Thunderstorm phenomena
Lightning and thunder
Lightning is the electrical discharge produced when voltage differences between the ground and a part of the storm (or between two separate regions of the storm) are large enough (several hundred million volts) to overcome the insulating effect of the air. Strikes can occur within the cloud, between clouds, or between clouds and the ground. Thunder is the sound produced by the explosive expansion of air heated by the lightning strike to temperatures as high as 30,000°C.
To see the annual variation in thunderstorm and lightning activity across Australia, have a look at the average annual thunder-day and lightning flash density maps
Fig 1: Thunderstorm over Melbourne City
Hail
Hail is solid precipitation in the form of balls or pieces of ice known as hailstones.
Hailstones can form in a thunderstorm with a strong updraught when frozen raindrops, suspended in the updraught, grow rapidly by 'sweeping up' small cloud droplets which freeze on contact. Their diameter can range from 5 to 50mm or even more, but most hailstones are smaller than 25mm. Hailstones larger than cricket balls have been recorded in Australia.
Fig 2: Giant hailstones measuring 5cm across
Wind Gusts
In a mature thunderstorm, the falling rain and hail drag the surrounding air downwards. More importantly, evaporation from the raindrops and melting ice cool the nearby air which creates a cold dense bubble of air that accelerates downward towards the ground. Upon reaching the ground, this downdraught creates a dome of cool air that can spread sideways very quickly, producing a cool, gusty wind that can cause damage. In situations involving winter fronts, the downdraught can be enhanced by strong winds in the atmospheric layers in the first 2000 m above the surface.
Tornadoes
Many people believe that tornadoes do not occur in Australia; this is not true, they do and have caused significant damage. These are the rarest but most violent of thunderstorm phenomena. A rapidly rotating mass of air (often called a vortex) which can range in width from a few metres to well over a kilometre, extends downwards in the well- known funnel shape from the base of a storm cloud. Weaker tornadoes, especially in drier conditions, may not be accompanied by a visible funnel, and can only be recognised by a debris whirl near the ground. A tornado usually rotates clockwise in Australia (viewed from above) and contains destructive winds that in rare cases can exceed 300 km/h.
Fig 3: Small tornado