What is a Severe Thunderstorm?

Thunderstorm photograph courtesy of Jane ONeill.

Literally hundreds of thunderstorms occur each year in Victoria. Although all thunderstorms produce lightning, that is a danger itself, not all of them are "severe" or likely to produce damage. In Australia, Severe thunderstorms are defined as those that produce any of the following:

• Hailstones with a diameter of 2cm or more
• Wind gusts of 90km/h or greater
• Flash flooding
• Tornadoes

Why do we get Severe Thunderstorms?

• See "The Australian Storm Spotter Guide"

Monthly Distribution of Severe Thunderstorms

Although severe thunderstorms can occur at any time, the distribution of events shows a marked pattern throughout the year. The graph below shows the frequency of severe thunderstorms for each month of the year. There is a marked tendency for severe thunderstorms (indeed all thunderstorms) to occur during the months October through to April. The increase in storms during this period is primarily due to the increase in energy provided by the sun during the warmer Spring and Summer months, coupled with Spring, Summer and Autumn weather patterns that are favourable for storm growth.

Hourly distribution of Severe Thunderstorms

Just as there is a marked monthly distribution of storms, there is also a marked daily distribution, peaking between 2pm and 6pm during the afternoon. Again, this is primarily due to the daily heating of the earth's surface by the sun reaching a maximum during the afternoon.