Question

Row 1 Outside in the early morning,
waiting for the school bus, you
notice that the grass is wet. The
water is cold on the grass.
Construct an explanation that
describes why the grass is covered
in water.

Answer 1

Why the grass is wet (dew):

- The air always contains some water vapor. During the day the ground and air warm, and that moisture stays as vapor.
- At night the ground and the blades of grass lose heat by radiating it to the cold sky. If the night is clear and calm, the grass cools a lot.
- The dew point is the temperature at which the water vapor in the air will start to condense into liquid. When the grass cools below the dew point, water vapor condenses on the cooler surfaces.
- The condensed water appears as droplets on the grass — that’s dew. The water feels cold because it has the low temperature the grass reached overnight and because evaporation of some of the water from your fingers removes heat, making it feel colder.

(Other possible reasons for wet grass are recent rain or sprinklers, but in a clear, calm early morning the most common cause is dew from condensation.)