I get TV over the air. I have some channels with marginal reception at my house and generally they are better in the day than at night.
Because I'm a geek, I will go into the settings of my TV to get the SNR values and compare 1) when I can receive certain channels well, 2) when it is indicating "no reception" at all and 3) when it's marginal (cycling between displaying a screen image and not). In general, the threshold where my TV stops showing a consistent image is around 19 +/- 1 dB SNR. Not really knowing much about digital TV, I assumed that it would need a better SNR than that, but apparently not.