ASK TOM: Does lightning start from the ground or from the sky?

Dear Tom,
When lightning strikes the ground, does it start from the ground or from the sky? My wife and I are constantly arguing about this.
Brian Miller, Morton Grove
Dear Brian,
It does both, but the great majority of lightning ground strikes involve the flow of electric current from cloud to ground. The installation of a nationwide lightning detection network in the early 1990s enabled researchers like lightning expert Ron Holle to count lightning bolts accurately, and Holle says the numbers are stunning: Just in the United States, the tally is 100 to 125 million bolts annually, of which about 25 million are ground strikes. Of those 25 million, 90 percent are termed “negative,” meaning the flow of electric current is from cloud to ground, and the remaining 10 percent are “positive,” current flowing from ground to cloud.