About Severe storms
For a thunderstorm to be created, it needs three main ingredients: air that is moist, an atmosphere that is unstable, and a weather event like a front, trough or area of low pressure.
A severe storm, also known as a thunderstorm, is a storm that consists of strong winds, heavy rain, lightning, thunder and possibly hail. They can result in tornadoes or cyclones and cause flooding and extensive damage. The Bureau of Meteorology defines a severe thunderstorm as a storm producing any of the following effects:
- hailstones that are bigger than 2 cm
- wind gusts of more than 90 km per hour
- flash flooding
- tornadoes or ‘twisters’.
How are severe thunderstorms created?
Severe thunderstorms are created when cooler air begins to push warmer, humid air upwards. As the warm air rises rapidly in an unstable atmosphere, the cloud builds up higher and higher and begins to spread. A thunderstorm can quickly develop when the atmosphere remains unstable or if it gathers additional energy from surrounding winds.
Hail can form within the thunderstorm when the air is much cooler than the water vapours that make the cloud. When the cooler air freezes the water, droplets fall as hailstones rather than raindrops. Hail can range from a few millimetres in diameter to the size of a tennis ball.
As the air becomes warmer, clouds build and expand in size. In every cloud there are positive and negative electrons (charges). The positive charges are at the top of a cloud and the negative charges sit towards the bottom. As a cloud grows, it creates a gap between the two charges, which causes a spark, known as lightning. Lightning can occur inside a cloud, between clouds, and between clouds and the ground.
Thunder is caused when the lightning rapidly heats the air that surrounds it, causing an explosive effect. The air around a lightning strike can be heated to temperatures as high as 30,000°C in just a fraction of a second.
Where do severe storms occur?
Severe storms can occur anywhere in the world, but Uganda and the Democratic Republic of Congo, both in Africa, have the highest frequency of lightning. Australia has several severe storms each year, with most occurring between September and March. Thunderstorms usually occur during these months as more of the sun’s energy is available and the weather patterns of spring and summer are favourable for storms.