Upward lightning from tall objects is a unique type of ground flash because it originates from a tall object attached to the Earth’s surface and has as a monopole and unidirectional leader propagate upward toward the overlying storm. Typically the upward propagating leader is positive, but upward negative leaders have also been observed. A thunderstorm charge region overlaying the tall object and inducing opposite polarity charge in the object is a necessary requirement for this type of lightning. However, it is the tall and narrow shape of the object that enhances the electric field locally at its tip that creates the conditions favorable for the initiation and development of an upward propagating lightning leader. Without the presence of the tall object, an upward lightning flash would not occur. The high-speed videos below show upward lightning flashes from television towers in Rapid City, South Dakota.
Upward flashes can be primarily identified in still images by the upward branching if branching occurs. Frequently, upward leaders will propagate to the cloud base and then turn horizontally and travel along the cloud base. The digital still image below show this behavior.
Our research has shown that upward flashes can either be triggered by preceding nearby lightning flashes or initiate without any triggering activity. Upward flashes during the summer months are most frequently Lightning-Triggered Upward Lightning (LTUL) and triggered during certain types of storms and specific types of preceding flashes. Horizontally extensive storms with large stratiform precipitation areas are the most favorable storm type for producing upward lightning. The horizontally extensive positive cloud-to-ground (+CG) flash is the most common triggering flash type for LTUL. When horizontally extensive positive charge is over tall objects, the passage of negative leaders through the positive charge can result in a large enough electric field change at the object tips to initiate an upward leader. The triggering negative leader activity must pass close enough and/or be strong enough to cause a fast electric field change that exceeds the threshold for upward leader initiation. The negative leader activity can arise from a cloud flash, but more often, the leader activity is driven by a positive cloud-to-ground flash with the negative leaders propagating over the objects following the return stroke. Additionally, the return stroke of a +CG flash which traverses a leader network that has formed near the objects can cause an extremely fast electric field change that initiates upward leaders nearly immediately following the return stroke (within 2 milliseconds based on our observations). The following videos show LTUL flashes where +CG flashes initiated the upward leaders from tall towers.
The following are animations showing the observed triggering processes.
A cloud flash causes negative leaders passing over the towers resulting in the initiation and development of upward positive leaders.
Following a positive cloud-to-ground return stroke, continued negative leader growth in the upper portion of the leader network propagates over the towers causing upward positive leaders to develop.
A positive cloud-to-ground return stroke traverses the bipolar leader network that forms near the towers causing a rapid electric field change and the development of upward positive leaders from the towers.
Self-Initiated Upward Lightning (SIUL)
Upward lightning can initiate without any preceding nearby flash activity. This most often occurs during winter months when robust snow events such as blizzards or heavy snow envelop the tall objects. Convection within these stronger winter systems causes charging and electrification to take place, but unlike the warmer summer conditions the cloud base tends to be much lower bringing the tall objects into the charge regions or exposing the objects to charged snowfall. Because SIUL usually occurs with the tall object shrouded in clouds or heavy precipitation, visually observing this type of upward flash is difficult. The same towers that experience LTUL in summer in Rapid City, South Dakota have experienced SIUL during three major snow events. Most notably, the blizzard of 4 Oct, 2013 created favorable conditions for 25 SIUL flashes from the towers. Although the towers were obscured for all of the upward flashes, a 2-dimensional interferometer system recorded some of these flashes. Below is a figure showing the upward development of the flash from that tower as the radiated energy from the leader tips was triangulated by the interferometer. In all cases, there was no preceding nearby flash activity prior to the upward leader initiating.