Mae Louise Gibson, aged 10, of Ova, Arizona for her question:

What is the difference between fog and smog?

A fog is really a cloud sitting on the ground. It is made of tiny droplets of water hanging in the air. Suppose all the droplets in a fair‑sized fog were gathered together. They would hardly fill a pitcher with water.

These moist water droplets gyre very sticky. Crowded together they tend to trip and hold other particles present in the air. They gather dust and specks of smoke from around cities and factory chimneys.

These smokey particles turn the misty white fog into a blanket of dark, dirty smog. Smog is a mixture of fog and smoke particles. Sometimes other waste particles are present also. Nature provides us with fog. The smog is created by man‑made dust and smoke added to the fog,

Smog never appears over the wide expanses of the clean ocean. It settles over cities and industrial areas. The smog over London town is sometimes so thick that high noon is as dark as a moonless night.