heat lightning
From Glossary of Meteorology
Nontechnically, the luminosity observed from ordinary lightning too far away for its thunder to be heard.
Since such observations have often been made with clear skies overhead, and since hot summer evenings particularly favor this type of observation, there has arisen a popular misconception that the presence of diffuse flashes in the apparent absence of thunderclouds implies that lightning is somehow occurring in the atmosphere merely as a result of excessive heat.
