fog under the Golden Gate Bridge, California
Clouds on the earth
Eeee-rump! Eeee-rump! The growly sound of a foghorn echoes in the
night. A thick, gray fog creeps in from the ocean and settles over the
waterfront. You can hardly see your hand in front of your face. It’s
like being in the middle of a cloud.
As a matter of fact, that’s just what fog is. It’s a cloud that touches
the earth, instead of floating high in the sky.
Like every other kind of cloud, fog forms when warm, moist air meets
cool air. Fog often forms when warm, moist air passes over the cold
water of an ocean, lake, or river. The warm air quickly cools. Then the
water vapor in it becomes millions of tiny droplets of water —a cloud
that rolls in from the water and spreads out over the land.
Fog forms over land in much the same way. This happens when ground that
has been warm all day begins to cool off. As the warm air above the
ground cools, the water vapor in the air turns into droplets of water.
Then there’s a fog that hugs the ground.
