Tornadoes and Other Funnels
Tornadoes are relatively rare events but they can happen with almost any severe thunderstorm. Of the 10-20 annual sightings in Australia, most are first seen and reported by members of the public. Various methods have been used for estimating maximum wind speeds in tornadoes including time-lapse photography, interpreting damage patterns and theoretical calculations. It is now generally agreed that maximum wind speeds in tornadoes do not exceed 450 km/h.
How and Why They Form
As you can imagine, it requires a narrow range of conditions for a thunderstorm to become tornadic. The key conditions are: an intense, sustained updraught; strong wind shear, i.e. turning and strengthening of the environmental winds with height (e.g. northeast at the surface and northwest aloft); and strong winds at cloud-top level. These conditions allow the thunderstorm's own air-flow pattern to interact with the environmental winds and produce rotation within the updraught.
Weak tornadoes (Photo 11) are formed primarily by tightening up of the rotating updraught, as the thunderstorm intensifies to a maximum, and are found right under the updraught core, sometimes without any significant cloud-base lowering. This form of rotation also accounts for most funnel cloud sightings, waterspouts and landspouts. Weak tornadoes are most likely during mid-summer thunderstorms but may also accompany the passage of intense cold fronts during the winter months in southern parts of Australia. They are mostly brief events and cause only limited damage.
Photo 11. Small tornado over grape vines near Beechworth, Victoria.
Photograph courtesy of Jan Osmotherly.
Stronger tornadoes (Photo 12), which typically occur with late spring/early summer severe thunderstorms, have a more complex formation. The updraught and downdraught operate together to enhance the rotating column within the cloud and we see this as a rotating cloud base and wall cloud. It is speculated that, at a certain stage, a particularly intense updraught burst partially blocks the prevailing winds aloft and deflects additional air down toward the surface. These various airstreams then interact in a very complex way to translate their opposing forces into a tight rotational motion, which then extends downwards. This can take from 10-20 minutes after the intense updraught burst has occurred.
Photo 12. Strong tornado near the highway between Nimmitabel and
Cooma New South Wales, 23 December 2008.
Photograph courtesy of Heather Leckie.
In its decaying stage, the tornado lifts and becomes thinner and rope-like. Debris will decrease suddenly and become diffuse or spread out horizontally. The condensation funnel will shrink upward and become twisted, curved, or less vertical in orientation. It continues to shrivel in this rope stage until it vanishes or retracts into the cloud base. The wall cloud will also lose structure and size but may rebuild again later.
Tornadoes move with the thunderstorm cloud at about the same speed and direction, typically around 30-50 km/h. However, tornadoes associated with thunderstorms near vigorous cold fronts may reach speeds of 80-100 km/h. Once formed, their motion is always dependent on the movement of the parent thunderstorm and is reasonably predictable, although some tornadoes will weaken and subsequently redevelop nearby.
What it's Like Near a Tornado
Mostly tornadoes occur near the back side of the thunderstorm, near a brightening sky usually to the west. Heavy rain and hail may precede their arrival but once the thunderstorm's main updraught is overhead, only a few scattered hailstones and an eerie calm will accompany their approach. The tornado itself produces a violent wind that begins and ends abruptly. It will last from several seconds to (at most) two minutes and be accompanied by a variety of sounds caused by the damage to buildings, trees, etc. Some people also hear a 'roar' but many tornadoes make no sound at all. If the sound is irregular it is likely the result of damage occurring nearby; but if it is a steady, softer sound that originates in the sky or toward the main thunderstorm cloud, it is more likely caused by large hailstones hitting the ground or colliding in mid-air. After its passage, some light rain or hail can occur, along with cooler winds, before clearing takes place.
Other Rotating Structures
There are several other rotating structures to watch for. They are not tornadic in the strict sense but are of enough concern to the public to merit reporting.
A funnel cloud is a visible vortex attached to a cumulonimbus cloud but not reaching the surface. You may sometimes see one that looks like a real tornado either in the wrong location or apparently unrelated to any lowering or updraught region. These mid-air funnels are very deceiving and the only way to discount their threat is to look at the surrounding clouds for clear signs of rotation. If not associated with tornadoes, they are very short-lived and quickly decay after formation.
Landspouts and Waterspouts
The landspout looks like a slender tornado over land, but is not associated with an organised rotating cloud, does not form below a wall cloud and is generally not intense enough to do much damage. The mechanism which forms a landspout is similar to the waterspout: relatively cool air passing over hot ground produces updraughts and cumulus clouds. Random swirls can be caught up in an updraught and so tighten up into a funnel which is made visible by raised dust, rather than by condensation of moisture.
Photo 13. Waterspout off Boat Harbour, northwest Tasmania.
Photograph courtesy of Martin Eadie.
A waterspout (Photo 13) looks like a slender tornado but occurs only over water. They are occasionally seen near the coast in the late summer and autumn. Cool, unstable air masses passing over the warmer waters allow vigorous updraughts to form, which can tighten up into a spinning column. The cool, moist air usually supports a full condensation funnel. Waterspouts can be dangerous for boaters and shoreline locations but are no threat farther inland since they collapse soon after they move onshore.
The gustnado has been accepted as a 'type of tornado' but is really a brief, intense vortex that forms on the leading edge of gust fronts. Scud and debris or dirt may be seen but a condensation funnel is usually absent. They will last from a few seconds to several minutes and are strong enough to cause minor damage. They are distinguished from a true tornado by their location under an advancing dark cloud bank, or shelf cloud ahead of the rain core. Although the air is rotating, this event is grouped more appropriately with straight-line winds (downbursts and microbursts).
Photo 14. A 'gustnado', a type of tornado on the leading edge of a wind squall,
just ahead of a ragged shelf cloud, Melbourne, Victoria.
Photograph courtesy of Andrew Treloar.
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