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). 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.