Air burst vs ground burst. As a good rule of thumb air bursts are fairly clean, ground bursts are very dirty.
About ten years ago Chinese general stated that in a nuclear war they would use a curtain of ground bursts on the Western coast of the US (and presumably also the UK) to destroy the population.
There are semi-classified maps from the late 50s which show that most of the UK would have been turned into a desert in an old fashioned fission bomb war. (They were posted online about fifteen years ago. I downloaded them but lost them in a drive crash.)
It's also known strategy to target power reactors with ground bursts to spread even more fallout and kill even more people for longer.
Anyone suggesting "Nuclear war - not so bad actually" is delusional.