Crystalline Winter Nights

November and December commonly are the cloudiest months in the Midwest, but January and February sometimes redeem themselves with several crystal clear nights that are great for deep sky observing. To a large extent, these nights are a result of less water vapor in the air, which then minimizes the scattering of light -- both light pollution and the light from deep sky objects. Weathermen refer to the clarity of the atmosphere as "turbidity". The amount of water vapor in the air is sometimes described in terms of the depth of water that would result if the vapor in the Earth's atmosphere were to be condensed into liquid water. Since cold air cannot hold as much water as warm air, the vapor content is lowest on cold, dry winter nights and highest on hot, humid summer nights. On winter nights, the atmosphere often contains less than 0.05 inch of water, while a humid summer night may average 2.5 inches. The vapor present on a humid summer night often steadies the atmosphere and improves the view of bright, extended objects, such as planets. In the winter, we often remark about the bright stars. While they are intrinsically bright and are physically nearer to us, a dry, cold night really does make them appear all the more brilliant.

Published in the February 1998 issue of the NightTimes