Particular problem sites include roadsides. The level of pollution is influenced by certain common weather factors. When pollutants are emitted, they spread out through the air and become diluted. This dispersal is greatly assisted by swirling movements of the air, called turbulent eddies. If the decrease is fairly rapid, large vigorous thermal eddies are generated.
Mechanical eddies occur because of frictional effects of wind moving over various surfaces.
Sometimes temperatures are colder near the ground and then increase with height. This is a temperature inversion. Pollutants emitted into this air disperse very slowly, often developing high concentrations around the emission sources. Australian weather is dominated by the high pressure systems or anticyclones that continually migrate from west to east around the southern hemisphere. As each anticyclone passes over, it usually brings fine, cloudless weather with little or no wind for two to four days.