The Effects of Meteorological Conditions

The seasonal climatic variations and long thermal inversions in the urban areas often make the difference between pure air and serious pollution. The meteorological factors that affect the dilution, dispersion and/or transformation of the emitted pollutants are: speed and direction of the wind, atmospheric stability, amount of sunlight or intensity of radiation, rainfall and temperature. These factors determine the temporal (hourly, daily and seasonal) and spatial variation of the resulting concentration of the emitted pollutants. The tendency of the atmosphere both to increment and to suppress the vertical movements affect the atmospheric concentration of the pollutants. A stable atmosphere tends to increase the concentrations, while an unstable atmosphere tends to minimize them. Stability is related to the vertical thermal structure and with the horizontal variation of the speed of the wind. Thermal inversion, which occurs when the normal vertical profile of temperature is reverted and layers of cold air remain trapped by warmer air, causes the increase in concentration of the pollutants collected in the cold air, given that the normal vertical current of air is impeded. In these circumstances, the production of both normal and photochemical smog  is enhanced. These phenomena could be more serious during the cold seasons when generally more fuel is used for heating (Bellomo and Liff, 1984; Faiz et al., 1990).

Another meteorological condition to consider is the maximum height of the mixing layer, i.e. the height below which relatively vigorous vertical air mixing occurs. This height may vary daily or seasonally in temperate climates. In the summer, during daylight, the mixing layer may be several hundred meters high, while in winter, it may be of only a few meters (Bellomo and Liff, 1984). If the same seasonal amount of pollutants were discharged throughout the year, we might expect higher pollution concentrations in winter, since there would be a smaller volume of air within which the pollutants could disperse.

References

BELLOMO, S. J.; LIFF, S. D.; 1984. Fundamentals of Air Quality for Highway Planning and Project Development, Training Manual. Federal Highway Administration, U.S. Department of Transportation; Washington, D.C., U.S.A..

FAIZ, A.; SINHA, K.; WALSH, M.; VARMA, A.; 1990. Automotive Air Pollution: Issues  and Options for Devoloping Countries. Doc. de Trabajo 492 de los Funcionarios del PRE. Banco Mundial, Depto. de Infraestructura y Desarrollo Urbano; Washington, D.C., U.S.A.