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An overview of Tornadoes and Thunderstorms by Paul Graham

Tornadoes are violent rotating funnels of air filled with dust and debris. There are different types of tornadoes that form for different reasons and some types are more destructive than others. The best known and most destructive type of tornado forms as a result of a severe thunderstorm and is particularly common in the US Midwest. Severe thunderstorms also produce other violent phenomena such as large size hail stones, strong wind gusts, or violent downburst winds sometimes as strong as a tornadoes rotating wind. Other types of tornadoes can occur because of locally intense convection perhaps as a result of a hot land surface and cooler air above and some sort of surface wind convergence zone (perhaps a sea breeze). For example, the dust devil (or willy - willy) is an intense vortex of dust that is common to the hot arid zones of the world such as central Australia but is usually fairly short lived. Waterspouts are tornadoes that occur over the water for the same reasons that one may occur over the land. Usually, the most violent and longest lived tornadoes will be associated with a severe thunderstorm.

Tornadoes produced by severe thunderstorms are a global phenomenon but mainly occur in continental mid-latitude zones where cold polar air meets warm tropical air. The US mid-west is well known as a tornado region. Here in Australia there are around 20 reported tornadoes each year. However, the figure is probably much higher because many storms occur in uninhabited areas and tornadoes are a very localised event so that they would never be seen by anyone. Australia has a long history of severe thunderstorms and associated tornadoes. A recent example of a tornado day is Semptember the 29th, 1996 when 2 tornadoes were reported in nothern NSW. Tornadoes are classified on a scale f0-f5, (known as the Fujita Scale name after Dr T. Theodore Fujita, Severe Storm Researcher from Chicago University) of intensity relating to wind speed and resulting damage with f0 (64km/h < wind speed < 117km/h) being the least destructive to f5 being the most devastating (wind speeds > 418km/h). Strong tornadoes are very rare but there have been some reported cases here in Australia. An example are the tornadoes that occured in November of 1989 in nothern NSW one of which rated around F3 (possibly F4) in strength (Interestingly, tornadoes here are often reported as "mini-tornadoes" or "mini-cyclones" by the media). However, it should be noted that tornadoes are the rarest thunderstorm phenomena, are very localised events and are generally short lived.

Thunderstorms form when there is atmospheric instability. This is usually the case when there is cold air in the mid to upper troposphere which encourages warmer air in the lower troposphere to rise. On a day when there is the potential for thunderstorms, the sky is often littered with very tall puffy cumulus clouds (known as cumulus congestus). These tall cumulus clouds are an indication that convection is occurring probably as a result of atmospheric instability (cold air over warm). You can imagine bubbles of air rising like the bubbles in a glass of lemonade.

Thunderstorms may be grouped into their different types. Firstly, single cell storms are usually the weakest and shortest lived of all thunderstorms. They may last for up to an hour but quickly dissipate after reaching maturity as their downdraughts stifle the incoming updraughts. Secondly, muticellular storms which are sometimes severe and usually much longer lived storms. As their name suggests, they are composed of lots of constituent storm cells. If they are severe they can bring large hail, destructive winds and occasionally a tornado. Sometimes, this type of storm system will form in a line with each cell's downdraught merging, forming a gust front. This occurrence is known as a squall line and may bring strong winds and heavy rain or hail. A squall line may last many hours and will usually move with the prevailing upper tropospheric wind. Multicellular storms can be just a whole lot of regenerating single cell storms where the downdraught of one storm enhances the updraught of another developing storm. So you have a situation where there are a whole lot of storm cells, some developing, some dissipating and some which are mature. The third type of storm system is known as the supercell. As the name suggests, this type of thunderstorm is the strongest type and enjoys the longest life. A supercell storm is composed of a single, rotating updraught known as a mesocyclone and tends to move at right angles to the prevailing upper tropospheric wind. This rotating updraught has a slight lean which means that a resulting downdraught won't stifle the incoming air to the updraught, hence enabling it to last for a long time, perhaps many hours. It is a myth that the rotation is a direct result of the Coriolis force since it occurs on a very localised scale. This rotation is caused by wind blowing in different directions at different altitudes and at different velocities. It is the rotating updraught of such a storm that can lead to a tornado. Sometimes, with such a storm, you will see a lowered rotating section at the base of the cloud. This is known as a wall cloud and may or may not lead to a tornado. Sometimes, there will be a visible rotating funnel that has not yet reached the ground. This is known as a funnel cloud and may or may not lead to a tornado. You will know when a funnel cloud has become a tornado if you see raised dust or debris. Note that the condensation funnel (or cone shaped cloud) may never reach the ground, but if you see raised dust or debris, then it has become a tornado. This phenomenon of a rotating wall cloud or funnel cloud will most likely be found at the rear of the storm, usually the north westerly or south westerly side in Australia. If you see this, you should seek shelter immediately in the most secure section of a building and put a mattress over you for protection. At the soonest possible time, you should report this to the Bureau of Meteorology.

The Bureau of Meteorology has a network of volunteer storm spotters who may report such occurrences if they occur. It is very important to understand what not to report before you actually make a report. If you decide to become a storm spotter, you will be given a guide which will have more information.

For more information about types of storms, take a look at the following excellent resource from the University of Illinois (the best I have seen):

NOAA Severe Storms Spotters Guide: online meteorology guide