On mornings after the temperature dropped the previous night, the grass outside is often wet. What causes this?

When the grass is wet from dew in the AM…it’s a sign the air temperature has fallen to the dew point temperature. When these two temperatures are equal…condensation is observed.

Floating water molecules, or water vapor, are always present in the air. The warmer the air is, the more water vapor it can hold. When air cools, it reaches a temperature at which it cannot continue to hold its moisture; this temperature is called the dew point. At this temperature water vapor will condense, becoming a liquid. At night, without the warmth of the Sun, the temperature of the ground and the grass that covers it becomes cooler. If it becomes cooler than the dew point of the air that touches it, condensation will occur. The grass will be covered with droplets of water by morning, even though no rain has fallen during the night. This moisture on the grass is called dew, and it often evaporates in the morning sunlight before we get to see it. (If the temperature at night drops to below freezing, the dew will become frost.) The same thing happens when the glass filled with a cold drink “sweats” on a warm summer day, or when your eyeglasses fog up as you come indoors from the cold.

Source(s):
http://wiki.answers.com/Q/What_causes_the_grass_to_be_wet_in_the_morning_if_the_temperature_dropped_the_night_before

When temperatures during the night reach dewpoint or sub-dewpoint the water vapor condenses thus shading the grass with a condensation (a.k.a dew).

Source(s):
My cerebral cortex.