It’s not where lightning did strike 2022 in the U.S. that concerned scientists — it’s where it didn’t.
A news story in the local newspaper got my attention with the above headlines, especially it’s where it didn’t. I would venture to say that 99.99% people in the world consider lightning as a dangerous phenomenon, however, as you will see that it’s happening is critical for environmental perspective. This post explores very basic scientific facts of lightning and thunder.
U.S. lightning data released by weather and environment monitoring company Vaisala Xweather showed portions of the Midwest and Texas — typical lightning hot spots — had a weaker display of activity in 2022 compared to its recent average. The lack of lightning was a signal itself of atypical weather across the country that year, including major drought across the Mississippi River basin.
“It was a decently active year for lightning, but the distribution of it was a little bit different than we normally see,” lightning researcher Elizabeth DiGangi said. Lightning detection is like measuring a person’s reflexes: the system can appear normal overall, but certain areas may show more signs of struggle than others. That’s what happened in 2022. After back-to-back years of concerning low activity in 2021 and 2020, the total number of lightning strikes was closer to average. Still, pockets of the country showed record low or high activity in ways that left researchers surprised.
Where lightning was low
The number of lightning flashes sits between 20 million and 25 million flashes each year in the United States, but it’s normal for there to be fluctuations within a certain range. Each year, lightning typically fluctuates about 7 to 10 percent across the country due to normal variation in the number of thunderstorms and weather patterns, Vagasky said. But “when you get down to smaller scales and you go down to the state level or county level … that’s when you see these really big fluctuations,” he said. For instance, lightning in the Mississippi River basin was down 11 percent compared with the 2015-2021 average, which can be devastating as the region receives around half of its annual precipitation from thunderstorms. Less lightning can mean there was less precipitation overall in parched areas.
Lightning is a large-scale natural spark discharge that occurs within the atmosphere or between the atmosphere and the Earth’s surface. On discharge, a highly electrically conductive plasma channel is created within the air, and when current flows within this channel, it rapidly heats the air up to about 25,000°C. The lightning channel is an example of terrestrial plasma in action.
The Airport Fence for Dare Devils
Seeing the Lightning
Lightning is visible as a flash of blue-white light. The extremely high temperatures -generated heat the air molecules to a state of incandescence (white hot) such that they emit a vivid white light. At the same time, nitrogen gas (the dominant gas in the atmosphere) is stimulated to luminesce, producing bright blue-white. The combination of light from luminescence and incandescence gives the bolt of lightning its characteristic color.
Lightning’s partner
Temperatures in the narrow lightning channel reach about 25,000°C. The surrounding air is rapidly heated, causing it to expand violently at a rate faster than the speed of sound, similar to a sonic boom. At about 10 m out from the channel, it becomes an ordinary sound wave called thunder.
Thunder is effectively exploding air, and when heard close to the lightning channel, it consists of one large bang. At about 1 km away, it is heard as a rumble with several loud claps. Distant thunder has a characteristic low-pitched rumbling sound. However, beyond 16 km, thunder is seldom heard.
Conditions needed for lightning to occur
It is the formation and separation of positive and negative electric charges within the atmosphere that creates the highly intensive electric field needed to support this natural spark discharge that is lightning.
The formation of electric charges in the atmosphere is due mainly to the ionization of air molecules by cosmic rays. Cosmic rays are high-energy particles such as protons that originate from outside the solar system. On colliding with air molecules, they produce a shower of lighter particles, some of which are charged.
Within a thundercloud, the rapid upward and downward movement of water droplets and ice crystals can separate and concentrate these charges. The negative charges accumulate at the bottom part of the cloud and the positive charges towards the top.
Lightning production
As the area of negative charge at the base of the thundercloud builds up, it induces a region of positive charge to develop on the ground below. As a result of this, a potential difference of voltage is created across the cloud-to-ground gap. Once the voltage reaches a certain strength, the air between the base of the cloud and the ground develops electrical conductivity. At first a channel, known as a step leader, is formed. Although invisible to the naked eye, this allows electrons to move from the cloud to the ground.
It is called a stepped leader because it travels in 50 to 100 m sections, with a slight pause in between, to the ground. As it nears the ground, a positively charged streamer fires upwards from the ground to connect with it. Streamers are most often initiated from tall objects on the ground.
Once connected, electrons from the cloud can flow to the ground and positive charges can flow from the ground to the cloud. It is this flow of charge that is the visible lightning stroke.
After the first discharge, it is possible for another leader to form down the channel. Once again, a visible lightning stoke is seen. This can happen 3–4 times in quick succession. All of this happens in a time interval of about 200 milliseconds.
Monitoring lightning
A worldwide lightning location network (WWLLN, pronounced ‘woolen’) was founded in New Zealand in 2003. Working with the collaboration of scientists from around the world, the network plots lightning discharge locations seconds after they occur.
Around the world, there are about 45 lightning flashes per second. Apart from generating the characteristic blue-white light, radio wave pulses known as sferics are also produced. The frequent crackles heard when tuned into an AM radio station during a thunderstorm are sferics from the lightning discharges.
These sferics are registered at the 60 WWLLN receiving stations around the world and provide a near real-time information dataset. This information is made available to scientists via a high-speed internet connection provided by REANNZ (Research and Education Advanced Network New Zealand).
Red sprites
High above thunderstorm clouds at altitudes of 50–90 km, large-scale electrical discharges can occur. These are triggered by thundercloud-to-ground lightning activity. They appear as fleeting, luminous, red-orange flashes and take on a variety of shapes. Unlike ‘hot plasma’ lightning, they are cold plasma forms somewhat similar to the discharges that occur in a fluorescent tube.
It is because of their fleeting nature, lasting mostly for only milliseconds, and ghost-like appearance that the term ‘sprite’ has been used.
Worldwide Lightning Distribution
Data from space-based sensors reveal the uneven distribution of worldwide lightning strikes. Units: flashes/km2/yr. Data obtained from April 1995 to February 2003 from NASA’s Optical Transient Detector and from January 1998 to February 2003 from NASA’s Lightning Imaging Sensor.