First here is a good site with a nice overview: http://www.cddc.vt.edu/host/atomic/atmosphr/index.html
The largest explosion ever unleashed by the US was the 15 megaton explosion called Castle Bravo.
The blast was far more powerful than scientists had anticipated, and fallout landed on inhabited parts of Bikini Atoll and on fishermen on a Japanese tuna boat whose name was, ironically, the “5th Lucky Dragon.”
And how Lucky was the Lucky Dragon? Here is an outstanding National History Day video created by a student named Lauren White in Maryland:
And the quest to limit nuclear weapons continues even in 2010. One of you all sent me this link: