[sbierwagen]

Note: due to the ratification of the Partial Test Ban treaty in 1963, every test after that year has been conducted underground. (With the notable exception of China, which isn't a signatory, and has detonated a couple nuclear weapons in the atmosphere post-1963.)

https://en.wikipedia.org/wiki/Partial_Test_Ban_Treaty

EDIT: China.

[abirkill]

The vast majority, but not all, I don't think?

Wikipedia states that the most recent atmospheric test detonation was carried out by China in 1980, although the reference link appears to be broken:

https://en.wikipedia.org/wiki/List_of_nuclear_weapons_tests#...

[sbierwagen]

This was news to me, but apparently China is not a signatory of the PNTBT.