Conditional instability and lightning incidence in Gaborone, Botswana

Summary

Lightning is more frequent in deep convective storms formed by conditional instability. It has been shown that conditional instability increases with the wet bulb potential temperature. The incidence of lightning in Gaborone, Botswana was monitored over a period of two years with a CGR3 flash counter. The data were compared with the measured wet bulb temperatures. The results indicate that the monthly lightning activity in Gaborone increases by an order of magnitude for every 2°C rise in wet bulb temperature. There is also evidence to show that, in general, the ratio of lightning incidence to rainfall is significantly reduced as the wet bulb temperature decreases. Periods of continuous rain over a few days were generally characterised by a fall in the wet bulb temperature with a corresponding decrease in lightning activity. In consistence, one such nine day period was observed where the lightning incidence was sustained right through when the wet bulb temperature did fall. However, there is some evidence to indicate that the relationship does not hold very well during unusual winter lightning activity.