A Dew Primer

Dew does not "fall" from the sky. It condenses from the surrounding air onto any object that's colder than the air's dew point. The dew point, often mentioned in weather broadcasts, depends on both temperature and humidity. When the humidity is 100 percent, the dew point is the same as the air temperature. At lower humidity, the dew point is below the air temperature. If the dew point is below freezing, you get frost instead of liquid water.

A familiar example of dew physics occurs when you take a bottle out of the refrigerator. If the bottle is colder than the air's dew point, it drips with condensation. Your telescope is the bottle.

"But my telescope can't get colder than the air!" a new Schmidt-Cassegrain owner once told me. "It was warmer than the air when I brought it outdoors. The Second Law of Thermodynamics says that can't happen!"

If only life were so simple. Objects do try to come to the same temperature as their environment and then stay there, as the Second Law says. But they don't exchange heat just with the air around them. They also exchange heat with distant objects by radiation. That's why the Sun can feel warm on your skin even though it's 93 million miles from your skin. At night the heat flow goes in the opposite direction. The effective temperature of the dark night sky is just a few degrees above absolute zero, and a telescope in an open field is exposed to a whole celestial hemisphere of this cosmic chill.