Unstable atmospheric conditions that are ripe for thunderstorms and sometimes tornadoes have increased across the northern hemisphere in the past 40 years. The finding is one of the best indications yet that strong storms are getting more frequent, as climate models predict – but this might not necessarily mean there will be more tornadoes.
“It favours more occurrence of severe storms such as tornadoes,” says study author of the University at Albany in New York. “But the actual formation of tornadoes, it depends also on changes in other factors.”
Storms are driven by warm, wet air rising into the atmosphere, usually seen as towering cumulonimbus clouds. This air movement, or convection, can occur when the atmosphere is unstable, meaning the “convective available potential energy” – essentially the buoyancy – of air masses is positive.
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Air buoyancy can be calculated from temperature and humidity measurements taken by balloons launched from weather stations, but stations have used a range of different sensor technologies since the 1950s, making it hard to compare data over time.
Dai and his colleague at the Jiangsu Meteorological Observatory, China, reworked the data to make trends over time easier to spot. In their analysis they found that the frequency of episodes of atmospheric instability increased by up to 32 per cent in most parts of the northern hemisphere from 1979 to 2020, due to rising humidity and often rising temperatures in the lower atmosphere.
Moreover, the strength of these episodes of atmospheric instability has grown in places like the south-eastern US and south-eastern China. At the same time, the negative buoyancy that an air mass must overcome to begin convection has been increasing, specifically in the early morning hours in North America and Europe. That is the “perfect recipe for generating severe storms”, say Dai, causing energy to build up longer before the air starts rising.
“It’s not a shocking new result, but rather a verification that what was predicted is now happening,” says , a climate scientist formerly at NASA.
On its own, however, the trend may not translate into a rise in the frequency of extreme storms like tornadoes. This is because other factors go into forming these storms. These include high wind shear, which is when winds blow a different speed or direction at a higher altitude, causing the storm to tilt or rotate.
Modelling has suggested that with climate warming, although the much larger increase in convective energy will still boost conditions for severe thunderstorms.
It still isn’t clear what this means for tornadoes. So far, scientists have in the northern hemisphere. Instead, they have found a “”, observing more days in which several tornadoes occur, but fewer tornado days overall.
Geophysical Research Letters
