When ice crystals form on the surface of our tender plants it is called extra-cellular freezing. When this happens, water is drawn out of the plants' cells through the cell membranes.

If the temperature drops slowly and if the freezing does not last too long, there may be some uninjured cells left to reabsorb the water when the temperature warms up again. However if a plant cannot reabsorb the water, the plant is irrevocably injured because of dehydration.

Varying Degrees of Freezing

A severe freeze also causes internal, or inter-cellular, freezing where the water freezes inside the cells and bursts the membranes making it impossible for the plant to recover. You can see this when you notice that a plant's leaves are black and mushy.

Frosts that are light and moderate in severity are usually called radiation frosts. They occur when the earth loses so much heat at night that the temperature at ground level drops below freezing (32 F).

An advection frost, on the other hand, occurs when a huge mass of freezing air, such as an arctic air mass, passes through a region. More water vapor occurs in warm air and as that air becomes colder it gradually reaches a saturation point. Vapor converts to liquid when the dew point is reached, and if the temperature is below freezing, ice crystals instead of dew form on the leaves of the plants, and we say there is a frost.

Reference:

Harnden, Phillip, A Gardener's Guide to Frost. (Chapter 2). Willow Creek Press, 2003.