[1] The Brazilian lightning detection network detected about 310,000 cloud-to-ground flashes over three large metropolitan areas in Southeastern Brazil during three summer seasons (from 2000 to 2002). It was observed an enhancement of 60-100% in the flash density over the urban areas compared to their surroundings. Conversely, a decrease of 7 -8% in the percentage of positive flashes was also observed. The spatial distribution of the flashes follows closely the shape of the São Paulo city heat island, thereby supporting the thermal hypothesis. The PM 10 concentration showed a positive linear correlation with the number of cloud-to-ground flashes and a negative correlation with the percentage of positive flashes, supporting the aerosol hypothesis. These results indicate that both hypotheses should be considered to explain the CG lightning enhancements over the urban areas. However, only the aerosol hypothesis could better support the decrease of the percentage of positive flashes.

In this report thunderstorm day monthly records obtained in three cities of southeast Brazil (São Paulo, Campinas, and Rio de Janeiro) since the 19th century are analyzed. The analysis is complemented by the spatial distribution of lightning in the last decade. For São Paulo and Campinas, data indicate a significant increase in thunderstorm activity during the period from the end of the 19th century to the present, simultaneously to an increase in the surface temperature well correlated to the population growth of the cities. This result did not match anything expected from natural climate cycles and gives strong observational evidence for the anthropogenic influence on the thunderstorm activity. For Rio de Janeiro, data did not show a significant positive trend from the middle of the 19th century to the present in spite of the increase in the surface temperature, suggesting that variations are most probably a result of a complex combination of local and large‐scale features. In addition, a statistical analysis of the data after 1951 shows that a significant increase (by a factor of 3.7) in the thunderstorm activity in Rio de Janeiro occurs for the simultaneous occurrence of a positive anomaly of the South Atlantic sea surface temperature and La Niña, compared to the simultaneous occurrence of a negative anomaly and El Niño, even though no significant variation was found when each large‐scale phenomena occurs isolated. The same occurs for São Paulo and Campinas, although with a lower amplitude.

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Abstract. An 8-year climatological analysis (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996) of lightning network data over Belo Horizonte, a large city located in Southeastern Brazil, and nearby surrounding areas has indicated a significant enhancement of approximately 100% in the negative flash density and of 50% in the positive flash density over and downwind of the city, compared with the other adjacent areas. A decrease of 25% in the percentage of positive flashes was also observed over and downwind of the city. No urban effect was evident in the peak current of both negative and positive flashes. These results are in agreement with the recent results obtained by Steiger et al. (2002) for Houston, except that the strength of the effect is twice larger than in Houston. The reason for this difference is not clear.