Try and keep this simple.

Ever seen a mirage on the road that looks like water? Or seem foggy conditions where you could hear say a train from 10 miles away?

The same thing happens at night to VHF and UHF radio waves that carry the television (both analog and digital) to your TV antenna.

During the day, the sun heats the ground, warming the air directly above it. As you go up in the sky (obvious if you have ever been in a small airplane without air conditioning). This is the normal situation in the day. Air is hotter near the ground and gets colder for the first 10 miles as you go higher from the ground.

Now comes night time. The opposite starts happening. The ground, lacking sunlight, cools. It cools the air above it faster than the air at altitude can cool. Air is a great insulator so the higher air doesn't cool as fast as air near the ground.

This is called some times an inversion because there is hot air above cold air at the surface. Normal is hot air at the surface and colder at altitude.

If this night time cooling at the surface is enough, the radio waves instead of going out into outer space hit the boundary between cool air at the surface and warm air above them and bend back to earth at distances greater than they normally would.

Its like the air becomes a big mirror over the earth at night. Some nights it's stronger than other.

So just like a hot road bends light back at you making it look like water, the hot air above the cold air at night, bends the radio waves back to earth.