Which of these two projects are you doing?
Watching Nuclear Particles: http://www.sciencebuddies.org/science-f ... p087.shtml
Particles in the Mist: http://www.sciencebuddies.org/science-f ... p086.shtml
I can think of a few factors that might lead to variation in your findings, particularly if you are doing the Watching Nuclear Particles project. First, the background radiation in your location simply varies with time. This could be due to a number of things. If, for example, you live in an area with lots of granitic rocks, variations in how much air is flowing through the room your cloud chamber is sitting in might lead to changes in the number of tracks you observe. Another possibility is that it was easier to observe the tracks at one time. For example, was the room darker at one time than another? It is also possible that there was more alcohol vapor in the chamber during one of your tests due to differences in how soaked the felt was.
There will always be variability in your data. Some of that variability will be due to random fluctuations. Other variability may be systematic. Have you tried doing the experiment multiple times in the afternoon and multiple times at night (over a period of several days and nights)? This would assess the reproducibility of your observations. If not, try it and see if you consistently observe more particles at night. If you do, then the cause is something systematic, and we'll have to think about that in more detail. But, if the number of particles is not consistently higher at night, then the cause is likely random fluctuations.