How does the CN Tower handle lightning strikes?
They say lightning never strikes in the same place twice, but try telling that to the staff at the CN Tower. The 553.33-metre freestanding structure gets hit on average 75 times a year, most often during the summer when pop-up thunderstorms fire down powerful electrical discharges with abandon.
First, a little about lightning. The brilliant white bolts are caused by massive electrical build-up in the roiling mass of a thunderstorm. Each strike contains upward of 100 million volts - enough to set you seriously askew - and between 10,000 and 200,000 amperes of current.
The crack and rumble of thunder is caused by air, superheated by lightning to around 28,000 degrees, five times hotter than the surface of the sun, moving in a shockwave.
A special weather station on the tower is programmed to detect lightning within in a 36 km area. That information is used to schedule exterior maintenance and the availability of the EdgeWalk, the circular catwalk around the top of the main pod.
The CN Tower has a series of long copper strips that run from the tip of the radio and cell antenna (the white bit above the upper observation deck,) down the building's hollow hexagonal core to the ground. When lightning strikes, the electricity is safely channelled to the soil south of Bremner Boulevard via a cluster of 42 grounding rods that penetrate 6 metres below the surface. Each one is 56 cms thick.
Irene Knight, the CN Tower's manager of public relations, says lightning has never damaged the tower and, thanks to the grounding system, the observation deck is allowed to stay open during inclement weather. "Visitors and staff inside the tower are very safe and not actually able to see the lightning strikes from within the building," she says.
Not so weird science.
Chris Bateman is a staff writer at blogTO. Follow him on Twitter at @chrisbateman.