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A new slow-motion video offers the best view yet of the split-second collision of electric currents that creates a flash of lightning.

The video captures a thread of electric current, or lightning leader, zipping down from a thundercloud to meet another leader reaching up from the ground. When the two touch, it triggers a much stronger current to surge between the cloud and ground, and lightning flashes.

A single strand of electricity, or streamer, at the frayed tip of each lightning leader is enough to forge the connection between the two currents, researchers report online February 1 in Geophysical Research Letters. Understanding this cloud-to-ground connection is important because it determines where lightning strikes.

Streamers at the tips of lightning leaders were known to link these currents together. But because streamers are so faint and lightning leaders merge in mere millionths of a second, it was unclear exactly how they fused together. The link could be forged by many overlapping streamers coalescing into a single channel of electric current, or by contact between single streamers from each leader.