What is a tornado? How does it occur?
By definition, a tornado is a violently rotating column of air in contact with the ground and pendent from a cumulonimbus cloud which is active for a very short time (Figure 1). In Bangladesh, a tornado is often misinterpreted as a nor'wester or an ordinary thunderstorm or even with a cyclone although none of these is similar to each other.

Nor'westers or ordinary thunderstorms have straight line winds, while in tornadoes and in cyclones, the wind is rotating. It can be tracked down easily by looking at their damage patterns even if there is no satellite or radar image available. It is easy to distinguish between a tornado and a cyclone as cyclones are formed in deep ocean, synoptic in scale and their duration is much longer than tornadoes. While tornadoes are usually formed in the ground, they can easily cross over the water and strike the nearest river or reservoirs. Tornadoes on water are commonly known as 'water sprout'.

Tornadoes are formed from a violent thunderstorm called 'supercell' which tends to be accompanied by large hail and lightning. Tornado forecasters always watch out for these supercell thunderstorms for the tornado forecasting as it is hard to pinpoint a tornado occurrence. There are several conditions required to be present in the weather to form a supercell thunderstorm. One obvious condition is the presence of a 'dry line'. A 'dry line' is an imaginary line on the ground which divides two immediate regions based on temperature and moisture differences. In the late spring and early summer months, such a dry line often exists to the western side of Bangladesh (Figure 2). During this time Northern India is heated and dried up quickly as it is closer to the desert condition and to the Himalayas, but in Bangladesh lots of moisture remain in the weather because of its proximity to the Bay of Bengal. Also, as shown in Figure 2, the mid-upper tropospheric flow known as the 500 mb jet splits around the Tibetan Plateau allowing the westerlies to be stronger than average in this region implies strong influence of regional topography on the local atmosphere.

In general, at the end of winter season in Bangladesh, the southerly warm wind tends to flow from the Bay of Bengal which carries a lot of moisture and when it confronts with the cold, dry winter air that still remains in the atmosphere, makes the weather very unstable. That is why many thunderstorms locally known as 'Kalbaishakhi' or 'nor'westers' are often seen in Bangladesh around this time which are the results of this unstable weather. On a given day, with the presence of necessary weather conditions especially the dry line, these thunderstorms can be turned into supercell thunderstorms and produce tornadoes.

Tornado risk months and area
As mentioned earlier, most of the tornadoes occur in Bangladesh in the late spring and early summer months i.e. from March through May; thus, it can be called as the 'Tornado Season'.

Jonathan Finch, a meteorologist in the National Weather Service, USA has been doing research on Bangladesh tornadoes for a while. Together with Ashraf Dewan, he documented 86 tornadoes occurred in March-May from 1838 to 2001 in Bangladesh and in East India. Half of these tornadoes (43) killed at least 30 or more people and 25 of them killed at least 100 people. Climatologically, first 20 days of April can be considered as the most dangerous in terms of the killer tornado occurrences. According to Finch, 32 out of 43 (75%) of the tornadoes that killed 30 or more people and 16 of the 25 (64%) tornadoes that killed 100 or more people occurred during the first 20 days of April. On a similar note, more than 900 people were killed in Dhaka and Comilla (660 in Dhaka) by two violent tornadoes in 1969 on the first day of Bengali Calendar, 14th April. Imagine, how terrible the consequences would be, if such a tornado strikes in the present day Dhaka which has now grown many times in population since then.

In the documented tornadoes, most of the tornadoes occurred in the afternoon or evening, especially the most catastrophic tornadoes with a peak around 4.30 pm. A few violent tornadoes occurred between 9 pm and 12 am. A general idea about the tornado risk areas in Bangladesh can be perceived from the documented tornadoes.

Tornado disaster management: points to ponder

Due to regional topography, tornadoes are natural phenomena in Bangladesh. We cannot prevent tornadoes or supercells from happening. But we can certainly take necessary actions to reduce damages and casualties from these killer tornadoes and that is the main principle of disaster management.

a) Building basements and protective housing: "Go to the basement"- is the motto of people in tornado risk areas of USA. Basement is the safest shelter during a tornado occurrence as it destroys everything in its path on the ground. Unfortunately, we do not see basements in the houses of Bangladesh. Government can build basements in public buildings such as in schools in tornado risk areas so that people can take shelter during emergencies. Also, local engineers and architects should come forward to design low cost housing with basement and other protective features for tornado risk areas in Bangladesh. Recently, researchers at Texas Tech University designed a 'storm safe room', which is an interior room in the house that can provide protection against deadly tornadoes. The idea was conceived from post-storm inspections of hundreds of homes in more than 90 towns and cities struck by tornadoes which revealed that in many instances a small room in the central portion of the house remained standing even when the house was severely damaged or completely destroyed.

b) Limiting the use of C.I. sheet: Corrugated Iron sheet locally known as 'dheu tin' is a popular roofing material in Bangladesh. But during windstorms like tornadoes, nor'westers and cyclones, it can be a perfect lethal weapon. Tin sheets tend to blow freely in the wind, causing many laceration wounds and fatalities. Studies showed that during windstorms, a good number of people die or injured from flying debris such as tin sheets. In tornado and cyclone risk areas, people should be encouraged to use alternate low cost roofing materials instead of C.I. sheet. Government should set policies immediately regarding this matter.

c) Enforcing building code in hazard risk areas: Bangladesh government published National Building Code in 1993 to safeguard public life. But unfortunately, only in big cities the codes have been enforced to some extent. In rest of the country, housing condition is so poor that these houses cannot withstand winds even from ordinary thunderstorms let alone tornadoes and cyclones. Government should enforce building codes mandatory at least in the tornado and cyclone risk areas. If necessary, government should provide easy loans or grants to comply with the building code. Also, although windstorms in general are considered in preparing the building codes, it needs to be revised specifically in terms of tornado and cyclone risks in Bangladesh.

d) Not to venture outside during thunderstorms: People should not venture outside during a thunderstorm. Large hail and lightning can be deadly. While the lightning threat occurs from March through the summer months, the large hail threat is confined to the spring. Also, many passenger launch disasters occur during March-May due to the high winds. So, people should be careful on boarding a passenger launch around this time especially during a thunderstorm.

e) Tornado awareness week: In order to create mass awareness about tornado safety, "Tornado Awareness Week" can be observed each year nationally in Bangladesh. Since, April is the deadliest month from tornadoes in Bangladesh, 2nd or 3rd week of April may be observed as the "Tornado Awareness Week". Mass media can play a vital role to create awareness through television, newspaper campaign about tornado safety in this week.

f) Tornado forecasting: In USA, tornado forecasting is given to the public at two levels. First, a tornado watch is issued by the government weather service for a given area and a given time, if conditions are right for possible tornado formation. It means people of that area should be watchful and alert of approaching tornadoes during this period. Second, a tornado warning is issued if a tornado has actually been sighted on the ground or indicated on meteorological radar. It means people have to go to the basement or take shelter immediately. Sometimes, a tornado siren is used to alert people of the possible danger. It is really unfortunate that Bangladesh Meteorological Department (BMD), which is the government weather agency, does not provide any type of tornado warning for the public. As a government weather agency, it is its sole duty to alert people on any type of weather hazard.

Jonathan Finch has been forecasting on Bangladesh tornadoes in www.bangladeshtornadoes.org during the tornado season for the last few years. To forecast on high end severe storms or possible tornado formation, he has to rely on weather data available on internet by the Indian Meteorological Department or other global weather stations. It is a great irony that with limited resources and on a voluntary basis, while Mr. Finch has been providing at least some sort of alert on Bangladesh tornadoes, being a full-fledged government weather agency, BMD is unable to provide any service at all. Government should come forward to solve this problem immediately. For technical assistance on tornado forecasting, BMD may ask for collaboration with the National Weather Service, USA or at least can utilize the expertise of Jonathan Finch.

Acknowledgement: Jonathan Finch, Meteorologist, National Weather Service, USA, for reference, pictures and discussions at different times.

Tanveerul Islam, PhD is a post doctoral research fellow, Texas A & M University-Galveston, USA.