Stars Above Horizon

We are interested in the visibility of stars for the naked eye. How much influence does the background brightness have? What is the exact influence of the air mass close to the horizon, its humidity and temperature on the visibility of stars? Does it dependent on the geographic latitude (i.e. the climate of the region) and how big is the range of human eyes’ visible abilities?
We do not collect personal data, we are only interested in the broad overview of the range of human eyes. So, please participate as often as you can and wish, at most various geographic places - on holidays and other travels, at home while camping, barbecuing or simply enjoying a summer evening, in big cities as well as in the country side. The app and data are not personalized, so feel free to invite your friends and family to participate in the same observational session or use the app on the same phone.
Please keep in mind that you will upload scientific data! So, feel free to take it as a challenge but please stay honest in uploading only real data!