Have you ever noticed the stars in the winter seem to shine brighter than in the summer? The key to the clarity of the sky is the amount of moisture it contains. The more moisture, the dimmer the stars will appear. Think of a hot, humid summer evening and the haze in the air. The higher the temperature is, the more moisture (and haze) the atmosphere can hold. Conversely, the colder the atmosphere gets, the less moisture it can hold, and thus the potential for the clarity and transparency of the atmosphere to be at its’ best is greater. The more transparent the atmosphere, the brighter the stars will be. Thus colder air usually means brighter stars. The difference in brightness of the stars on cold nights vs. summer nights should be easily noticeable.
Low moisture also typically means more twinkle. Because moist air is harder to heat and cool then dry air, it tends to lessen temperature differences across the column of air. In the summer, the moist air robs transparency, but it creates stability. Thus the best time to view the moon and planets in telescopes at high power is during the summer. In the cold dry air, temperature differences are much more common. Different temperature air has different densities, and it is this difference in density that bends star light at very, very slight angles. Add some wind to move these subtle temperature differences around and the twinkle increases.
Thus, cold, clear nights mean brighter stars with more more twinkle. With the coldest nights of the season upon us, we’ll probably have the brightest, most energetically twinkling stars of the year. Just dress warmly!