Lightning Strike Over
Portland, Maine
(image © Max Riseman)
This day featured numerous Severe Thunderstorm Warnings throughout the afternoon; the National Weather Service issued a Severe Thunderstorm watch that expired at 5:00PM. The rough weather came through Portland right around that time, with very heavy rain and much lightning. At our observation point near Back Cove, there was heavy rain as well as several very close lightning strikes. No hail or extreme winds were noted.
There were two lightning strikes in particular that were extremely close. I had my DV camcorder running during this time, and recorded both close strikes on tape; at least the flash and the resulting thunder. The first one was very loud, and sounded much like a gunshot or explosion; in this particular one, the windows of the car were closed. The second clip was taken from the window of my car, looking toward Portland in-town. The open windows allowed for a much nicer representation of the 'electric slap'. The sound map of this thunder's onset is below.
Thunder Sound Intensity Over Time
(from Sequence 2)
In calculating the approximate distance from us to the lightning strike, the frame time of the first sound (the 'slap') was used.
The photo at the top of this report is taken from a third movie sequence with three distinct bolts that were recorded on tape. The slow-motion animation link below shows the three strikes - they are not visible during the full-speed loop of sequence 3; only two were visible in the full-speed DV playback.
In an attempt (afterward) to figure out where the bolts in Sequence 1 and 2 hit, I used the time stamp on the camera, combined with the frame time of the flash and the frame time of the first hint of thunder from that stroke; relative positions of objects in the picture were used for timing. Weather values from Weather Maine (South Portland) were used to calculate the speed of sound. At 28°C/82°F and 85% Relative Humidity, the speed of sound is 350m/s. Combining that information with the timing of the video frames (1/2 to 3/4 second), I calculated that the approximate distance to the strike (at least the origin of the sound) to be between 500 and 850 ft (175m to 260m) away! Upon referencing a local topographical map (1:25000), I looked for recognizable buildings. Given the location of the buildings on the map, the apparent location of the flash in the footage, as well as the approx. 500ft between us and the strike, I surmise that both strikes hit the communications tower by Hannaford and Arby's, near Back Cove. It is unclear where the third sequence of strikes hit.
Movies: Sequence 1 | Sequence 2 | Sequence 3 (340Kbps WMV)
Animated GIF: 24 frames taken from the DV camcorder. There is approximately a 0.07 second interval between frames, 24 frames total. (654KB). This is a slow-motion shot of Sequence 3 above:
