Also I think lot of this data is processed then only key parts are kept. At least that's my guess.
For example it shows the squares around people where it detects movement. It probably only saves each square as a separate video vs saving the entire image. Then other info like GPS info and date/time is stored, and maybe an overall picture is saved every couple minutes. Basically enough info to pin point each person at each location at each time of the day. Then all the mediocre people eventually get deleted and they only keep records of "suspicious" people longer. Ex: they see a guy go into a record store, and later that day that guy is now buying blank CDs at walmart. Huge red flag. Or a guy buying 10 propane tanks, etc.
As for cloud cover, remember most of this is classified. Chances are those cameras have an infrared mode. They can probably track people right through the roof of Walmart too. Though thermal imaging is not as sharp so it's harder to define a specific person. But they probably have ways. Each person probably has a slightly different thermal signature, movement speed, etc so if they can catch that person with the normal camera then they can probably continue to keep track even if clouds roll in.
They probably don't track every single person, but rather randomly pick and choose. Guessing they'll use these to catch speeders too.