AbstractThis study examines diurnal variations of lightning flash characteristics observed by the Lightning Mapping Array in São Paulo, Brazil. The diurnal flash counts exhibit the typical afternoon convective maximum. The mean source altitude demonstrates a discrete increase that is temporally coincident with the local sunrise. The mean horizontal and vertical flash extents each attain a maximum (minimum) around local sunrise (afternoon, i.e., 13:00–17:00 local solar time). In addition, joint histograms of flash horizontal and vertical extents show that the majority of the flashes occurring during the afternoon convection are shorter and more comparable in size, and the differences between the horizontal and vertical extents are reduced. Conversely, flashes preceding and following the peak in afternoon convection are less symmetric, with larger horizontal than vertical extents. We discuss whether these observations could be partially explained by the diurnal variations in the convectively induced mixing regimes that control thundercloud charge regions and associated charge separation distances. The documented diurnal flash characteristics closely match recently published findings on the diurnal variation of the peak currents of cloud‐to‐ground flashes. Possible physical mechanisms for these observations are discussed.