Mark Mazer
Tue, 13 Nov 2007 13:09:28 PST

>How are others using these words? 

Frost is ice formed from water vapor.  Hoar frost is an equivalent term. Rime is supercooled fog droplets that freeze onto exposed objects.
Frost forms when the temperature of the object is lower than the dew point. The temperature of the object must be at or below freezing. Raditional cooling (infrared heat loss) will lower the objects temperature relative to the air temperature.  The air temperature will be higher than the dew point.

Cool air is denser and heavier than warmer air and this air tends to sink into low spots such as valleys.  Exposed valleys are more prone to occurrences of frost than hilltops.

For maximum nocturnal radiation cooling to occur skies must be mainly or completely clear and the breeze must be light to calm.  Urban area will provide up to 10F protection and readings that are taken from such areas are one of the reasons that global warming type statistics are suspect to interpretation. 

When I lived in CT, it was not uncommon to see frost form in a small area on the property that was simply several feet lower than the surrounding area.  Frost will form just as the sun begins to rise as the sun will warm the upper part of the atmosphere driving cooler air downward.  Frost forms from the outer edge of a leaf, thus it is not uncommon to see frost damage on edges only.

Frost damage is damage that occurs when frost forms on a leaf surface.

Freeze damage is damage that occurs when  air temperature is below freezing.

Nonetheless, we had a frost several nights ago and the tomato and basil in the garden have turned black. In the greenhouse, the tomato and basil plants are just fine, even if they were subject to temperatures of 34F.

Mark Mazer
Hertford, NC who became a frost maven while living along the fall line on the south side of a river valley  in NW Connecticut for several decades.

More information about the pbs mailing list