With global warming and warmer nights, one might expect the potential for frost damage to decrease. However, there are indications that global warming is leading to increased variability with more extremes. In addition early warm periods can advance the crop phenology to earlier in season and make the crop more sensitive to frosts.
While some rough rules govern frost potential of a given site, most knowledge of microclimates is valid primarily for sites with very simple topography (uniform slope or flat terrain) and large expanses of uniform surface vegetation, soil type and soil moisture. Even then, row orientation complicates the temperature distribution and airflow.
At more typical site, the spatial distribution of microclimate becomes more murky and the knowledge of a perceptive field manager, if one exists, becomes useful.
1. The ground cools RADIATIVELY which cools the air adjacent to the surface. Clouds effectively reduce the net radiative loss of energy. Haze and pollution to some degree reduce the net radiative cooling.
2. AIR MOTION causes vertical turbulent mixing which brings down warmer air to the surface. Both the airflow and turbulence may be too weak for human detection without instrumentation but there is always some airflow and mixing. The weaker the turbulence, the greater the cooling at the surface.
The cooling is therefore greatest on clear calm nights.
3. Upward heat flux from the SOIL also reduces the surface cooling. This upward heat flux is greater with wet soil and heavier soils such as clay. Organic debris at the surface (leaves, matted grass) significantly reduces the heat flux from the soil.
cooling on slopes leads to drainage of cold air down the slope which generates mixing and downward transport of warmer air. Rows oriented down the slope and short or no between row ground cover promotes cold air drainage. Brush can slow the motion but not stop it. Air motion is weak in low lying areas where cold pools form. Cold air drainage may flow over the top of the even colder air at the bottom of the slope. Often cold air drainage is complex and simultaneously generated by multiple slopes.
There are numerous other influences on horizontal variations of nocturnal surface temperatures but attempting to include too many factors can be counter productive.