3. The regional climate and air situation
Information on the dispersion of pollutants


The dispersion of air pollutants is influenced not only by the wind conditions but also by the vertical thermal stratification of the atmosphere and by air turbulences. For the calculation of the dispersion (mathematical immission prognosis), the relevant parameters are summed up in dispersion categories (diffusion categories) depending on wind direction and wind speed.

Tables 3.7 and 3.8 show these categories on the basis of data for Stuttgart airport from the German Meteorological Service.

Wind-  direction	Dispersion categories
	I	II	III1	III2	IV	V
350 - 010	0,7	1,4	1,6	1,3	0,6	0,4
020 - 040	0,7	1,2	1,6	1,1	0,8	0,5
050 - 070	0,7	1,1	1,5	1,2	1,0	0,6
080 - 100	0,7	1,2	1,2	1,3	1,1	0,6
110 - 130	0,2	0,5	0,3	0,4	0,4	0,2
140 - 160	0,3	0,7	0,4	0,4	0,3	0,2
170 - 190	0,7	1,3	1,5	0,8	0,3	0,1
200 - 220	1,0	1,9	5,9	1,4	0,6	0,1
230 - 250	1,8	2,9	8,9	2,1	0,5	0,2
260 - 310	1,2	1,8	3,5	1,3	0,3	0,1
290 - 310	1,0	1,4	2,2	1,1	0,3	0,1
320 - 340	1,1	1,6	1,9	1,4	0,5	0,2

Table 3.7: Frequency distribution of dispersion categories according to the wind direction in % (for Stuttgart-Echterdingen)

Wind  speed  in m/s	Dispersion categories
	I	II	III1	III2	IV	V
< 2	19,0	19,1	4,1	4,2	5,2	2,2
2,1 - 5	-	3,8	16,5	9,2	2,3	1,4
> 5	-	-	10,4	0,9	0,2	0,0
Sum	19,0	22,9	31,0	14,3	7,7	3,6

Table 3.8: Frequency distribution of dispersion categories according to the wind speed in % (for Stuttgart-Echterdingen)


The application of dispersion statistics requires a thorough verification of the wind situation and, if necessary, a modification of the statistics, especially in valleys.

The tables illustrates the high frequency of the stable dispersion categories I and II, representing the poor mixing of the air between sunset and sunrise. These occasions with bad dispersion conditions (ground inversions) are nearly as frequent as the "normal case" with a neutral stratification, represented by the dispersion categories III1 and III2. Situations with an unstable stratification and therefore a high degree of turbulences in the atmosphere (categories IV and V) are relatively rare (e.g. on warm summer afternoons with the formation of cumulus clouds).

With the exception of the unstable categories IV and V, the wind direction sector 230 to 250 degrees is by far the most frequent for all other dispersion categories in our climatic region. This means that the prevailing wind direction is predominant in all dispersion cases relevant for air hygiene. This statement is to invalidate the idea that only easterlies are responsible for air pollution during the winter. What is right, however, is the fact that the annual peak values of air pollution occur during the generally very cold "easterly weather conditions", which are rare in fact and are exceeded in frequency by about 98 % of all measurements during the year.

© City of Stuttgart, Office for Environmental Protection, Section of Urban Climatology

• Kontakt
• Sitemap
• Suche
• Hilfe
• Intranet Login
• Intranet
• Impressum