VI. The urban climate of Budapest: trends and perspectives . 117
VII.5. Evaluation of the urban impact on air humidity, fog, heavy rains and
VII.5.2. Hails
The number of days with hails in large cities is higher than in other settlements of the countryside (Figure 34). This is explained by the fact that the atmosphere in cities is polluted by aerosols. Buildings, roads, highways and other urban elements are getting dirtier than soil and grass within a day. The convection increases above large cities and bigger cloudiness is formed than in the surrounding areas.
Figure 34: Long-term changes of annual number of days with hails per year for large cities and other settlements of Belarus
Source: calculation of the author
Besides, the city is ‘heterogeneity’ height. It can be considered as the roughness of surface. The latter transforms vertical and horizontal movements of air. It means that conditions in large cities are more favourable for the formation of hails. In Figure 34 the long-term course of change of mid-annual number of days with hailstones can be seen for large cities and also for the countryside.
From Figure 34 it follows that the number of days with hails per one meteorological station is 40 percent higher.
VII.6. Conclusions
The estimations of the amounts of anthropogenic heat fluxes in urbanized areas of Belarus and some other countries were made using methods developed by the A.M. Obukhov Atmospheric Physics Institute. The highest anthropogenic heat fluxes were revealed in The Netherlands, Korea, Belgium and Luxembourg.
For instance in Hungary, due to the greater population density, anthropogenic heat fluxes are more than twice as high as in Belarus.
The contribution of the ‘heat islands’ to the temperature change of Minsk from 1961 to 1990 was about 0.5 ˚C, and during 1991–2009 it reached almost 0.7 ˚C. In the period from 1961 to 1990 warming was greater at night, but in recent years this feature is not visible in the temperature pattern. Observed summer temperature rise tends to be more intensive than the winter one.
A significant feature of ‘heat islands’ of large cities is their seasonal variations of intensity and clear dependence on the diurnal temperature, especially in cloudless weather. Probably, it is defined more frequent inverse temperature stratification during the cold season and at night. Associated with thermal emission temperature increase is ‘locked’ in a thin layer in winter, and convection ‘smearing’ it in a thicker layer of the atmosphere in summertime.
Excluding the effect of urbanization growth in the amount of
VII. The estimation of the impact of urbanization on climate and extreme weather phenomena
during the last years (2005–2013) confirm the existence of a fourth area.
Analysis of differences in humidity ‘large city–countyside’
shows that during the winter months the humidity in a city is higher than in surrounding areas. The burning of organic fuels leads to the formation of additional water vapour in the urban territory. Total transpiration by plants in a city is lower than in the surroundings, with larger vegetated areas. Therefore, humidity in a city is lower than in neighbourhood, especially during daylight hours.
Growth of cities like Minsk and Gomel is accompanied by an increased number of foggy days. However, in very large cities like St. Petersburg, the number of foggy and misty days still decreases, due to the higher temperature in large cities in comparison with the countryside. Therefore, to obtain conditions necessary for the formation of fog and mist, more water vapour is required.
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