If the tower is a distance away, and the atmosphere is disturbed, you can be seeing atmospheric changes of signal strength. Even the difference between the heat of day and the cooler night air in the summer can cause that over a long enough path. (Electromagnetic wave propagation is a fascinating subject to some, but it takes more than just a few large websites to really learn enough about it to understand it - it's definitely not something that can be explained on a forum. But it can change the signal from 5 bars to no bars on a long path.)