On Dec. 29, 2018, a weather satellite peering down over the Pacific spotted a cluster of towering thunderheads with extremely cold cloud tops. A recently released study confirms one of the clouds was measured at minus-168 degrees Fahrenheit, establishing a record for the coldest cloud-top temperature reliably measured globally.

For comparison, that’s almost 40 degrees colder than the lowest temperature ever measured on Earth’s surface — minus-129.3 degrees in Antarctica in 1983. The coldest reading obtained in North America was minus-81 degrees, measured in Canada’s Yukon Territory in 1947.

The results offer new insight into exactly how cold the tops of clouds embedded in deep thunderstorm activity can get, as well as highlight the enormous technological strides making these modern-day observations possible.

The paper, written by Simon Richard Proud at the National Center for Earth Observation in Oxford, Britain, and Scott Bachmeier of the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin at Madison, was published March 22 in the American Geophysical Union’s Geophysical Research Letters.

It details the rarity of temperature readings that low but also indicates that ultralow cloud-top temperatures may be becoming more common.

“The idea for this collaboration began when Simon contacted me about the [minus-165 degree] cloud-top temperature sensed by [a NOAA satellite] with Typhoon Kammuri on 30 November 2019,” wrote Bachmeier in an email. “We had both taken a look at this event using geostationary and polar-orbiting satellites. [We] thought it might be good to take a look back in time at all available satellite data and make sure there were no other tropical cyclone or thunderstorm events over the tropical West Pacific that were possibly colder.”

Indeed there were. After combing through years of data, the pair of scientists happened upon one lone thunderstorm that exhibited shockingly low temperatures.

“In general, the further back you go, the less reliable the temperatures,” Proud said in an email. “To look at historical statistics, we used the MODIS sensor onboard NASA’s Aqua satellite as it is one of the more reliable, giving more than 16 years of very consistent temperature data.”

The temperature readings were made by satellites peering down from above at the anvil clouds of thunderstorms. Anvils are the ice clouds that result when a thunderstorm updraft ascends and then flattens out at the tropopause. The tropopause is a level in the atmosphere that marks the transition from the troposphere, the lowest level of the atmosphere, and the stratosphere, which surrounds it. Temperatures rise with height in the stratosphere, “capping” the vertical development of storm clouds.

Counterintuitively, the tropopause is coldest over the equator and comparatively mild over the poles. That’s because the atmosphere is taller at the equator where the surface is warmer, and subsequently the tropopause is much higher up.

Vigorous thunderstorms can occasionally produce “overshooting tops,” or a bulge atop a flattened anvil where a strong updraft has so much momentum that it carries air pockets above where they would otherwise stop rising. In a sense, the air is rising quickly enough that it can’t put the breaks on. Sometimes those overshooting tops puncture the tropopause.

In the southern west Pacific, the tropopause is known to be very cold, averaging around minus-120 degrees.

“The Western Pacific usually has a very cold tropopause, on average more than [25 degrees] colder than over the USA for example,” Proud wrote. “This together with the very warm seas makes perfect conditions for these massive, cold storms.”

He said more than three-quarters of the coldest storms the pair investigated occurred over the ocean.

“We saw no really cold storms at all over the USA,” Proud wrote. “We were only looking for those below [minus-139 degrees] and found none that cold. US storms can be very nasty in terms of hazards, but they’re also … warmer than the tropical/oceanic beasts!”

The record-cold thunderstorms in question formed between 163 and 165 degrees east longitude, about four degrees south of the equator in December 2018. Over the course of four hours, storms developed and towered upward, their tops ultimately cooling below the tropopause temperature. That’s largely because they radiated more heat to space than they absorbed from the surrounding environment, resulting in net cooling.

That’s around the time that the NOAA-20 satellite flew overhead. There are two main types of satellites — geostationary, which are fixed over a given location, and polar-orbiting. The NOAA-20 satellite is a polar-orbiting satellite; it traces lines of longitude in the skies some 512 miles above the ground, passing over both poles and scanning one slice of the Earth every 101 minutes. Onboard the satellite are five key instruments, among them the Visible Infrared Imaging Radiometer Suite, or VIIRS. That’s the apparatus that measures the temperature of cloud tops.

One of the pixels returned by the VIIRS satellite pegged the most extreme cloud top as being at 161.96 degrees Kelvin, or minus-168.42 Fahrenheit — the coldest cloud-top temperature ever ascertained by satellite. According to the paper, it beats out the minus-152 degree measurement obtained over Tropical Cyclone Hilda in 1992 and narrowly edges out the minus-165 reading over Typhoon Kammuri, which slammed the Philippines at Category 4 strength in December 2019.

As thunderstorm activity continues to blossom in a world of gradually warming sea surface temperatures, the pair has already documented an uptick in the frequency of ultracold thunderstorm cloud tops. They’re unsure exactly the cause — but agree the phenomenon merits further study.

“[It’s unclear] whether it’s a climate trend or whether it’s coincidental weather conditions … coming together,” Proud wrote.

Whatever the case, something bizarre is going on.

“There have been as many [super-cold cloud tops] in the last 3 years as in the 13 before that,” Proud wrote.