METEOROLOGY

The lightning capital

Lake Maracaibo, in Venezuela, is the world’s top lightning hotspot

Lightning strikes once every 10 seconds over Lake Maracaibo, in Venezuela

Imagem: Ruzhugo27 / Wikicommons Lightning strikes once every 10 seconds over Lake Maracaibo, in VenezuelaImagem: Ruzhugo27 / Wikicommons

Somewhere around the globe, lightning is striking right now. Forty-four lightning flashes occur every second on Earth – nearly four million times a day. Estimates are that only 20% of these bolts touch the ground, while the rest occur inside clouds. The place on the planet where the most flashes occur is Lake Maracaibo, the biggest lake in South America, located in western Venezuela. The clouds that form over its 13,000-square-kilometer surface – stretching across 160 kilometers at its widest point – produce about 8,000 bolts per day, according to a study published in February 2016 which identified the planet’s top 500 lightning hotspots.

These brilliant lightshows are so frequent over Maracaibo that Spanish writer Lope de Vega mentioned the lake and its numerous flashes in his poem La dragontea, written in 1598: the lightning – the epic goes – held back a British invasion of the city of Maracaibo, near the lake. According to some accounts, the lake once served as a lighthouse for Caribbean navigators because it illuminated the sky at night.

The main reason for the high rate of strikes in this spot is the pattern of convergent windflow, explains meteorologist Rachel Albrecht, professor with the Department of Atmospheric Sciences at the Institute of Astronomy, Geophysics, and Atmospheric Sciences of the University of São Paulo (IAG-USP). She is first author of the world ranking of lightning hotspots that was compiled in partnership with researchers from the United States and accepted for publication in the Bulletin of the American Meteorological Society. This convergence is the result of a particular combination of environmental factors: a large tropical lake surrounded by very rough terrain.

During the day, the land heats up much faster than the lake water and this temperature difference causes the wind to sweep landward across the lake, toward the mountains that close off Maracaibo Bay. At night, the breezes reverse direction because the mountains and land cool off before the lake, whose warm waters supply the atmosphere with moisture. As the water vapor rises, it condenses and forms massive thunderclouds filled with ice crystals and hail. These frozen particles collide with each other millions and millions of times inside the clouds, creating electric charges and thus building up large electric fields that set off lightning. “Flashes occur more often around three in the morning, local time,” says Albrecht. According to the meteorologist, lightning flashes peak in the afternoon in most continental regions, especially during summer storms.

MeteorologyLightning around the world
The study that defined Lake Maracaibo as the world’s lightning champion – the region sees an average rate of 232 lightning bolts per square kilometer per year – used data collected by the NASA Tropical Rainfall Measuring Mission satellite from 1998 to 2013. Orbiting 405 kilometers above the Earth, the satellite has imaging sensors that capture optical pulses generated by the interaction of lightning with gases in the clouds. It should be stressed that these sensors record both intra-cloud and cloud-to-ground lightning. The current study used a resolution five times greater than previous studies, enabling the detection of lightning over areas gridded at 0.1o of the Earth’s surface – near the equator, each grid is equal to a square whose sides measure about 10 kilometers each.

Although South America is the site of the planet’s leading lightning hotspot – featuring the largest number of flashes per square kilometer per year – Africa is home to the greatest number of hotspots. Of the 500 locations that were analyzed, over half (283) are found on the African continent, mostly in central and western Africa. Asia has 87 hotspots; South America, 67; and North America, 53. Oceania ranks lowest, with only 10.

The African region displaying peak flash rate density – 205 per square kilometer per year – is the Democratic Republic of Congo. The skies over this country, which has an area of 2.3 million square kilometers, produce 95 million bolts per year. Although regions within Brazil appear well down on the list, starting in 191st place, with a landmass nearly four times that of Congo, it is the world’s title holder in absolute terms: 108 million bolts per year.

The region in Brazil with the highest flash rate density lies northwest of Manaus in the middle of the Amazon Forest, where there are 68 strikes per square kilometer per year, according to the study signed by Albrecht. An earlier survey, conducted by the Atmospheric Electricity Group (ELAT) of Brazil’s National Institute for Space Research (INPE), identified the city of Porto Real, in Rio de Janeiro State, as the lightning capital of Brazil, with a flash rate of roughly 20 per square kilometer per year.

The use of different measurement strategies accounts for the disparity, according to Osmar Pinto Júnior, ELAT coordinator. ELAT relies on monitoring networks that are set up only in the South, Southeast, Central-West, and Northeast regions of Brazil. Not only is the distribution of these networks more limited; they are also made up of surface sensors that detect radiation invisible to the naked eye and that is generated by cloud-to-ground strikes. So while the figure recorded northwest of Manaus corresponds to the total number of flashes, including both intra-cloud and cloud-to-ground, the flash density measured by ELAT in Porto Real represents only the total of cloud-to-ground bolts. “These maximum rates reflect different yardsticks for different regions,” explains Pinto Júnior. Despite this difference in measurement strategies, flash rate density is quite high in the region of Porto Real. “Resende, a neighboring town,” says Albrecht, “ranks 396th worldwide and 10th among regions in Brazil.”

Scientific article
ALBRECHT, R. et al. Where are the lightning hotspots on Earth? Bulletin of the American Meteorological Society. Online. Feb. 17, 2016.