SMOG is a term developed by combining the words FOG and SMOKE. Smog has characteristics of both fog and smoke. The smoke is the product a human air pollution and the fog is a product of moisture condensing onto the air pollutants or the air pollution's whitish color reducing visibility. There are several factors that when combined together produce smog. Each of these is covered below.
Cars, factors and burning add pollutants to the air. In the urban environment these pollutants are concentrated over a small area of the earth's surface.
Forest fires put large amounts of pollutants into the air. The burning of wood gives off large amounts of smoke.
Light wind keeps the pollutants concentrated over a certain area. The pollutants can not be dispersed until wind and other factors mix out and dissolve the pollutants.
Widespread rain will remove air pollutants from the troposphere. In periods of no rain this can not occur.
INVERSION, HIGH PRESSURE SYSTEM
A high pressure system will have stable sinking air. This prevents the pollutants from mixing out with air aloft. An inversion (temperature increase with height) results in stability also and will trap pollution near the surface.
WARM AND HUMID AIR
This is the factor that makes the smog reduce visibility more. At warm temperatures the air can evaporate large amounts of water vapor into it. If the relative humidity is high at the same time that it is warm then that is an indication large amounts of moisture are in the air. With the relative humidity high, moisture will condense on the pollution particles. This occurs even when the relative humidity is less than 100% on certain particle types. Rain will not occur even when the air is very warm and humid if high pressure is preventing the air from being lifted to condense out rain. This situation produces hot, hazy and humid weather.
The sun's radiation will react with the air pollution to produce new pollutants such as ozone that reduce visibility further.
If certain of the factors above persist over time the tendency will be for the smog to get worse each day.