Why Is Grass Sometimes Wet Outside in the Morning?

Dew

• Dew on your lawn in the cool early morning is just a vast accumulation of individual water droplets. The amount of dew or wetness on your grass is directly related to your lawn's surface temperature. You often see dew in warmer summer months, when exposed surfaces store high amounts of radiant heat from the sun. Once air temperatures dip below surface temperatures, stored heat begins radiating back outward. This can result in the condensation of water on those exposed surfaces.

Condensation

• Dew is a direct result of condensation, which occurs when a vapor changes to a liquid. In this case, that means water. As the grass on your lawn begins to cool when the sun goes down, its surface temperature starts dropping to match the night air. The fast-moving molecules in the radiated heat leaving the grass start slowing down as they cool. This creates a change phase that results in the creation of the condensate water.

Temperatures

• Dew never forms on grass when surface temperatures match surrounding air temperatures, which is one reason why you never see dew on the grass in the winter. Similarly, in the fall, you often see frost rather than dew; as the daytime surface temperatures of your grass begin to approximate surrounding air temperatures, any moisture that develops quickly freezes. Early morning dew depends almost entirely, in fact, on the surface temperature of the grass being higher than the surrounding atmosphere's temperature.

Humidity

• If you've ever watched a weather broadcast, chances are good you've heard the meteorologist mention dew point. This is basically the temperature at which relatively humid air has to be cooled for water vapor to condense into water. This is also another reason why you'll never see dew in the winter; the air's just not humid, or wet, enough. In the summer, however, when humid air's the norm, early morning grass will almost always develop dew.