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
downward. When the tip of the stepped leader approaches the ground, one or more upward-moving
leaders initiate 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 current
upwards through 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.
Unlike cloud-to-ground lightning which starts inside the thunderstorm, a
ground-to-cloud lightning flash begins from a tall ground-based object and moves
upward. This type of lightning is common with strikes to towers and skyscrapers:
read more.
This animation depicts a type of upward-moving ground-to-cloud lightning striking a tall television tower. AT RIGHT: Photo of ground-to-cloud (upward) lightning striking a television broadcast tower.
