WHAT IS SMOKE?

Smoke is the result of incomplete combustion of the certain fuels. This means that if most of our common fuels were able to burn completely, we would have no smoke!

Most fuel consists of carbon, hydrogen, oxygen nitrogen, a little sulphur, and perhaps some mineral ash. Now if these fuels would burn complete, the final product would be carbon dioxide, water vapor, and free nitrogen, all of which are harmless. If sulphur is preset, small quantities of sulphur dioxide are also given off, and when this comes in contact with air and moisture, it becomes a corrosive acid.

For complete combustion, a fuel must have enough air for full oxidation at a high temperature. These conditions are difficult to obtain, especially with solid fuels, and the result is smoke. Anthracite and coke can be burned without producing smoke because they have no volatile matter.

But bituminous coals decompose at rather low temperatures so that gases and tarry matter are freed; they combine with dust and ash and produce smoke.

The air in any city is full of suspended solid particles, but not all of this is smoke. It may contain dust, vegetable matter, and other materials. All of those gradually settle under the force of gravity. In small towns or suburbs, probably about 75 to 100 tons of these deposits settle down per square mile during a year. In a big industrial city, the deposits may be 10 times as great!

Smoke can do a great deal of harm. It damages health, property, and vegetation. In big industrial towns, it lowers the intensity of the sunshine, especially the ultraviolet rays which are essential to health.

If the wind didn’t spread the smoke, big industrial towns would probably have fog every day. In fact, where smoky fog occurs, it often happens that the death rate goes up for lung and heart diseases.

The effect of smoke on vegetation is especially harmful. It interferes with the “breathing” of plants and screens off needed sunlight. Quite often, the acid in the smoke destroys plants directly!

Today, many cities are waging active campaigns to cut down on smoke or to prevent it from doing damage.

WHY IS A DESERT DRY?

What is a desert? A desert is a region where only special forms of life can exist. All deserts have a shortage of moisture, which means the life that exists in them must be able to get along with almost no water.

The amount of rainfall largely controls the amount and kinds of plan life in a region. Forests grow where there is much rain. Grassland grows where the rainfall is less. Where there is still less rain, only scattered, specialized desert plants can live.

Sahara in Africa

The hot deserts near the Equator, such as the Sahara in Africa, lie in subtropical land where the air settles downward, becoming warmer and drier as it does so. Lands in these areas are dry, even though they are next to the ocean. The same is true of deserts in northwest Africa and Western Australia.

Deserts farther away from the Equator are generally caused by the great distance from the sea and its moist winds, and by the mountains between the desert and the sea. These mountain barriers may catch rainfall on their seaward side but the interior, leeward region remains dry.

This is known as "the rain shadow" effect. The deserts of Central Asia lie in the rain shadow of the great Himalayan ranges and the plateau of Tibet. The deserts of the Great Basin, in Western United States, lie in the rain shadow of high mountain ranges on their west, such as the Sierra Nevada.

Deserts differ greatly in appearance. Where sand is abundant, the winds may build sand hills or dunes. These are sand deserts. Rock deserts consist mostly of bare rock, which forms fantastic cliffs and hills, or rough, jagged plains. Other deserts, like much of Southwestern United States, include barren Rocky Mountains and dry plains of soil and gravel. The wind sweeps away finer soil and the stones which are left form a gravelly surface called "a desert pavement."

Most deserts have some sort of plant and animal life. Desert plants often have little or no leaf surface to reduce evaporation of water from the plant. Or they have thorns or spines to discourage animals from eating them. Animals which can live in deserts can go for long periods without water and get liquid from plants or night dew.