The Atmosphere Has Become Increasingly Unstable During 1979-2020 Over the Northern Hemisphere

Atmospheric instability affects the formation of convective storms, but how it has changed during recent decades is unknown. Here we analyze the occurrence frequency of stable and unstable atmospheric conditions over land using homogenized radiosonde data from 1979 to 2020. We show that atmospheric stable (unstable) conditions have decreased (increased) significantly by similar to 8%-32% (of time) from 1979 to 2020 over most land areas. In boreal summer, the mean positive buoyancy (i.e., convective available potential energy [CAPE]) also increases over East Asia while mean negative buoyancy (i.e., convective inhibition [CIN]) strengthens over Europe and North America from midnight-dawn for unstable cases. The increased unstable cases and mean CAPE result from increased low-level specific humidity and air temperature, which increase the buoyancy of a lifted parcel. The stronger CIN results from decreased near-surface relatively humidity and decreased lapse rate in the lower troposphere. Our results suggest that the atmosphere has become increasingly unstable, which could lead to more convective storms. Severe weather events such as tornados and intense thunderstorms often cause significant loss of life and property. Their formation requires instability or unstable conditions in the atmosphere. Climate models project increased unstable conditions under greenhouse gas-induced global warming, but how atmospheric instability has changed during recent decades is unclear. Through analysis of newly homogenized radiosonde data, here we show that the atmosphere has become increasingly unstable over most land areas over the Northern Hemisphere from 1979 to 2020, with an increasing number of unstable conditions (i.e., with positive buoyancy) but a decreasing number of stable conditions (i.e., with zero or negative buoyancy). These changes result mainly from increased low-level moisture content and warmer air temperature. Such instability changes favor increased occurrence of convective storms. Newly homogenized radiosonde humidity and temperature data allow quantitative assessment of historical changes in atmospheric instabilityThere are increasingly more unstable conditions with positive buoyancy in all seasons over most land areas from 1979 to 2020The increased unstable conditions mainly result from increased low-level moisture content and warmer air temperature