One phenomenon I've found peculiar on frosty mornings is that the grass directly under my (apple) trees doesn't freeze, leaving a noticeable silhouette on the l

One phenomenon I've found peculiar on frosty mornings is that the grass directly under my (apple) trees doesn't freeze, leaving a noticeable silhouette on the lawn. I've assumed it was due to the cool, heavy air falling and the tree acting as a shield but this article makes me think there are more complex factors at play.