Lightning, despite being a common and readily-detected phenomenon, is nevertheless difficult to study. One reason is the difficulty of predicting when and where lightning will strike; tall structures do attract more lightning strikes, but it’s hard to move them into a storm’s path. Instead, researchers often use small rockets carrying a fine wire to trigger strikes, an approach [Electron Impressions] recently replicated (more details).
The science of this is less than straightforward: even in calm weather, there’s a surprisingly strong atmospheric electric field, about 100 volts per meter off the ground. During a thunderstorm, though, this can build up to kilovolts per meter, and may reverse polarity. When an updraft carries supercooled water, ice crystals, and graupel (ice particles formed by supercooled water freezing on a snowflake) upwards, the heavier graupel falls relative to the other components. As it collides with ice crystals, it builds up a negative charge and the crystals accumulate a positive charge; across a storm, this leads to positive charge building up near the top of clouds and negative charge near the bottom. Lightning equalizes this imbalance. In the relatively few cloud-to-ground strikes, a dielectric breakdown begins from both sides of the stroke, with leaders rising from the ground and descending from the cloud. The wire trailed by a rocket creates an artificial leader, ideally triggering a controlled strike.
To consistently get a strike, the rocket needs to be launched under a strongly-negatively-charged region of the cloud. An electric field mill measures the local field strength; these are usually quite expensive, but [Electron Impressions] managed to build his own. The rocket itself was 3D printed and designed to fly well under stormy conditions. It carried a strand of thin copper wire wound onto a plastic spool meant to minimize friction and prevent broken wires. The rocket’s igniter was waterproofed for storm conditions and remotely triggered using a walkie-talkie for safety.
After many attempts, [Electron Impressions] finally managed to trigger a strike and video the results. The first stroke created a plasma channel, along which several more strokes followed. This indicated that the cloud had probably been negatively charged, which was in agreement with the field mill’s measurements. The wind blew the plasma channel slightly to the side, where it caused a second rocket to explode on the ground. Both ignition systems were destroyed, and the remains of the rocket were never found. Sadly, the strike doesn’t seem to have formed a fulgurite, but it did fracture the ground as water flash-boiled.
For a more reusable solution, some researchers have also experimented with using drones to trigger lightning. There’s good reason for more study; the theory of lightning formation still has some major open questions.
