Last night the National Weather Service issued a frost advisory for our area, and we did indeed get a frost. They are issuing a freeze warning for tonight. What’s the difference?
I contacted a member of the local weather station staff. He told me that
Both [the frost advisory and the freeze warning] are only issued during the growing season. A Frost Advisory is issued when the predicted temperature is expected to fall to 36 degrees or lower in the next 3 to 30 hours during the growing season. So temperatures 35 to 40 range would also dictate a frost advisory. A Freeze Warning is issued when there is an 80% or greater chance that the temperatures are expected to fall to 32 degrees (F) or lower in the next 3 to 30 hours during the growing season. If the temperature is expected to fall below 28 degrees (F) this is considered a Hard Freeze.
Another way to look at it is that a frost is caused by radiational cooling, that is, the earth loses enough heat that temperatures drop below the freezing point at ground level. Sometimes when there is a very light frost, the freezing temperature occurs just a bit above ground level. Our first frost this year was a very light one, where the tops of the basil plants got damaged, but the plants themselves survived.
A freeze, on the other hand, is caused by advective cooling, where a mass of cold air comes into the area from somewhere else (like the Arctic). This usually signals the end of the growing season for all but the hardiest plants. When the weather service issues a freeze advisory, they are telling you the party’s over. It’s time to get serious about fall clean up and winter preparation.
Time to Get Serious with the Chores
I need to empty and wash the bird bath and bring it in. Oh, and I better make sure the rain gauge is empty. What’s on your must-do list?