I had written something else for today but I since frost is the hot topic I thought I’d talk about the weather instead. I know-everybody talks about it but nobody ever does anything about it!
All the signs were pointing to frost yesterday afternoon but we dodged a bullet here in suburbia. It’s mighty nippy out there right now at 6:00 am, (about 38°F) but I’m not seeing any frost. Since it’s always coldest (and darkest) just before dawn, I think the danger has passed. Yesterday we had falling temperatures through the afternoon with clear skies and no wind last night, and these are usually sure signs that frost is on the way, so I’m surprised.
Clouds act like a blanket and keep temperatures from falling too fast, but clear skies allow radiational cooling, which just means that all the warmth escapes into the atmosphere. Wind stirs up the atmosphere and keeps cold air from settling and staying in one spot, but on windless nights the cold can pool in low spots and cause leaf surface temperatures to cool rapidly. When the surface of a leaf reaches 32° F water vapor can form ice crystals on it, and that is frost. Because cold air sinks, a thermometer 5 feet off the ground might read 40° F, while at ground level where plants are it can be freezing. This is when people ask how we can have frost when it’s so warm.
A fact I find interesting is that cold air flows downhill much the same as water does. I once had clients who lived at the top of a hill and their first frost was always a week or two later than the unlucky folks at the bottom of the hill. I was also able to plant their vegetable garden much earlier in the spring because their higher elevation warmed earlier.
Speaking of vegetables, I hope everyone has their tarps or sheets ready. There was light snow on top of Cannon Mountain up in the White Mountains yesterday morning at an elevation of 4,080 feet, so it won’t be long before we see frost.