Thunderstorms: Their Phenomena and Unique Aspects
Understanding the Phenomenon of Lightning Strikes
BERLIN - A flash of light illuminates the night sky, a rumble of thunder resonates overhead, and a distinctive scent fills the air – such scenes are not uncommon during the warmer months. But how does a lightning strike occur? What happens within a storm cloud? And how far away is the approaching thunderstorm?
Unraveling the Mystery of Lightning
Lightning results from powerful electrical charges within developing storm clouds. Tiny ice crystals accumulate a positive charge through friction and collisions, while larger water droplets become negatively charged. As a consequence, the upper, cooler section of the cloud is generally positively charged, while the lower portion with water droplets assumes a negative charge [1, 2].
This voltage can discharge as a lightning strike within the cloud, between two clouds, or between a cloud and the ground. Rarely, the latter takes place, although it is the most observable form. Upon impact, temperatures in a lightning bolt can reach around 30,000°C [2].
A lightning strike can extend several kilometers, with the longest ever recorded by the World Meteorological Organization (WMO) in 2020 spanning over 750 kilometers across the USA – roughly the distance between Hamburg and Vienna [2]. In terms of duration, a lightning strike in South America purportedly lasted approximately 17 seconds in the same year [2].
Frequency of Lightning Strikes in Germany
According to the lightning tracking company Nowcast, 2024 saw more than 1.5 million lightning strikes with a strength of at least 5 kiloamperes (kA) in Germany, marking the highest number recorded per year since 2018 [3]. Most lightning does not reach the ground, with an estimated 210,000 ground strikes – equating to one every two and a half minutes on average [3].
The Cause of Thunder
Thunder, which follows lightning, is caused by the immense heat generated during the lightning strike. This rapid expansion creates a shockwave that propagates in all directions and is perceived by our ears as thunder [4]. Depending on the angle of the lightning relative to the observer, thunder produces different sounds. If lightning runs vertically toward an observer, the sound waves arrive simultaneously, resulting in a loud bang. For oblique lightning, sound waves from various parts of the lightning reach the ear sequentially, resulting in a rhythmic rumble [4].
To estimate the distance between an observed lightning strike and the observer, the so-called seconds rule can be applied: sound travels around 330 meters per second [4]. By counting the seconds between the lightning's flash and the subsequent thunder and dividing by three, one can roughly calculate the distance to the strike in kilometers.
Peak Lightning Occurrences
While many associate the strongest lightning strikes with the onset of a thunderstorm, Nowcast reports that the most powerful strikes often occur near the storm's conclusion. Although weaker discharges within the cloud or close to the ground tend to occur earlier, particularly potent ground strikes often follow as the storm moves away [5]. These discharges typically start at the upper part of the cloud and travel a greater distance to the ground.
Power levels of up to 150,000 amperes are possible, with isolated instances exceeding 300,000 [5]. By comparison, most household appliances consume less than ten amperes [5]. This indicates that a lightning strike can carry tens of thousands of times more current than typical household appliances.
The Aroma of Rain
After a prolonged dry spell, the initial raindrops often carry a unique scent known as petrichor. Coined in 1964 by two Australian researchers, the term originates from the ancient Greek words "petros" (stone) and "ichor", the ethereal fluid in the veins of the gods [6]. The scent arises when rain combines with dry soil. Plants release an oil during drought, which adheres to soil particles and is released upon impact by raindrops, thereby filling the air with the aroma [6].
Safety Measures During a Thunderstorm
According to the German Weather Service, the best course of action during a thunderstorm is to seek shelter in solid structures or vehicles [7]. If caught outside, the experts advise avoiding isolated trees, tall towers, masts, metal fences, and bodies of water. Crouching in a low spot, keeping feet close, and hugging one's legs are effective actions to minimize exposure [7].
References:
- http://www3.nrcresearchpress.com/doi/abs/10.1139/p71-349
- https://www.dwd.de/EN/service/lightning/lightning_node.html
- https://www.dwd.de/EN/com/nwc_bolt/welcome.html
- https://www.dwd.de/EN/service/lightning/lightning_thunder.html
- https://www.dwd.de/DE/com/nwc_bolt/files/tornado/NWCReportThunder_2019-10-30.pdf
- https://royalsociety.org/science-policy-and-economy/podcast/what-causes-rain-smell/
- https://www.dwd.de/DE/whatfor/warnwetterinformation/warnwetterinformation_node.html
The electrical charges within developing storm clouds, specifically the collision and friction between tiny ice crystals and larger water droplets, contribute to the occurrence of lightning, which is a fundamental aspect of environmental-science. The science of weather helps us understand that thunder, following a lightning strike, is caused by the immense heat generated, which creates a shockwave that propagates as thunder, a unique phenomenon observed over time and space.
