How Does Dew Form?
On clear summer mornings the grass is often wet with dew. Dew is liquid water droplets that form on grass, spider webs and other things in the early morning or late evening. Dew does not fall like rain. It forms where it is found.
Dew forms when water vapor in the air condenses as it touches something cold. At night, after the sun stops warming the earth, grass and other objects out in the open cool off quickly. The air alongside cools, too, and moisture is released.
The moisture collects on the cold surface as dew. When the sun rises, the grass and other things on which dew has formed soon get warm. Then the dew evaporates and disappears into the air. Dew only forms under certain conditions.
If a warm, clear day is followed by a cool, clear evening, dew will likely form. On a normal warm day, water evaporates from the warm ground into the air. That means it turns from a liquid into a gas called “water vapor.”
When evening comes, the warm ground continues to radiate heat into the air. As the ground begins to cool, the air will not be able to hold all the moisture.
At a certain point — a temperature called the “dew point” — water vapor in the air will begin to condense (turn back to liquid water) faster than water is evaporating.
When this happens, dew forms on surfaces that aren’t warmed by the heat radiated from the ground. That’s why you mainly see dew on things like grass, leaves and even car roofs.
If the temperature gets low enough in the evening, dew may undergo yet another transformation. When temperatures are low enough, dew may freeze to a solid form that we then call “frost.”