14. Blue Crawlers: The Horizontal Lightning Phenomenon
A rare kind of blue lightning, blue crawlers spread horizontally either inside or slightly above the cloud layer. Blue crawlers move laterally, producing complex patterns of blue light that seem to “crawl” across the heavens unlike vertical discharges such as blue jets or sprites.
The same process—excitation of nitrogen molecules by electrical discharges—produces the blue hue of these crawlers, same as other blue lightning events. But the horizontal migration of blue crawlers produces a unique optical impression, sometimes likened to a network of blue veins over the cloud surface.
Usually connected with big, intricate thunderstorm systems, blue crawlers could help to distribute electrical charge inside the storm. Though less frequent than certain other blue lightning events, blue crawlers offer important new perspectives on the three-dimensional electrical structure of thunderstorms.