I'm not in agriculture but I am familiar with thermal radiation an normal heat flow. I had investigated thermal radiation some time ago and I recall a clear sky "looks" like -40 °C (or °F at this temp).
I would think for a bud to freeze it would depend on:
- combination of temperature and time at temperature.
- radiant cooling added into the effect of #1
I don't have any experience to suggest an actual solution. However I would consider thermal sensors (probably 1-wire) located at different places in the field, as well as some measurement of sky radiation.
These coupled with the time of day and some integral of time at temperature should be a good start.