Grades K–2 · Earth & Space
How can grass become wet when no rain has fallen?
Grass can sparkle with water drops on a morning when no rain has fallen. Those drops may be dew. The water was already nearby as invisible water vapor mixed with the air, before it gathered on the grass.
In the diagramDrops can form without rainWater vapor is already in the airCool-enough bladeBecomesliquid waterThe drops are drawn much bigger than life.
After sunset, a blade of grass can lose energy and become cooler. If its surface becomes cool enough for the surrounding moist air, some water vapor changes into liquid on the blade. More water can join the little drops as this change continues.
Dew forms on the surface instead of falling from a cloud. A warmer blade may stay dry in the same air, so every leaf does not have to collect drops. A quiet-looking lawn can be changing the form of water right at the boundary between grass and air.
About this explanation
Starting knowledge: Recognize water drops and compare warmer and cooler surfaces.
Sources
- NOAA National Weather Service — Dew
- NOAA National Weather Service — Dew and frost development