Last week we had the first hard frost of the season. There was nothing unusual about that, this being November, but I noticed some chatter over Facebook between friends in the area who were surprised about the effects on their gardens. Some had damage everywhere on their property but others had little or none. Why wasn’t the damage more uniform?
According to the weather experts, the freezing nights of fall are usually caused by an inversion layer. Anyone who has lived where smog is common (L. A. for example) will recognize that term. In this case the weather conditions are trapping cold air (not smog) near the ground. The folks at Cornell University describe it this way:On overcast nights, cloud cover acts like a blanket on the Earth, trapping radiant heat from the ground. Any wind mixes the air thus trapped, creating a uniform temperature. However, clear skies and calm winds allow radiant heat from the Earth to rise to the upper layers of the atmosphere. Lack of wind prevents mixing of the air and an inversion layer develops. An inversion means that atmospheric conditions are inverse or opposite of normal daytime conditions when air temperature decreases with height. In an inversion, cold air collects near the ground while warmer air lies above this trapped cold layer.
The cold air trapped at ground level does not, however, lay evenly on the ground. It flows like an invisible river downhill, collecting in pockets. It also moves away from objects that give off heat, like the brick walls of homes. Add to this the fact that different plants are more or less affected by cold air, depending on their hardiness and you wind up with a situation where plants growing close together have different levels of damage. To illustrate this I took a few pictures of frost damage on my property. Keep in mind these pictures were all within a 100-foot radius of each other.
In these three examples I showed how very frost-tender basils can be affected by where they are planted. Being close to a source of heat is the biggest factor. Even the difference of 2-3 degrees in air temperature is significant when the frost is relatively light and short-lived. Remember also that cold air tends to flow downhill. Walls, fences, and other barriers not only give off warmth, they can re-route the flow of cold air during the night. I’ll conclude this post by giving you the link to the Cornell University article on frost plus two other articles.