Cloud-to-ground (CG) lightning is a lightning flash that connects charge in a thunderstorm to the ground. It is the kind that strikes the ground, trees and structures, even though it makes up a minority of all flashes.
Negative and positive flashes
Most CG flashes lower negative charge. In the typical case a negative stepped leader works down through the cloud toward the ground, and an upward return stroke follows.
Positive CG flashes lower positive charge, usually from the positively charged upper part of the storm. NOAA's JetStream puts them at less than 5 percent of strikes, while an NWS lightning page gives about 10 percent, so the share is small and should not be treated as a fixed number.
Because positive flashes often start away from the main updraft, from the cloud edge or the anvil, they can land well away from the rain. JetStream notes they can strike more than 25 miles from precipitation.
Where positive flashes show up
JetStream notes that positive lightning can be the dominant CG type in winter and during a thunderstorm's dissipating stage, and that its peak charge and potential can be up to ten times that of a negative flash.
These are tendencies, not rules. A run of positive strikes is a reason to look closer at a storm, not a forecast of what it will do next.
How CG lightning is detected
Ground-based networks such as the National Lightning Detection Network (NLDN) use more than 100 sensing stations across the US. NSSL notes that within seconds of a strike, the location, time, polarity and amplitude are processed.
The Geostationary Lightning Mapper (GLM) works from orbit and maps total lightning, in-cloud and cloud-to-ground together. The two answer different questions: GLM shows how electrically active a storm is overall, while a CG strike map shows where flashes reached the ground.
What a CG map leaves out
A CG strike map undercounts a storm's lightning, since NSSL notes cloud flashes outnumber CG flashes roughly 5 to 10 times. It also cannot tell you where the next strike lands, and positive flashes can reach the ground away from the rain, so the NWS rule still applies: When thunder roars, go indoors.