Fog is a mixture of air and tiny droplets of floating water. The clouds that drift above our heads are made of exactly the same kind of misty material. And a fog is no more than a lazy cloud resting on the ground. It may be a pale, thin haze reaching high above the ground or it may be thick enough to hide the scenery. A filmy fog is white or pearly grey, but sometimes the tiny droplets of water become mixed with grubby grime and the fog turns to smog.

Filmy fogs are most common near the seashores and smogs tend to form from fogs that hover over busy cities and smoky factories. We are not sure how it happens, but smog may be caused by specks of soot and grubby grime in the air. In any case, smog is a danger to health, for it spoils the very air we breathe and, for this reason, many cities are trying to control it.