There is also less atmospheric interference (from the sun) at night and thus radio signals are able to travel farther and stronger. The ionosphere plays a part in the reflection of radio signals as well as it's composition is different at night with the lack of the sun. That is probably the real reason you are getting better signal strength.
This is also why many AM radio stations are regulated to to turn down their power output at night. AM radio is affected much more by this phenomenon than the high frequency FM signal the cellular phones use.
This probably actually isn't going to have much of an effect on cell phone transmissions. While the frequencies cell phone use are highly attenuated by the atmosphere, the main impact this would have is if the signal were reliant on any kind of bounce of skip. Reasonably speaking, I don't think signals will bounce above frequencies of 400 or 500 MHZ, and cell phones use MUCH higher than that now.
The UHF frequencies used in cell phones generally won't bounce or skip, so this basically takes the state of the atmosphere completely out of the equation. This means that while at night the frequency might be highly scattered, during the day it would just pass harmlessly out into space. In both cases it would never be reflected back to earth and in either case the signal is useless to the cell phone tower.
The AM radio station thing you mention is something completely different. AM radio stations are a MUCH lower frequency, and at those lower frequencies can travel thousands of miles at night. In fact, at some of those lower frequencies and in the right conditions particularly with some tropospheric ducting, amateur radio operators have picked up their own signal coming from 180 degrees in the opposite direction, meaning the signal bounced all the way around the earth.
Chances are as someone above mentioned this is just due to congestion.
Yes, you are correct. I'm quite tired as I am up past my bedtime and wasn't thinking clearly. I recall back when I was in the US Marines, we were able to talk from California to Kuwait on HF from our MRC-138. I also was able to talk from Australia to Okinawa too.
I just neglected to remember the cellular freq's being so much higher up on the spectrum and not affected by the ionosphere.
Skywave at night no impact on cellphone frequencies. In fact, radio signals above ~30 MHz are not reflected by the ionosphere at all, and since AT&T GSM/UMTS signals are at significantly higher frequencies (850/1900 MHz, etc.) there is absolutely no impact on signal strength whatsoever.
That's not entirely true. Signals above 30 MHZ will bounce, just not as readily. The whole 6 meter band is a favorite of amateur operators and it's in the 50 MHZ range, though there are only certain windows when it opens. Many times I have had frequencies in the 100 MHZ (radio stations) to the 144 MHZ range (2 meter HAM) skip off the ionosphere via E-skip.
I noticed this w/ my old phone...made AT&T's EDGE network feel so fast (relatively)