THIRUVANANTHAPURAM: The unusual light seen in the eastern skies of the capital city on Sunday night was ‘heat lightning’, R Jayakrishnan, Honorary Director of the Astronomical Observatory of the University of Kerala, has said.
The rare phenomenon had stirred up a discussion among residents and the observatory had received several phone calls from the public seeking explanation regarding the phenomenon.
“Heat lightning is a term commonly used to describe lightning from a distant thunderstorm which is too far away that the actual cloud-to-ground flash cannot be seen nor the accompanying thunder can be heard,” he explained.
The movement of sound in the atmosphere depends on the temperature and density of air. As temperature and density vary with height, the sound of the thunder is refracted through the troposphere. This refraction results in spaces through which the thunder does not propagate.
“Many think that heat lightning is a specific type of lightning. But in fact, it is simply the light produced by a distant thunderstorm. Often, mountains, hills, trees or just the curvature of the earth prevents the observer from seeing the actual lightning flash,” Jayakrishnan added.
The faint flash seen by the observer is the light being reflected off higher-level clouds. Also, the thunder can only be heard for about 10 miles from a flash. This sound often reflects off the Earth’s surface and the resultant rumbling sound is partly due to these reflections. This reflection and refraction can leave voids where thunder cannot be heard.
According to the scientist, the troposphere usually reflects the light, and leaves out the sound. In such cases, some fraction of the light emanating from distant thunderstorms, whose clouds may be so low to the horizon as to be essentially invisible, is scattered by the upper atmosphere and thus visible to remote observers. Scattering by different constituents in the air results in various colors, he added.