Warm air will hold the most vapor.

When warm air becomes colder, what is done with the vapor?

The vapor is condensed into small particles of water.

When this takes place near the earth, what does it produce?

It produces a fog.

Where do fogs mostly occur?

Fogs mostly occur over low grounds and along the course of streams.

When the vapor is condensed higher up in the air, what does it produce?

It produces a cloud.

What is a cloud?