In the games, the climate seems relatively normal. Did a nuclear winter come and pass, is it a really mild nuclear winter or did it not happen at all? If it didn't happen at all, why not?
The latest version of the Fallout Bible contains a number of FAQ quotes that seem to address this. The game's designer was initially minded to say that there was a 'nuclear winter':
Some areas of the world were not hit by nukes, and even though many cities promptly went to hell out of starvation and rioting (New Reno), there were still plenty of cubbyholes and old shelters for people to eke out an existence. Survivalists and some isolated military units in Power Armor (and other equipment) also were able to hole up and survive the Nuclear Winter that followed.
But then changed his mind...
Most classic theories on nuclear war include a nuclear winter after the attack, where particulates in the atmosphere cause a substantial drop in temperature,along with snowing, freezing and the like. There is a 'Great Winter' listed in the Fallout timeline, but it doesn't immediately follow the nuclear exchange. Was or wasn't there a nuclear winter? If not, why not? If so, why wasn't it mentioned before? Mind, it is quite possible I missed mention of it. Presumably, though, it would have had some sustained impact on the world.
Nope, I just assumed there was a nuclear winter and that was a bad assumption - I just assumed with hundreds of bombs flying around a nuclear winter was pretty inevitable, but I don't think one ever occurred in the Fallout universe.
As for why not, I don't know enough about nuclear warfare at the moment to say why not - I'll make the assumption that the nuclear warheads used in the Fallout universe were of comparable tonnage to the nukes in the real world (early) 1950s era, in which case, many of them could have been used without causing a massive blackout.
and after discussing the point at great length with fans, ultimately decided that it definitely didn't happen:
Hello. One question realy pisses me off... Fallout2 is a "GURPS post nuclear adventure", but after nuclear war there must be global winter and very cold... and in Fallout there are desert everywhere.
Actually, based on feedback I've gotten, I don't think the world of Fallout had a nuclear winter. It's part of the genre - miles upon miles of scorched earth, inhabited by slathering, bloodthirsty mutants.