I live in SC. In winter when it's cold, like now, I seem to get better reception with my indoor antenna than I do in summer when it's warmer. In summer I get a lot of breaking up on certain channels, which doesn't happen in winter. At least it sure seems that way. Does that make sense?

Less humidity?
Depending on the types of trees surrounding your home, less leaves?

Most likely leaves. Tons of possibilities.

Your best bet is to go to the local reception thread for your area, follow the directions at the top of this section and let the guys who know your area figure out the best ways to help you get your channels year 'round.

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.

Leaves, aha! Lots of trees in my area so that's probably it. I will check the linked threads for suggestions, looks informative. Thanks all.

Serious ... humid air is thinner ... Water molecule weighs less than 78% nitrogen molecules, 21% oxygen molecules.

Humidity makes it a little harder for a plane to fly.

It's probably trees, in this part of the country where most trees don't lose their leaves fall can have worse reception than the summer. Not so much with VHF but UHF can be a real mess with leaves and the Santa Ana winds. Multipath can be a four letter word..

Sometimes i get a chan 80 miles away just fine.

More cloudy days also a possibility?

What you're describing is almost certainly caused by tree leaves. The FCC assumed an outdoor antenna at 30' for OTA DTV. An indoor antenna is at a huge disadvantage. They all suffer from signal attenuation and some amount of multipath.

The only weather that affects reception is temperature inversions which can enhance or degrade signals. Hot, cold, dry, humid, wind or precipitation in any form has no affect on VHF/UHF signals. If you think weather is affecting your reception then something else is going on. There are many possibilities.

The reception prediction programs over estimate signal strengths when vegetation or buildings are in the way. They under estimate signal strengths on 1 and 2 edge paths when nothing else is blocking the signals. They are most accurate when the antenna has an unobstructed view to the transmitter.

The most likely fix for your problem is to go with the FCC minimum recommended antenna.

Basically they assume a two story house with a rooftop antenna.

But apparently in DC trees are much shorter. Here in NH a 30 foot high antenna is still 40-50 feet bellow the trees. That is our situation, plus here in terrain challenged NH even without trees we would not have line of sight (LoS) to Boston. Having said that with a good outdoor antenna we are able to receive all the Boston stations 40-ish miles away on either 2-edge or Tropo. Pretty amazing when you think about it.

/tom

Why is it when the FCC assumes something it never turns out like it should.

What are the dominant tree varieties in the immediate vicinity of your antenna? If they are nut-bearing trees (oaks, pecans, etc.), it's possible that during the summer growing season as those annual fruits are growing larger and more dense, they might block some of the signal strength for more distant channels in line-of-sight azimuths because of that blockage that becomes greatly reduced as the nuts mature and drop off.