Why do stars look brighter in winter?

Geza Gyuk, director of astronomy at the Adler Planetarium in Chicago, explains that it's all about the atmosphere - or rather what's in the atmosphere.  In winter, there's less getting between the stars and us.  There tends to be less dust since dirt and plant debris freeze and tapped down to the earth's surface.  So there's less in the air blocking the stars' light.

Also, warm air holds more humidity - water vapor.  Think of a sultry Midwest day: there is a haze hanging over the land.  We would have to look through the haze to see the stars.

The colder temperatures in winter causes the water vapor precipitate out of the sky and fall to the earth in the form of sleet or snow leaving the sky clearer for star watchers.  Stars twinkle more on a clear night - or so it appears.  In winter the a large body of water such as Lake Michigan is warmer than the air above it (especially at night when the air cools down).  The warmth bubbling up through the air unevenly acts almost as a magnifying glass or a de-magnifer.  The resultant waving of the air makes the stars appear to be twinkling.