Tornadoes

Tornadoes and their Damage

How do mesocyclones form?

Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.

Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

From mesocyclone onwards
Tornadoes often develop from a class of thunderstorms known as supercells. Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 2–10 km across. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms.
When the mesocyclone approaches the ground, a visible funnel appears to descend from the base of the storm, often from a rotating wall cloud, creating winds that can cause damage a good distance from the tornado. Usually, the funnel cloud becomes a tornado within minutes. This is when the tornado is at its most powerful stage, this can last for only minutes, up to several hours.

When cold winds have completely wrapped around the tornado's air supply, the vortex begins to weaken, becoming thin and rope-like. This often lasts no more than a few minutes, after which the tornado fizzles. During this stage, the shape of the tornado becomes highly influenced by the winds of the parent storm, and can be blown into fantastic patterns.

As the tornado weakens, its associated mesocyclone often weakens as well. In particularly intense supercells tornadoes can develop cyclically. As the first mesocyclone and associated tornado weaken and disappear, the storm's inflow may be concentrated into a new area closer to the center of the storm. If a new mesocyclone develops, the cycle may start again, producing one or more new tornadoes. Occasionally, the old mesocyclone and the new mesocyclone produce a tornado at the same time.

Though this is a widely-accepted theory for how most tornadoes form, live, and die, it does not explain the formation of smaller tornadoes, such as land spouts, long-lived tornadoes, or tornadoes with multiple vortices. These each have different mechanisms which influence their development. However, most tornadoes follow a pattern similar to this one.

The scale of damage
Tornadoes have their own scale of damage, since the Beaufort wind force scale (wind intensity) only goes up to (wind) force 12. The tornado scale is called: The Fujita scale of tornado intensity, it is named after its designer Tetsuya Fujita.

The original scale as derived by Fujita was a 13-level scale (F0-F12) designed to smoothly connect the Beaufort scale (wind intensity) and the Mach number (speed of sound) scale. The gap between F0 and F1 corresponds to the eleventh and twelfth levels of the Beaufort scale, "violent storm" and "hurricane" respectively. On the original scale, the wind speeds for F11 and F12 corresponded to Mach numbers 0.9 and 1.0 (once the speed of sound) respectively. This provided a smooth relationship between the three scales. From these wind speed numbers, descriptions of damage were made for each category of the Fujita scale, and then these descriptions were used to classify tornadoes.

At the time Fujita derived the scale, little data was available on damage caused by wind, so the original scale presented little more than educated guesses at wind speed ranges for specific tiers of damage. After the scale was derived, it was soon realized that F7-F12 levels did not correspond to actual tornado damage. Later, the National Weather Service decided not to use the F6 level either, as identifying the damage caused by a tornado stronger than F5 would be next to impossible.

Thus there were only 6 categories left:

SCALE WIND ESTIMATE *** (Km/H) TYPICAL DAMAGE
F0 < 116 Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.
F1 116-180 Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.
F2 181-250 Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
F3 251–330 Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
F4 331-415 Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
F5 416–510 Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.

Tornado Types

Of course, there is not just one kind of tornado, there are different kind of types. Every type of tornado has several characteristics and takes place on a certain area.

Landspout
Landspout is the most common form of a tornado. It is a narrow vortex which forms over land under a growing Cumulus Cloud. The winds this tornado creates might reach up to 225 km/h. It lasts several minutes before it disappears again.

Waterspout
A waterspout is a column of rotating wind that develops downward form a line of Cumulus clouds to an area of water. A Cumulus cloud and an area of water are both essential for a waterspout to occur. Cold air which overlays the warmer water is a critical element leading to a waterspout event. The strong winds inside the tornado bring the water of the water area up. These spouts can contain winds of 320 km/h and move across the water with a speed up to 15 to 25 km/h.

Dust Devil
Dust Devils are spouts that go come from the ground and go upward to fizzle out in clear air, they aren’t attached to clouds. They’re most commonly found in deserts and form when air at the ground becomes hotter than the air above it, which happens by the warmth of the sun. The lighter, hot air begins rising and begins making a whirling motion that carries the sand and dust upward. The top speeds of these winds seem to be around the 90 km/h.

Snowspout
A Snowspout is an extremely rare phenomenon in which a vortex resembling a waterspout is created under a snow squall. For a snowspout to occur there are three criteria; extremely cold temperatures present over an area of water warm enough to produce fog or steam above the waters surface, lake effects snows in a clustered or banded formation must be present and on going, and the environmental winds must be extremely light.

‘Supercell’ Tornadoes
A ‘Supercell’ Tornado is a long-lived thunderstorm containing within its structure a continuously rotating updraft of air. When warm and moist Mexican air combines with cold America air, ‘Supercells’ can develop. The tornadoes inside this ‘Supercell’ can remain contact with the ground for more than an hour, are more likely to be violent and carry winds of more than 300 km/h.

Tornado Occurrence

Theoretically a tornado can occur on every spot on this planet, but in most areas this chance is very small. For example, in the Netherlands we can expect a tornado once in 30 years.

Tornadoes have been observed on every continent except Antarctica; however most of the world’s tornadoes occur in the United States. Other areas which commonly experience tornadoes include New Zealand, western and south-eastern Australia, south-central Canada, north-western and central Europe, Italy, south-central and eastern Asia, east-central South America, and Southern Africa.

As just said, most tornadoes occur in the United States. Between the states of Texas to North Dakota and the states of Arkansas to Colorado the most tornadoes come into being. The reason for this is the Western winds at high heights which carry warm and moist air of the Gulf of Mexico to America; those winds collide with cold fronts coming from Canada. This area, which is a couple of states big, is therefore called Tornado Alley.

Tornado Alley
In the heart of this Tornado Alley, building codes are stricter than those for other parts of the United States. Buildings should require strengthened roofs and more secure connections between the building and its foundation. Other common precautionary measures include the construction of storm cellars, and the installation of tornado sirens. Tornado awareness and media weather coverage are also high

Statistics
When you look at the tornado statistic, when land area is taken into account, most tornadoes occur in Texas, Florida however has the highest density of tornado occurrence in the country. Nearly all of these tornadoes are weak and not produced by ‘Supercell’ Thunderstorms. In total between 800 and 1200 tornadoes occur each year.

The time of year of highest tornado frequency, also called ‘Tornado Season’, shifts geographically throughout the year. The Tornado Ally tends to shift farther north as the weather warms across the United States. The central parts however can be considered having no real ‘tornado season’, its more all–year-round, since this is where temperature changes between warm and cold are most common.

According to this report it almost sounds like tornadoes only occur in America, and not in Europe. In Europe about 300 to 700 tornadoes occur each year. About 45 in Great-Britain, and therefore it has the highest density of tornadoes per area in the world. The Netherlands is also high on that list with about 30 tornadoes each year. The ‘most famous’ one happened in 1967, a tornado went trough the villages of Chaam and Tricht (Northern-Brabant). 7 people died, and some injured. The chance of giant tornadoes occurring in the Netherlands is very small, because we less often have heavy thunderstorms.

Stormchasing

Why chase storms?
Chasing storms was first done by scientists to study the thunderstorm environment. Little about the dynamics and physics of thunderstorms, as well as lightning, is well understood yet, so in the interest of research storm chasing is necessary to keep track of and investigate a storm and watch it go through its lifecycle.
There is a second group of stormchasers who chase storms for a hobby, because they like storms and find them beautiful. A well-developed, severe thunderstorm is not an everyday sight and the power being displayed by lightning, downbursts and tornadoes can be truly awe-inspiring.
Finally, there is a group of stormchasers who track down storms for the media, or otherwise commercial interests. This group of chasers usually takes the most risks to get as close to the storm as possible in order to get the best and most dramatic footage.

Equipment
If you plan to chase storms seriously there are a few things which can be handy, or are even necessary, to be successful.
• A car or van.
• A laptop computer with cell phone + mobile internet.
• A still camera and video camera
• Film
• A GPS receiver with route mapping software (for the laptop)
• Some paper maps
• Miscellaneous things like food, water, torch, etc.

Prepatations
You can chase storms in a variety of ways. You can just go out and watch a storm passing by; not really stormchasing, and the chance of seeing something interesting is low, but stormchasing as a hobby usually starts this way.
However, waiting for a suitable storm chasing environment is probably the most time consuming part of the preparations you will need to make for a chase. Depending on where you are this can take quite a while! The best storms to chase, by the way, are slow moving, isolated thunderstorms, the so-called air-mass thunderstorms.
When the weather is looking good, make preparations by discussing where to go, and, if there is some sort of home station, who will be available to give weather updates over the cell phone. When using wireless internet, a home station is not really necessary, but then make sure everything is working and all batteries are charged. If necessary the batteries can, of course be charged in the car with the cigarette lighter, but it is nice to have everything ready at first!
Furthermore, make sure to have lots of film and video tapes ready. Nothing is more frustrating than getting a good storm and running out of photography supplies!

Stormchasing precautions
There are basically two dangers to stormchasers:
- Traffic accidents
- weather hazards, like lightning, hail, gusty winds and tornadoes
As far as the first danger is concerned try not to chase on your own so you can rotate driving duties. Try not to speed but if you really feel the urge than at least only do it when the roads are clear so you will only endanger yourself and not other road users. Take care to follow traffic rules when stormchasing; you don't want to have an accident or be pulled over and receive a ticket wasting money and valuable chase time.
Another danger on the road is aquaplaning. Because of the nature of storm chasing the risk of aquaplaning is higher than normal. Make sure you are aware of this hazard and know how to act to avoid this. Make sure you have a good set of tires to minimize the risk.
Weather hazards can be avoided by chasing in a responsible way. This means, for example, that you should not feel; the urge to come as close to a tornado as possible - viewing a tornado from 1km is close enough, if not already too close. Do not try to drive through a storms downdraft; if it is severe you never know what you might encounter when reaching the other side of the downdraft - you might find golf ball (or bigger!) sized hail or in a supercell storm you might even drive straight into a tornado!
There is also a chance that you, or your car, may be struck by lightning, especially on open roads. So to be safe when a storm is close sit in the car with the windows closed. Other situations to be avoided when a storm is close are:
- being near or under single trees;
- being near or in water;
- being on top of a tall building, hill, knoll etc;
- being in the open field;
- being near to long metal wires or fences.
- being in a building and close to a window;
- taking a bath, washing hands;
- Making phone calls.

Protection
Inside
You have to know where the nearest tornado shelter is and how to get there the quickest way. If you are not in a building with a shelter, go to a room in the middle of that building without windows or skylights or go into the basement or a crawl space and close the entrance to that space.

Outside
If you are outside and a tornado is approaching, go to the nearest substantial building if there is one close by, do not try to outrun the tornado or drive from it. I there is no building nearby, find a low place (roadside ditch) well away from buildings, your vehicle and trees (lightning danger), and lie down in the low area with your head covered. Highway overpasses are extremely bad shelters during tornadoes, because they can be destroyed and fall down.