When the charges have enough energy to begin to ionize the air a the free electrons will form a negatively charged “stepped leader” that will go from the cloud and make its zigzagged and often branched trip toward the ground. This process is slow, taking a few tenths of a second. The leaders are also weak and usually invisible. The atoms in the air near the ground, feeling the attractive force from the electrons in the stepped leader, separate into ions and free electrons. The positively-charged air ions from tall objects, such as trees, buildings, and towers leave in streamers. When a stepped leader and streamer meet, a channel of ionized air is created, allowing large amounts of charge to move between the cloud and the ground. The return stroke of charge back to the cloud is the brightest part of the process.