Last night San Francisco, and the entire state, was riveted by a spectacular lightning storm. According to the National Weather Service, more than 11,000 strikes were reported in the Bay Area. Across the state, 3,400 strikes caused fires that burned 150 acres, according to the California Department of Forestry and Fire Protection. A Red Flag Warning of fire danger is still in effect for the Berkeley Hills until at least 11 AM today. Lightning strikes even enlivened the Giants 7-0 loss to the Boston Red Sox, with bolts visible from the stands at AT&T Park.
It was hard to miss the dry lightning if you were outside last night. Several series of strikes were visible over Bernal Hill, near North Beach, Sutro Tower, and the San Bruno Mountains. But the web was transfixed too, as the Twittersphere lit up as well. KarltheFog cracked wise that, "Don't mind me, just shooting lightning bolts out of my sides." @angryjewishbetc complained, "I almost got struck by lightning Monday. Officially more scared of nature than the average skill of muni drivers." And coleen just shook her head: "It makes me feel better that no one can spell lightning in San Francisco, apparently." (For the record, there's no e in the word.)
But what actually happened last night? The kind of lightning that we saw is called dry lightning, which means that it occurs in the absence of rain that reaches the ground. Nobody was wet, but we still saw the bolts. How does that work? According to this exhaustive account in the Southern California Fire Journal, dry lightning is a misnomer. These strikes occur during conditions in which cloud precipitation evaporates before it hits the ground. Dry lightning is likely to occur when the air temperature is warm, but conditions are dry. The clouds that form under those circumstances tend to be relatively high off of the ground, which is one of the reasons that the rain evaporates before it reaches us. Much of the lightning that we saw last night was discharged from a point within the clouds to another point in the clouds, not the ground, in a situation known as a "cloud flash." (In general, lightning forms when charged water ions within the cloud polarize, with positive-charged ions at one end and negative ones at the other.) Dry lightning often forms in what is known as the "North American Monsoon" region over Mexico and California. Dry lightning is also more likely to cause fires than the kind with rain associated with it (because the water in the rain tends to lower the chance of flames), which is why fire authorities are on such high alerts recently.
Bonus Explainer: Dry Lightning, off of his solo album The Ghost of Tom Joad, is also the title of one of Bruce Springsteen's most under-rated songs. Listen to an acoustic version of it here.