Weather and climate are not the same thing. Weather refers to the conditions of temperature, cloudiness, windiness, humidity and precipitation that prevail at a given moment or the average of such conditions over time periods ranging from a few hours to a few days. The weather can change from hour to hour, from day to day or from week to week.

Troposphere is the medium in which the phenomenon of weather takes place. The structure of the troposphere with the warmest air at the bottom promotes vertical instability leading to hot air rising and cold air sinking. This is because hot air is lighter than the cold air, hence the troposphere tends to be vertically well mixed. Under certain circumstances, however, the usual temperature profile of the troposphere near the ground is altered so that temperature increases with altitude. In this situation, called an ‘inversion’, with warm light air above and the cold heavy air below, vertical mixing is suppressed over the altitude range where the temperature has increased. Vertical mixing is also suppressed in circumstances where temperature decreases with altitude but not enough to overcome the stratifying effect of density variation. Large quantities of particulate matter in the air can lead to inversion.

Inversions are of special importance in environmental science because they inhibit the dilution of pollution causing what is commonly known as smog. The word ‘smog’ was coined in England for the ‘smoke-fog’ pollution associated with the country. In December 1952, London was covered by smog for ten days. Though deaths occurring at the time were not directly attributed to the smog, later statistics confirmed that 6,000 more people, especially the sick and the elderly, died at that time of the year than usual. This smog was caused not only by fog, smoke and ash particles, but also by the presence of a large quantity of sulphur dioxide gas in the air due to burning of coal in the city for heating and industrial purposes.

The “London smog” is very different from the “Los Angeles smog” or “photochemical smog”. The later is a newer variety of smog caused by automobile exhaust. Formation of photochemical smog, is a case of ‘synergism’ which in layman’s expression, is equivalent to saying two plus two is greater than four. Let us now consider synergism in the formation of smog. You know that oxides of nitrogen and unburnt hydrocarbons are both emitted in the automobile exhausts. In the presence of ultraviolet radiations in sunlight, they interact to produce Peroxyacetyl nitrate (PAN) and ozone. PAN and ozone are more harmful than either nitrogen oxides or hydrocarbons or even the two put together. Because of the wind patterns in the eastern Pacific Ocean, and the ring of mountains surrounding the Los Angeles basin, it is an ideal place for the formation of inversions, usually about 2,000 feet above the floor of the basin. Another climatic feature that contributes to Los Angeles air pollution problem is its abundant sunlight which, as discussed above, acts to produce photochemical smog.

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Precipitation, i.e. rain, snow, sleet, and hail, too, bring down to the earth’s surface, pollutants suspended in the atmosphere. Though pollutants such as chlorofluorocarbons cannot be washed down, sulphur dioxide molecules have a strong affinity for water molecules. They combine to precipitate as acid rain. Acid rain is destructive not only to human health and plant and animal life, but also to buildings.