Scientists have just published research showing the top of one tropical storm cloud system in 2018 reached -111C

Earth‐orbiting satellites have long been used to examine meteorological processes. In the context of severe weather, brightness temperatures (BTs) at infrared wavelengths allow the determination of convective cloud properties. The anvils of cumulonimbus clouds, for example, typically produce BTs close to the tropopause temperature.

Particularly severe storms generate overshoots that penetrate the stratosphere and are cooler than the anvil. In this study, we describe clustered storm overshoots in the tropical West Pacific on December 29, 2018 that resulted in the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard NOAA‐20 measuring a temperature of 161.96K (−111.2°C), which is, to our knowledge, the coldest on record. We describe the local meteorological conditions, examine the VIIRS overpass that produced the cold temperature, compare VIIRS with other sensors that observed the region and, finally, analyze the historical context provided by two other satellite instruments to show that such cold temperatures may be becoming more common.

VIIRS I5 brightness temperatures (BTs) for 13:38 UTC on December 29, 2018. The cluster of particularly cold BTs is slightly to the left of the image center. VIIRS, Visible Infrared Imaging Radiometer Suite.

What is a overshooting top?

An overshooting top is a dome-like protrusion shooting out of the top of the anvil of a thunderstorm and into the lower stratosphere. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the storm is severe

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Dr Proud and Scott Bachmeier, a research meteorologist at the University of Wisconsin-Madison, US, report the event in a paper in the journal Geophysical Research Letters.

They describe the contributing factors. Although big storms in that part of the Pacific are frequent around the December-January timeframe, this one seemed to get an extra boost.

In part, this came from some very warm ocean water in the region, but there was help too from a phenomenon meteorologists refer to as the Madden-Julian Oscillation. The MJO is an eastwards-moving zone of winds that can accentuate wet and dry weather.

What's noteworthy, however, is the hint in the long-term data that these super-cold thunderstorms may be increasing in frequency.

There have been as many such events across the globe in the past three years as there were in the 13 years before that. That's relevant as, in general, the colder the storm, the more likely it is to produce hazardous weather, such as lightning and flooding.

"The storm we report on was in the middle of nowhere, and just as well," says Dr Proud.

"If you'd been there you would have got drenched and very probably a load of hail on your head as well - and a lot of lightning.

"Over the last 20 years, it seems these super-cold thunderstorms are becoming a little bit more common. It's interesting that in this part of the world, the tropopause is actually getting warmer, so we might expect to see warmer clouds, not colder clouds, which likely means we're seeing more extreme storms as we're getting even bigger overshoots than we used to."

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