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GRASS TEMPERATURE VS AIR TEMPERATURE

Ground frost occurs when the "grass temperature" falls to zero, even though the air temperature may still be above zero.   Plotting the grass temperatures against air temperatures shows a correlation between the two. 

The trendline obtained has the equation:

air temperature =  ( 0.82 * grass temperature  +  2.57 )  deg C

which suggests that on average when the air temperature falls below 2.57 deg C, it is likely that there will be a ground frost.

However, do note that there is a lot of spread in the data, e.g. when grass temp = 0 on a particular day, the air temp might lie anywhere between 0 and 5 deg C.