Why is there dew on the grass in the morning?
(Lansing State Journal, April 12, 1995)


There is always a certain amount of moisture in the air.  This moisture comes from water evaporating from oceans, streams, lakes, etc.  The warmer the temperature is, the more moisture the air can hold.  This is why it often gets humid in the summer.  The colder it is, the less moisture the air can hold.  This explains why the air gets so dry in the winter.

As we all know, it is generally warmer during the day than it is at night.  During the day when it's warm, water evaporates into the air and the amount of water the air can hold depends on how warm the temperature is.  At night as the temperature cools down, the moisture that evaporated during the day is still in the air, but the air is no longer warm enough to be able to hold as much moisture as it could during the warmer day.  As a result, the water begins to condense on the cooling ground.  In other words it comes out of the air forming dew.  When the temperature warms up again in the morning the air can contain the water again and the water evaporates back into the air.

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