This time of year, the words "frost" and "freeze" get used a lot, often interchangeably, but to meteorologists they mean different things.

Here's an example: during our first chilly blast of the season two weeks ago, a friend of mine had frost three nights in a row. Her tomato plants survived the first two nights, but were history after the third. The explanation? On the first two nights, freezing temperatures were confined to a thin layer of air near the ground or just above objects on the ground, such as cars. So frost formed mainly on grass and windshields.

The word "freeze" implies that the temperature drops far enough below 32^oF in a much thicker layer of air and for a long enough time to injure vegetation. Water inside plant cells actually freezes, bursting the cell walls. A "light" freeze damages only tender plants, while a "hard" or "killing" freeze destroys all of them, ending the growing season. That's what did in my friend's tomatoes on that third frosty night. Ironically, a light coating of ice can, in some cases, protect a plant from more severe damage, an interesting topic for a future Franklin Fact.