You are correct when you say that the "official" screen temp taken 4' off the ground, and the ground temp will be different.

We have seen a short lasting dip to 29 that will 'blacken' the tops of a late soybean canopy, but there will be enough heat in the canopy to resist the cold - especially if the soil is damp underneath - and the vascular system will continue to function.

We've also observed an 'official' low of 33 that will put one heck of a white frost on a lawn, especially if it hasn't rained in a while...

To summarize: altitude, shelter from air movement, soil moisure, canopy density, duration of the low temp, cloud cover, and a whole plethora of factors will determine the severity of any given temp around the freezing mark.

Regardless, after a hot summer, anything under 50 is darn cold - let alone the "F" word.

Luke     aka Ken SWOnt