Recently, Brazilian researchers were successful in photographing the upward positive electrical discharges from lightning conductor rods that travelled to connect with the downward negative discharge from lightning in the clouds. The phenomenon, also known as “upward lightning” or upward flashes, has long been observed, but the researchers were able to capture it on camera using high-speed video cameras and very high resolution, according to The New York Times.
The researchers recorded the intense activity in So José dos Campos, a Brazilian city northeast of So Paulo. They published their research and photos in the open access journal Geophysical Research Letters in December last year.
Storm electrification and the resultant existence of a cloud charge area are necessary conditions for this to occur. The electric field locally on the ground is enhanced by the vertical elevation of tall objects, creating favourable conditions for the initiation of an upward streak (known as a leader) from a tall object. A leader can also form in response to an electric field change caused by a nearby preceding lightning flash.
There have been numerous upward lightning studies conducted in Rapid City, South Dakota, USA, and Sao Paulo, Brazil, between 2011 and 2016, during the summer thunderstorm seasons, according to a paper published in Nature on the triggering mechanisms of upward lightning by Carina Schumann and others (July 2019).
Characterizing the triggering of upward positive leads from tall objects as a result of earlier adjacent flash activity was one of the studies’ main goals. This included the use of a variety of high- and standard-speed video and digital still cameras, electric field metres, fast electric-field antenna systems, and lightning mapping arrays to observe a total of 110 upward flashes.
These data sets and correlated lightning location system data were analysed to identify the triggering flash type that initiated upward leads from towers. The propagation of the in-cloud negative leader during the continuous current that follows a positive return stroke, according to the article, is the most efficient triggering element.
Simply explained, the development of the stepped leader, which is effectively a channel of negative charge that goes downward in a zigzag pattern from a cloud and is practically imperceptible to the human eye, causes the positive charge on the ground to intensify in a nanosecond.
The electrical charges between the leader’s tips and the tops of tall objects on the ground keep growing as the stepped leader’s streaks continue to streak in the direction of the ground. These pressures eventually lead to the ionisation and subsequent increase in conductivity of the air over these towering structures.
An upward streamer is created when the positively charged channel of air immediately above the tall objects turns positively charged as the negative charge repeatedly moves towards the earth. In due course, one of the emerging positively-charged upward streamers makes contact with the negatively-charged, downward-moving stepped leader.
According to the National Oceanic and Atmospheric Administration (NOAA), it only takes a fraction of a second to go from the stepped leader initiation to the final connection being made with an upwards streamer. When contact is eventually made, the lightning channel is complete and charges can flow rapidly from the cloud towards the ground.