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Breakdown: Why is the grass wet in the morning when it didn’t rain?

Published: Aug. 25, 2021 at 9:02 AM CDT
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MEMPHIS, Tenn. (WMC) - Have you ever woken up after a clear summer night to find the grass is wet, even though you know it didn’t rain overnight?

The moisture on the grass is called dew.

Dew is the moisture that forms as a result of condensation. Condensation is the process a material undergoes as it changes from a gas to a liquid. Therefore, dew is the result of water changing from a vapor to a liquid.

Dew forms as temperatures drop and objects cool down. If the object becomes cool enough, the air around the object will also cool. Warm air can hold more water vapor than cold air, so if a mass of warm air is cooled, it can no longer hold some of its water vapor. This forces water vapor in the air around cooling objects to condense. When condensation happens, small water droplets form—dew.

Each night the weather report includes the 1) temperature and 2) the dew point temperature.

The temperature at which dew forms is called the dew point. The dew point varies widely, depending on location, weather, and time of day.

So, if the two temperatures are close, it is likely that dew will form during the night.

Dew tends to form on calm nights that come with clear skies. Windy conditions and cloudy skies keep the ground from cooling.

Humid locations, such as here in the Mid-South, are more likely to experience dew than arid areas. Humidity measures the amount of water vapor in the air. Warm, humid air is full of moisture that can condense during calm, cool nights.

Cold weather can also prevent the formation of dew. As temperatures drop below freezing (0°Celsius, 32°Fahrenheit), a region may reach its frost point. At a frost point, water vapor does not condense. It sublimates, or changes directly from a gas to a solid. Moisture changes from water vapor to ice.

As mentioned, dew is most likely to form at night, as temperatures drop and objects cool. However, dew can form whenever a dew point is reached.

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