A seasonal–diurnal analysis of land-sea contrast in lightning activity over Bulgaria and the Black Sea over 10 years is presented here. The maximum number of flashes over both surface types is registered during the summer (with a peak over Bulgaria in June and over the Black Sea in July) and a minimum number in winter (December/February, respectively). During spring, the maximum flash density is observed over Bulgaria (in May), while in autumn, it is over the Black Sea (in September). The results show that only in autumn lightning activity dominates over the Black Sea compared to over land (Bulgaria), while in winter, spring, and summer is vice versa. For this reason, an additional investigation was conducted to determine whether there is a relationship between lightning activity and the sea surface temperature (SST) of the Black Sea in autumn. The analysis reveals that the influence of SST on the formation of thunderstorms over the Black Sea varies depending on the diurnal time interval, with the effect being more significant at night. At nighttime intervals, there is a clear trend of increasing mean flash frequency per case with rising SST (linear correlation coefficients range from R = 0.92 to 0.98), while during the daytime, this trend is not as evident. This indicates that, during the day, other favorable atmospheric processes have a greater influence on the formation of thunderstorms than sea-surface temperature, while in the autumn night hours, the higher SST values probably play a more significant role in thunderstorms formation, in combination with the corresponding orographic conditions.

The analysis of land-sea contrast in lightning activity in autumn on a regional scale is presented here. The lightning data over Bulgaria and Black Sea in September, October and November for 10 years (2005-2014) are used. Data for the time and the position of lightning are provided by the ZEUS network operating at the National Observatory of Athens. The investigation reveals that flash density in autumn during the studied 10 years is about 2 times higher over the Black Sea than over land (Bulgaria) and the maximum flash density occurs in September over the Black Sea. Diurnal variation of flash density (at 3-h time intervals) shows that the maximum of lightning activity in autumn is observed over the maritime area (Black sea) in the morning interval (0600-0900UTC). According to the results, lightning activity in most of the considered time intervals is higher over the Black Sea than over land, with the exception for the afternoon hours: in time interval (1200-1500 UTC) it is vice versa and in (1500-1800 UTC) - the flash density values are equal. The spatial distribution reveals that there are much more number of lightning over the southern part of the Black Sea, compared to the northern part of the basin. Over Bulgaria, the centers with the maximum number of lightning are over the mountains.

The lightning activity over land (Bulgaria) and over maritime area (Black sea) during summer period (June, July, August) of 10 years is analyzed. Data for the location and time of detection of lightning provided from the ZEUS network operated by the National Observatory of Athens (NOA) are used. The results shows that summer-time flash density for each of the analyzed years is higher (between 1,5-3,5 times) over Bulgaria than over the Black sea, although the large differences in lightning activity during each year is established. However diurnal variation of flash density (at 3-h time interval) shows that during the night and morning hours lightning activity is higher over the Black sea than over Bulgaria (land), while between 9000 and 1800 UTC there is a predominant presence of lightning over land (Bulgaria). The analysis of spatial distribution of flashes at 3-hour time intervals reveals that the maximum of lightning activity during the afternoon (1200 UTC – 1500 UTC) correlates well with the location of mountains.