The answer is BOTH: There are distinct types of lightning strikes to earth that can travel in either direction - cloud-to-ground lightning and ground-to-cloud lightning. For each of these types of lightning, current flow and leader development can also take place in both directions.

For a negative cloud-to-ground discharge (the most common type of lightning striking the ground), the stepped leader (the channel of ionized air) begins in the lower section of a thunderstorm cloud and propagates. When the tip of the stepped leader approaches the ground, one or more upward-movinginitiate from the ground. The two opposide-moving leaders meet in midair, usually at a point about 300 feet or less above ground. When the stepped leader and leader meet, they provide a conducting path for charge flow, like a wire connecting the cloud and the ground. There is then a tremendous flow of currentthrough this established channel , brightly illuminating it.

This animation depicts the stepped leader descending to meet the upward leaders extending from the ground, and the first and subsequent return strokes. This is an extremely slow-motion animation- the actual process takes only a small fraction of a second. AT RIGHT: Photo of cloud-to-ground lightning.

Fig. 1: From high-speed video of a cloud-to-ground strike near Arian, Texas: The stepped leader descends, followed by the bright return stroke in the last 3 frames.