The number of stars visible on a cloudless night is primarily determined by two factors: light pollution and atmospheric conditions.
Light Pollution: Light pollution refers to the excessive artificial lighting in urban and populated areas that brightens the night sky, making it difficult to see stars. When there is high light pollution, fewer stars are visible because the artificial light overpowers the faint light emitted by distant stars. The level of light pollution varies depending on the location and time of observation. Areas with less human development, away from cities and artificial lights, tend to have darker skies and thus allow more stars to be visible.
Atmospheric Conditions: The Earth's atmosphere can also affect the visibility of stars. Atmospheric conditions such as humidity, air pollution, dust, and haze can scatter and absorb starlight, making the stars appear dimmer or less visible. Additionally, atmospheric turbulence can cause stars to twinkle and appear less steady. This effect, known as atmospheric seeing, can make it more challenging to observe faint or distant stars.
In regions with low light pollution and favorable atmospheric conditions, such as rural or remote areas with clear skies and minimal air pollution, more stars will be visible. Conversely, in highly populated areas with significant light pollution or during nights with poor atmospheric conditions, the number of visible stars will be reduced.