Why do stars twinkle more on certain nights, compared to others?

My understanding is that the particulate matter in the various layers of earth’s atmosphere get in the path of the photons traveling toward us; the twinkling is the particles briefly diffracting the light as they pass that path; if the atmosphere is more dense with material (dust, ions, water vapor, smog, etc.) the apparent effect would be more twinkling. I have heard that in outer space, there is little-to no twinkling effect as there are far fewer particles in zero atmospheric conditions.

The stars always look brighter to me on nights with a new moon (no visible moon).

The concept in astronomy is called seeing—the better the seeing, the clearer (and less twinkly the stars). @sndfreQ is really close—it’s actually thermal diffraction (like heat coming off a road), because the star’s light is coming from such a singular point, the tiniest heat differentials in the atmosphere make things twinkle.

@steve6 you are also correct—the moon tends to light up the atmosphere (at least in lower altitudes & lower humidites), which reduces the contrast of the stars against the sky—I was up at 6000ft on a cool, very dry night—the moon was quite lovely against a nearly black sky.

the clearer the path between you and the star
the brighter it looks