On the night of 12 Apr 2012, the Bay Area of California experienced a storm that literally lit up the skies with upward lightning. Some iconic photographs and video were taken during this event which provided evidence that numerous tall objects developed upward leaders in response to nearby flashes. The foremost images that illustrated what happened that night were taken by Phil McGrew. He had his Canon 5D Mark III camera running continuously using 20 sec exposures. During two of these exposures, his camera captured upward leaders that developed from the Bay Bridge and additional structures on the east side of the Bay. For each of the two photographs that he posted, it is likely that all the upward leaders developed during the same flash that probably lasted less than one second. He was located in a tall building on the east side of San Francisco downtown looking east along the Bay Bridge.
Below are embedding links as provided by Phil’s Flickr page where he has posted two images. Click on the images to go to his Flickr page. The exif data on his Flickr page indicates that the first of the two images was taken at 8:38:29 pm PDT using ISO 100, f/10, 20 sec exposure and a 28 mm lens. In this image there appears to be 5 upward leaders from the Bay Bridge structure and 2 upward leaders from two separate structures on the east side of the bay (likely in the Oakland area).
Again based on the exif data, the second image that Phil captured was at 8:42:41 pm PDT (4 min and 12 sec later) and used the same camera settings. This image (which has rightfully received international acclaim) appears to show 6 upward leaders from the Bay Bridge structure and 4 additional leaders beyond the Bay Bridge likely from structures on the east side of the bay.
Phil’s photographs indicate they were separated by 4 min and 12 sec. Not know the time accuracy of Phil’s camera, we compared the indicated times and time difference between the two images with National Lightning Detection Network (NLDN) data. Based on previous research findings, we suspected that these upward leaders were triggered by positive ground flashes (+CG) within 50 km of the Bay Bridge. Two very large estimated peak current +CG strokes were recorded at 3:39:59.425 and 3:44:12.332 pm PDT. They had estimated peak currents of +129.8 kA and +270.7 kA respectively and were separated by 4 min and 12.907 sec. There was a +27.8 kA stroke at 03:39:22.773 (37 min earlier of the first big +CG) and 2 -CG strokes at 03:40:32.843 and 03:42:21.957 fell within the time spaning the two large +CGs.
Below are GIS plots of the NLDN indicated return strokes and cloud events. The first figure shows the event location, event type by symbol (see legend) and estimated peak current based on relative symbol size. Notice the size of the +CG return stroke symbols relative to the other events.
The next figure shows the NLDN event locations and their times.
The last figure shows the NLDN events and a label of the estimated peak current.
We suspect that the upward leaders that developed from the Bay Bridge were positive polarity and developed following the large estimated peak current positive cloud-to-ground return strokes that occurred inside the Bay. These are examples of lightning triggered upward lightning in which the field change resulting from a preceding flash causes the development of upward leaders from nearby tall objects.
There were a number of other images from other people that showed upward leaders from tall objects during this same night and the other locations included the Golden Gate Bridge and tall buildings in Oakland. We suspect that these upward leaders also developed during the same triggering flashes that caused the upward leaders to develop from the Bay Bridge.