Originally Posted by ncsercs
Cooler air is thinner, warmer air (if humid) is thicker?
Other way around. Cooler air is denser than warmer air, and dry air is denser than humid air. (H2O molecules weigh less than just about every other component of air: N2, O2, Ar, CO2, etc.)
But you're right to consider the effect of the atmosphere. Light/radio waves slow down a bit in denser air, so if there's a gradient
in density, it can cause light/radio waves to bend (refract). This is responsible for the "mirage effect" in summer, when light headed toward the ground can be bent back up by a layer of thinner, warm air near a hot surface like a road, reflecting the sky much as a puddle of water would.
At radio frequencies, it's also responsible for "tropo" events. If you get a temperature "inversion," where air temperature rises with height (usually it's the other way around), radio waves can bend around the curvature of the earth, occasionally letting you receive TV stations from much farther than they would normally reach.
But I agree with the others: if your reception is better in winter, it's usually because the trees don't have leaves on them. Leaves scatter radio waves, reducing the amount of signal that makes it from the TV transmitter to your antenna.