Australian Storm Spotters' Guide

1. Thunderstorms and their main parts

Severe thunderstorms are very localised events that don't usually affect as wide an area as tropical cyclones and floods. Often, their devastating impact in Australia is under-estimated. However, severe thunderstorms are responsible for more insured loss (on average) than tropical cyclones, earthquakes, bushfires and floods combined and may result in death and injury.

This spectacular photograph (1.1) taken from an aircraft provides a "birds-eye" view of a thunderstorm over Point Lookout in NSW. The isolation of the storm system is a fairly common feature of severe thunderstorms.

1.1 Photograph by P. Mackey.

Thunderstorms have a three-dimensional structure, but it is best to think of them as a constantly evolving process, rather than just an object. The diagram below represents a vertical cross-section through a thunderstorm moving towards the left of the screen. Each storm, or cluster of storms, is a self-contained system (1.2) with organised regions of upward moving air, known as updraughts, and downward moving air, downdraughts. The warm, moist air which fuels updraughts, typically rises in successive cumulus towers along a flanking line that leads to the storm's main core. As air rises within the core, it cools and condenses into rain or hail which then falls in the downdraught. Near the ground, the downdraught spreads out in what is known as the outflow region. Often, the leading edge of the outflow is marked by an abrupt, cool wind surge, the gust front.  

1.2 diagram of a thunderstorm.

This photograph (1.3) from Gladstone, Queensland shows a good example of a flanking line. Note how the towers grow progressively larger towards the main storm updraught which is off to the left of the photograph.

1.3 Photograph by P. Kearton.

The top of a thunderstorm cloud, known as an anvil, is seen as bright and tall, reaching up to an altitude of 10-16 kilometres. It may appear to be "boiling" with cauliflower-shaped lumps but more often has a fibrous, frozen appearance because it is primarily composed of ice crystals. This photograph (1.4) from Brisbane shows a crisp thunderstorm anvil which is a good indicator of a strong updraught.

1.4 Photograph by A. Smallegange.

The boundary between cool storm outflow, here indicated (1.5) by heavy precipitation on the left, and rising warm inflow, here on the right, is sometimes marked by spectacular shelf cloud (or Arcus). Sometimes this cloud feature may become detached and move out ahead of a storm as a horizontal roll cloud. 

1.5 Photograph by A. Husted.