Weather

How does the ground get wet without rain?

Ben Domensino, 15 May 2019

Have you ever wondered why the grass is wet in the morning when it didn't rain overnight? Or why your car is covered in water as the sun rises but the road it's sitting on is dry?

While it doesn't rain every night, there's always moisture in the air. Sometimes this moistyre falls to the ground as liquid (rain) or solid (hail and snow), while other times it floats there as an invisible gas called water vapour.

For gaseous water vapour to turn into liquid, the temperature of the air needs to drop to a level called its dew point. This is simply the temperature at which water vapour will condense from a gas into a liquid.

On most nights, the ground and other objects get colder than the air around them. When the temperature of a surface like your car or grass gets colder than the dew point of the air around it, water vapour in the air will condense into liquid drops on that surface. This is called dew. If the surface gets cold enough to freeze this water, frost will form.

Some surfaces get colder than others. For example, when the metal and glass on your car gets colder than the asphalt on the road, you might see a dew-covered car on a dry road.

Dew can last well into the morning until the rising temerature causes it to evaporate, turning the liquid drops back into water vapour.