Tropical depression Imelda has caused nearly 1 million lightning bolts to form in and around the Houston area since Tuesday as the storm battered the region, causing heavy rainfall, over 20 mile-per-hour winds, and fatal flooding.
A known but understudied effect following bouts of extreme weather, this lightning cluster comes just weeks after parts of Oklahoma saw over a million lightning events in the span of 12 hours.
Chris Vagasky, a meteorologist and manager at Vaisala Solutions—a for-profit environmental and industrial measurement company that tracks lightning with over 100 sensors around the country—posted maps of the Houston area on Twitter with black and red dots denoting lighting strike events. Between Tuesday at noon to 4 p.m. on Wednesday, roughly 900,000 lightning events occurred within 150 miles of Houston.
“Having looked at lightning data pretty constantly for the last five years, numbers like this are very significant, they’re very big, but it’s a question of exactly how big is it,” Vagasky said, adding that this cluster makes up a “fairly large percentage” of the lightening the Houston area will receive in a given year.
Because each storm system is different and few people have looked into quantifying these lightning events, it is difficult to know how abnormal the cluster in Houston really was, said Vagasky, who wrote his master’s thesis on tropical cyclone lightning.
Max Vinetz, a graduate student at Rice University who lives in Houston, said it had been “storming all day” on Thursday, enough for his school to cancel classes. He added that the thunder was loud enough to set off car alarms near his apartment but that the rain was the most striking part of the storm for him.
“I'm sure you’ve been in heavy rain before, but sometimes there’s like a foot and you’re like, ‘It can't get heavier than this,’ and then the drops just get bigger and you're like, ‘OK, I guess we’re in the Bible or something,’” he said.
Vagasky explained that weather events like Imelda perturb the air and have the potential to cause intense and long-lasting lightning clusters. Upward rising air motion from hurricanes, storms and volcanoes creates a charge imbalance in the atmosphere; to rectify this, the sky sends out static discharge, otherwise known as lightning.
“When you think about it, all weather is, is just the atmosphere trying to get back into balance,” he said.
In Houston, Vagasky said, tropical depression Imelda literally created a perfect storm for lightning: moisture, a low-pressure system to move air upward, and instability in the atmosphere caused by warm temperatures close to the surface of the Earth and cooler temperatures higher up.
The lightning events took two forms: in-cloud and cloud-to-ground lightning, represented in the map by black and red dots, respectively. In-cloud lightning dissipates within the clouds and does not reach the ground, while cloud-to-ground strokes are the Zeus’ bolts that we typically think of. According to Vagarsky, 50-80% of all the lightning in a given storm is in-cloud, while the other 20- 50% are cloud-to-ground.
If caught in a lightning storm, the two best places to take shelter are inside cars with metal roofs and in modern buildings that have electrical wiring and plumbing running through the walls, Vagarsky advised.
“One of the things that we preach as meteorologists is, ‘When thunder roars, go indoors,’ or, ‘If you see a flash, dash inside,’” he said. “You don’t want to be outside during a lightning event.”
This article originally appeared on VICE US.