Can I ask why you want to know Cookygirl? Only, if its because you're worried about ground frost and plants, some plants are affected by air frost and some by 'grass frost' as well as a proper ground frost. Anyway, a simple answer to your simple question is, ground frost is possible when the air temperature falls to 5 deg C because the ground may be colder - but it depends on the time of year, because the ground is warm in autumn, but cold in spring and early summer.

Yes it is because of my plants but it just all confuses me.

Well, its not a simple picture, is it, depends on lots of factors, but basically, at this time of year in Britain, if the temperature falls to 4 deg C, I'd be protecting tender plants. Anything over 6 deg C and they should be fine because the nights are short.

oh thanks bamboo...tht simplified it for me :)