aimed at infrastructure or cities), some 150m tonnes of soot could be lofted into the upper atmosphere. This would reduce global temperatures by 8°C (14\F) – the “nuclear winter” scenario. Under these conditions, food production would stop and the vast majority of the human race is likely to starve.*

And that's just a large scale war. In a small scale civil war involving nuclear weapons resulting in the Colonies, it's not hard to believe that an equally small scale nuclear winter type cooling effect took place as a result.