Given your part of the country, I'm betting they're big old oak trees.
A signal meter behaving in this way is detecting short-delay multipath interference. In UHF reception (10.x is actually broadcasting on channel 51), trees are a notorious source for this interference, particularly in the presence of a breeze or wind. Light breezes rustle the leaves, while wind moves entire branches. Either way, you get temporary interruptions in the signal. Some say this issue is worse with deciduous (leafy) trees than it is with coniferous ("evergreen") species.
Tilting the antenna a few degrees toward the sky *might* help capture signals from above the trees, but your mileage will probably vary, and this isn't easy to do with a 4228.