The results of experimental studies of tens to hundreds millisecond long optical flares in the nighttime atmospheric emission in mid-latitudes are presented. Luminous emittance of optical flares in different regions of the optical spectrum was estimated, and distributions of the flare duration and the probability of their observation during a night were obtained. The moments of optical flares occurrence in time were compared with those of the gamma-ray bursts according to data from the BATSE catalog. It was concluded that the questions on the nature and sources of optical flares are still open.