Thunderstorms require 3 main ingredients; a source of moist air, an unstable atmosphere and a trigger mechanism. Moist air is important because when it condenses to form cloud, heat energy is released making the rising air more buoyant and fueling further cloud growth. An unstable atmosphere is necessary so that developing cloud is able to rise freely to great heights in the atmosphere. And trigger mechanisms are important as they serve as a focus for storm development. Typical triggers are cold fronts, heat troughs and regions of low pressure. Hills and mountains may also enhance storm development.
The severity of any subsequent thunderstorms will depend largely on the buoyancy of the rising air within the storm and the structure of the wind within the atmosphere. Wind direction and speed is rarely constant, and generally tends to increase in speed and turn anti-clockwise (in the southern hemisphere) with increasing altitude. The change in wind direction and speed as you move upwards through the atmosphere is known as “wind shear”.
Non-severe thunderstorms generally occur within environments possessing only low to moderate instability and minimal wind shear. If the atmosphere is very unstable with light winds and little shear, storms develop a “pulse-like” character, rising strongly upwards then collapsing over the period of half to one hour.
Severe thunderstorms in these conditions may produce large hail and strong bursts of wind but rarely produce widespread damage. An increase in wind shear produces storms with additional opportunities for regeneration. This allows several storm “cells” at different stages of their life cycle to be found within the one storm system, increasing their overall lifetime and the area they may affect. These “multicellular” thunderstorms may produce severe hail and wind, with the added possibility of flash flooding and weak tornadoes.
Multicellular storms forming south east of Busselton, 17 April 2013 © Ian Trigwell/Perth Weather Live
Certain instability and wind profiles can lead to the development of long-lived thunderstorms with strong rotation within their cores. These storms are known as “Supercells” and are responsible for the majority of damage caused by severe thunderstorms. Supercell thunderstorms may produce very large hail, extraordinarily strong wind gusts, powerful tornadoes and heavy rainfall.
This excellent example of a supercell was captured near Kalgoorlie on 17 March 2013
This article is an except used with permission as granted by the copyright owner. © Commonwealth of Australia 2011