Tornadoes: the telltale signs

Commanding Officer of the Naval Oceanography Command Facility, Lt. Cmdr. Hank Pomeranz has written the following article in an attempt to explain the phenomena and advise how local residents can best protect themselves.


Commanding Officer of the Naval Oceanography Command Facility, Lt. Cmdr. Hank Pomeranz has written the following article in an attempt to explain the phenomena and advise how local residents can best protect themselves.

Said Lt. Cmdr, Pomeranz: " In my meetings with the Emergency Measures Organisation and other individuals, and after reading your Royal Gazette editorials and being questioned by the media, I became convinced that some education regarding tornadoes was necessary for public good. On March 21, 1992, a tornado hit the Island's East End, causing damage to buildings on the Naval Air Station and property in St. David's.

On Tuesday morning, April 15, 1993, a tornado touched down on the Island's West End. Major damage was caused to several homes before the twister dissipated over the Great Sound. Fortunately, in both instances, no one was seriously hurt.

In this article I'll present some general facts about tornado activity, a description of a "typical'' tornado, our capabilities with regard to forecasting them, some hints to help you look to the skies for telltale signs of possible tornado generation and finally, some measures to take in case you are directly threatened.

First, here are some hair raising facts.

Tornadoes occur in many parts of the world, but no country experiences more tornadoes than the United States, which averages 781 annually! About three quarters of all tornadoes develop from March to July. On average, 92 people die in the US each year as a direct result of tornadoes although there have been several instances of over 100 people dying in a single day.

One of the most violent outbreaks ever recorded occurred on April 3 and 4, 1974. During a 16-hour period, 148 tornadoes cut through parts of 13 states, killing 307 people, injuring more than 6000, and causing an estimated $600 million in damage.

There are many other incidents as terrible. On March 18, 1925, 695 people died as seven tornadoes travelled a total of some 703 kilometres across Missouri, Illinois and Indiana.

On Palm Sunday, 1965, more than 30 tornadoes moved through five midwestern states, inflicting great damage and killing 256 people. Finally, more recently, on March 28, 1984, a series of at least 36 tornadoes marched across North and South Carolina, claiming 59 lives and causing hundreds of millions of dollars in damage. One tornado was enormous, with a diameter of at least 4000 metres (2.5 miles) and winds that exceeded 200 knots. Think of it! How wide did you say Bermuda was? Enough of the gory details, what do we know of the storms themselves? Tornadoes are rapidly rotating winds that blow around a small area of intense low pressure. Sometimes called twisters, or cyclones, tornadoes nearly always begin as a funnel-shapped cloud that looks like an elephant's trunk hanging from a large cumulonibus (thunderstorm) cloud.

This funnel cloud is called a tornado only after it touches the ground. They generally, but not always, spin anti-clockwise. Their diameter is usually between 100 and 600 metres, although some are just a few metres wide and others exceed 1600 metres.

Most move at speeds between 20 and 40 knots. However, some have been clocked at greater than 70 knots. On average, they last just a few minutes and travel a distance of about 7 kilometres. Because of the destructive nature of the tornado, it was once thought that it packed winds greater than 500 knots.

However, recent studies show even the most powerful of these storms seldom exceeds 200 knots and most probably have winds less than 125 knots.

Our knowledge of the furious winds of a tornado comes mainly from observations of the damage done and analyses of motion pictures, not from direct measurements inside the storms themselves.

Finally, although everything is not known about the formation of a tornado, we do know that they tend to form with severe thunderstorms and that unstable air is essential for their development. Springtime in Bermuda, while lovely, is a prime time for instability as the local atmosphere heats up and cold fronts continue to invade our shores from the continent.

Now let's talk about the state of the science (my term) or state of the art (as you're likely to call it) for forecasting tornadoes. The bottom line is that it's not very good. It's not good in the United States where they have the most dense pattern of weather observations on the planet, the most highly sophisticated equipment and where, as previously stated, they have the most tornado activity.

It's worse here, where tornadoes are rare and there are no other observing stations around. Consider this: the National Severe Storms Forecast Centre in Kansas City issues a tornado "watch'' when tornadoes are likely to form within a few hours. They have no way to forecast which thunderstorms will produce tornadoes. Many communities have trained volunteer "spotters'', who look for tornadoes after the watch has been issued.

Once a tornado is spotted, a tornado "warning'' is issued. Let me say that again. It's not until after a tornado has been seen that a warning is issued! No such luxury exists in Bermuda. Once a tornado is seen and reported, it's too late to issue a warning. The storm will probably dissipate before word ever gets out to the public.

Compare this to hurricanes, which can be tracked by satellite and aircraft for days before they become a threat to populated areas.

There is some heartening news to all this. A new generation weather radar, called Doppler radar, can uncover many of the features of severe thunderstorms, from which tornadoes are likely to develop. These new radars will assist forecasters in determining which severe thunderstorms will likely spawn tornadoes.

In addition, they should give advanced and improved warnings of an approaching tornado. As an added benefit, because the Doppler radar shows air motions within a storm, it can help to identify the magnitude of other severe weather phenomena, such as gust fronts, microbursts and wind shears that are dangerous to aircraft.

What can we do now? From the standpoint of the meteorologists here at the Naval Oceanography Command Facility, we will place a concerted effort towards identifying conditions likely to result in severe thunderstorms while giving as much public notice as possible. As usual, notice will be given through the media as the quickest way to get the word out.

Like our stateside counterparts, we cannot forecast which thunderstorm will produce a tornado. We therefore cannot say with any degree of certainty that a tornado will strike anywhere on the Island.

Here's what you can do. First of all, when thunderstorms are in the area, be alert and look skyward. The appearance of bulging pouches of clouds extending below the base of a thunderstorm cloud might be the first indicator of possible tornado activity. These bulging pouches are called mammatus-type clouds and always indicate severe thunderstorm activity. Mammatus-type clouds are not funnel clouds because they do not rotate.

The first sign that a thunderstorm is about to give birth to a tornado is the sight of rotating clouds at the base of the storm. If you see this sort of severe weather occurring report it immediately. We will issue a warning and contact the media, etc. On land, call 293-5339. Afloat, call Harbour Radio on Channel 16.

If you are threatened, take shelter if possible. Since homes in Bermuda generally don't have basements, what about that water catchment area under your house. If you are in your home, stay away from windows and, if possible keep your family in a small interior room (one with no outside walls). In a large building, the safest place is usually in a small room, such as bathroom, closet or interior hallway, preferably on the lowest floor. In schools, move to the hallway, away from windows.

The bottom line is that you are your own best protection. You can't move your home away from a tornado's path, but you can protect yourself to some degree.

One thing is for certain, tornadoes, while rare in Bermuda, will occur again.

Hopefully, this article will make us all a little more knowledgeable and, as such, safer when the next one strikes.

DEVASTATED -- Residents in Bob's Valley, Sandys, begin the clean-up operation after roofs were ripped from several homes during last week's tornado.

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