A 553-meter lightning rod
The CN Tower's great height of 553.33 meters makes it a predictable target for an otherwise unpredictable atmospheric event. It receives an average of 75 lightning strikes annually, making the structure a unique, high-altitude laboratory for studying the physics of lightning. Scientists began systematic observations here in 1978, just two years after the tower was completed. Comprehensive investigations have been ongoing since 1991, allowing for long-term data collection.
To capture the fleeting details of a strike, the tower is fully instrumented. Equipment includes high-speed digital cameras, electric field sensors, and Rogowski coils to measure the lightning's current. Since 2015, a high-resolution data acquisition system has been in use, capable of recording the electrical current of an entire flash with a time resolution of 4 nanoseconds. This equipment helps researchers analyze the complete lifecycle of a lightning flash, from its initiation to its final discharge. The data gathered helps improve lightning protection systems for other tall structures, such as wind turbines and skyscrapers. The tower's lightning protection system itself is robust, consisting of long copper strips that channel the electrical charge down the building's core to a network of 42 massive grounding rods buried deep in the earth.
The upward-moving spark
A primary focus of the research at the CN Tower is "upward-initiated lightning." Unlike cloud-to-ground strikes, these flashes start at the top of the tower and propagate upwards toward the storm cloud. The extreme height of the tower in the presence of a strong electric field from a thunderstorm above allows it to launch a positively charged leader into the air. This upward leader then connects with the negatively charged region of the cloud, creating the main lightning channel.
High-speed video analysis revealed that in some cases, lightning even strikes the tower below its tip. The detailed current measurements have also identified two distinct types of negative lightning impulses. The vast majority have fast wavefronts, characteristic of upward-initiated strikes. A smaller number of impulses are slower and more powerful, which are believed to be the result of more conventional downward-initiated lightning hitting the tower. The electromagnetic pulses generated by strikes to the tower are also significantly stronger than those from other lightning, which helps protect sensitive electronics in the vicinity of very tall structures.