In this paper I am mainly concerned with the surface soil temperature, but as we are accustomed by long usage to think of the grass minimum temperature as determining the occurrence or non-occurrence of frost, a few words of explanation on this point is perhaps necessary at the outset.

The grass minimum on nights of rapid radiation certainly does fall considerably below the surface soil minimum, but this is due to the very fact that it is the grass minimum, i.e. the air temperature just on the grass.