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date: 20 November 2017

Formation and Development of Convective Storms

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.

Deep cumulus clouds are omnipresent around the globe and play important roles in the transfer of energy through the atmosphere. Under certain conditions these cumulus clouds may evolve into convective storms. The qualifier “convective” implies that the storms have vertical accelerations that are driven primarily (though not exclusively) by Archimedian buoyancy. Such buoyancy in the atmosphere arises from local density variations that differ from a horizontally averaged—or environmental—density. Quantifications of atmospheric buoyancy are typically expressed in terms of temperature and humidity and allow for an assessment of the likelihood that convective clouds will form or initiate. The canonical mechanisms of convection initiation are weather fronts and orography, but a wide variety of other initiating mechanisms exist.

As modulated by the environmental temperature, humidity, and wind, weather fronts also facilitate the transition of convective clouds into storms with locally heavy rain, lightning, and other possible hazards. For example, in an environment characterized by winds that are weak and do not change much with distance above the ground, the storms tend to be short lived and benign. The structure of the vertical drafts and other internal storm processes under such weak wind shear are distinct relative to the drafts/processes when the environmental wind shear is strong. In particular, strong wind shear, combined with large buoyancy, favor the development of squall lines and supercells, both of which are highly coherent storm types. Besides having durations that may exceed a few hours, both of these storm types tend to be particularly hazardous: squall lines are most apt to generate swaths of damaging “straight-line” winds, and supercells spawn the most intense tornadoes and are responsible for the largest hail. Methods used to predict convective-storm hazards capitalize on this knowledge of storm formation and development.