What are tornadoes?
Tornadoes are formed when warm, moist air collides with cool, dry air to create powerful thunderstorms known as supercells. As the warm and cool air collide they form a spinning tube of rising winds (called 'mesocyclone') which becomes a tornado when it touches the ground. The base of the tornado (i.e. near the ground) appears dark due to dust and other materials it picked up from the ground.
Not all local severe thunder storms are tornadoes. In tornadoes the wind whirls around a central air funnel that resembles an elephant’s trunk. They take a zigzag route and cause severe damage within minutes. Other storms are not as severe.
Dr Samarendra Karmakar, director of the country’s meteorology department
Tornado are measured using the Fujita Scale created by Dr. T. Theodore Fujita in 1971, the year Bangladesh gained independence. The Fujita Scale range from F0 (light damage) to F5 (extreme damage).
Today, experts use the Enhanced Fujita (EF) Scale which looks at both a tornado's level of damage and its estimated wind speeds.
For some, a tornado might just be a thrill-seeking spectacle they watch from the comfort of the sofa. But for many, tornadoes not only wreck their towns, villages and livelihoods...they wreck lives as they leave death and destruction in their wake.
Types of tornado
Based on their intensities and power of destruction, tornadoes can be categorised into three groups - weak, strong and violent. And though violent tornadoes constitute only 2% of all tornadoes, they are the cause of most death (around 70%).
|Category||% of total||Deaths||Duration||Wind speed|
|Weak||70%||5%||1-10 min||Less than 180 kph|
|Strong||28%||25%||20+ min||180 - 350 kph|
|Violent||2%||70%||Over 60 min||Over 350 kph|
Generally, tornado-producing thunderstorms form during the afternoon hours.
Tornadoes are a regular occurrence in Bangladesh, with experts warning of more on the way.
Why is Bangladesh susceptible to tornadoes?
Bangladesh is the world's worst place for deadliest tornadoes
Very little information is available about the tornadoes that took place in Bengal historically. This changed after 1960, thanks mainly to the detailed reports in the Pakistan Observer newspaper (renamed to Bangladesh Observer after 1971 independence).
Bangladesh is considered the only other part of the world outside the United States where strong and violent tornadoes are prevalent.
Six of the ten's worlds worst tornadoes ever recorded in terms of casualties have occurred in Bangladesh, including the top two most deadliest. Jonathan D. Finch in his 'Bangladesh and East India Tornado Prediction' website listed 67 tornadoes that struck Bangladesh between 1838 and 1998. Of the 67 events, 19 were reported to have caused deaths of 100 or more each - almost half the total of the rest of the world - while at least 6 incidents caused death of more than 500 people each.
Bangladesh's location is perfect for storms
As one of the world's most fertile country and a country made of rivers, Bangladesh is perfect for agriculture. Thus it's no surprise that Bangladesh is predominantly an agrarian economy.
But its geography is also its downfall. Bangladesh is in a prime spot for twisters. Cold air from the Himalaya Mountains to the north meets hot, humid air coming from the Bay of Bengal located in the south. The clash brings strong thunderstorms. The storms lead to about 60 tornadoes each year. The country’s peak tornado months are April and May.
The geographic location of Bangladesh, with the Bay of Bengal to the south and the Himalayan Mountains to the north, makes a tornado a natural phenomenon for Bangladesh.
During the hot weather period, predominantly during the pre-monsoon season from March to May thunderstorms with great destructive potential occur.
More local severe storms are likely to hit Bangladesh as the cold weather approaching from the northeastern Himalayas meets the existing hot and humid weather system for the next six to eight weeks.
Dr Samarendra Karmakar, Director of the country’s meteorology department
The Bay of Bengal and the coastal mountain range just inland in East India restricts the area susceptible to tornados. Only a narrow strip along coastal East India is usually threatened by tornadoes, since spring moisture return over the Bay of Bengal is generally directed to the north and northeast across Bangladesh, instead of to the west into India.
In fact, most tornadoes in east India have occurred very close to the Bangladesh border.
Since most of Bangladesh to the south of 22.5N is swampland and less populated, many tornadoes in this area have undoubtedly gone undocumented.
Most people in Bangladesh are poor. In 1989, of the 110 million population, 65 million were landless day labourers. They rely on manual labour to survive. When there are no fields to tend, there is no money to earn.
There has been a population explosion in Bangladesh in the past 50 years. Dhaka's population alone has increased 10-fold since 1960. The population density combined with its status as a 'third world' country makes Bangladesh extremely vulnerable to natural disasters. Also many homes are not built to withstand powerful winds. The country does not have a reliable tornado warning system. And with the population ever growing, people are forced to live close together especially in the 'more affluent' towns and cities. When a twister strikes, it can kill dozens of people instantly.
It [Bangladesh] is one of the most densely populated countries of the world with almost 1,000 person in each of the 147,570 square kilometer area. Agriculture is the single largest producing sector of Bangladesh economy since it comprises about 20% of the country's Gross Domestic Product (GDP) and employs around 45% of the total labor force. Agricultural growth has accelerated from less than 2.0% per year during the first two decades after independence in 1971 to around 3.0% during the last decade. Despite such a steady growth in agriculture as well as in food production, Bangladesh has been facing persistent challenges in achieving food security. This is mainly due to natural disasters and fluctuations in food prices from the influence of volatile international market for basic food items.
The impacts of climate change on agriculture are global concern but for Bangladesh, where lives and livelihoods depend mainly on agriculture, are exposed to a great danger. Bangladesh is one of the most vulnerable countries to climate change because of its disadvantageous geographic location; flat and low-lying topography; dense population; high levels of poverty; reliance of many livelihoods on climate sensitive sectors, particularly crop agriculture and fisheries; and inefficient institutional and poor infrastructure. Many of the anticipated adverse effects of climate change, such as sea level rise, temperature increase, enhanced monsoon precipitation, an increase in cyclone intensity etc., will aggravate the existing stresses that already impede development in Bangladesh.
Bangladesh is the perfect spot for tornadoes to form. It receives cold air from the Himalaya Mountains, and then the cold air meets with the hot, humid air in the Bangladesh area.
Therefore it's no surprise that tornadoes are fairly common in Bangladesh. It's one of the most frequently hit countries in the world, behind the United States and Canada. In 1989 Bangladesh was hit with a drought - Manikganj district itself had been in a state of drought for six months, possibly generating tornadic conditions. In late April the President ordered the nation to pray for rain, little knowing the destruction that was to follow.
Two tornado seasons in Bangladesh
Although tornadoes have been recorded as early as January, Bangladesh has a very short tornado season running from mid-March to mid-May with a strong peak in April. A weaker and smaller tornado season also occurs later in the year around October.
Due to the orbital movement of the earth the wind changes direction twice a year - once when the Tropic of Cancer starts moving towards the sun in March-May, and once when it starts moving away from the sun in October-November. The transitional periods are usually referred to as pre-monsoon (March-May) and post-monsoon (October-November).
These two transitional periods are characterised by local severe thunder storms - including severe local storms, tornadoes and cyclones.
The frequency of devastating pre-monsoon local storms usually reaches its peak in April, while a few occur in May and the least in March.
Post-monsoon storms are usually weaker and they are fewer in number. They are smaller in size, and hence cause less devastation.
Although the country’s tornado season is much shorter than that of North America, it is far deadlier. This is because of an absence of a sophisticated warning system plus the lack of tornado shelters and buildings capable of withstanding the destructive winds.
That the Daulatpur-Saturia tornado struck during the month of April was no surprise.
A study of 85 tornadoes revealed that 80% of the tornadoes that killed 100 or more people and 75% of tornadoes that killed 30 or more people occurred during the first 20 days of April.
These findings strongly indicate that the violent tornado season for Bengal is very short, covering mainly the first 3 weeks of April. Almost all of the documented tornadoes occurred in the afternoon or evening, especially the most catastrophic tornadoes, with a peak around 16:30 BST (Bangladesh local time). Very few violent tornadoes occurred between 21:00 and 12:00 BST.
In the past few decades there has been a shift in the timing of violent tornadoes. Between 1963-1977 most of these occurred in early to mid-April. However, since 1985 several violent tornadoes have occurred outside the peak season.
The 13 May 1996 tornado in Tangail that killed more than 700 people in northern and central Bangladesh was extremely unusual in that it occurred so late in the season.
On the other hand, the tornado on 19 March 1961 in central Bangladesh that killed 210 people was unusually early in the season. In fact, this is the earliest spring tornado on record that killed 30 people or more.
Unfortunately, with the country’s population density continuing to increase, the risk of a similar event occurring in the near future is all too real.
The preferred movement of the tornadoes was to the southeast area. A few of the tornados moved to the south, and a few to the east or northeast. The paths were not always along a straight line. The Muhammadpur tornado on 11 April 1964 moved to the southwest, then curved to the southeast in a horseshoe fashion. The Manikganj tornado on 17 April 1973 moved in a zig-zag fashion to the southeast. The Saturia tornado on 26 April 1989 initially moved east, then hooked north before lifting.
Central, southcentral and southeastern areas have highest chance of tornadoes
Southern and central Bangladesh have the highest incidence of tornadoes. The areas which is specially tornado prone is around Dhaka and 80 km south of Dhaka, and the area between Dhaka, Faridpur, Madaripur and Chandpur. Fewer tornadoes have been reported in other parts of the country.
Around 75% of the most deadly tornadoes have occurred across central, south-central and southeast Bangladesh, covering an area of 8,000 sq miles approximately.
A comprehensive climatology of violent tornadoes was developed for the Bengal region. The primary data sources included the Bangladesh Observer (Pakistan Observer prior to 1972), previous Bengal tornado studies, and world-wide web databases.
A list of 85 tornadoes to strike Bengal, along with the date, time, location, latitude and longitude, number of people killed, path width, path length, direction of movement, and tornado justification criterion.
...75 out of 85 of the tornadoes occurred between 22N and 25N. All but five of the tornadoes occurred between 88E and 91.5E. The highest concentration of the most violent tornadoes (30 or more deaths) occurred in a 4,000 sq mi area of central Bangladesh,or along and between 23N and 24N and between 89.5 E and 90.6 E. This is a little smaller than the state of New Hampshire in the United States.
Although a few tornadoes were documented for Bengal in early to mid-March and in mid to late-May, they are most likely in early to mid-April. Most violent tornadoes occur in the afternoon and evening, and moved to the south-southeast or southeast. All tornadoes occurred between 21N and 26.5N and 85.7E and 91.5E.
Indian tornadoes are less common, with most occurring close to the Bangladesh border. The highest concentration (20 out of 43) of the most violent tornadoes (30 or more deaths) occurred in a small area (4,000 sq. km.) in central Bangladesh, generally along and north of 23N and along and south of 24N and between 89.5E and 90.6E.
The distribution of devastating tornadoes in Bangladesh indicate that about 8,000 sq mile of the country across the central, south central and southeast region are particularly prone to this atmospheric disturbance. Almost 75% of the incidents have been reported to have occurred in these parts of the country.
Tornadoes are a common natural hazard. The annual economic loss caused by tornadoes and severe thunder storms is perhaps second only to that of the annual floods. They kill and maim people, destroy houses, factories, schools, livestock and plantations. They form so randomly and quickly and hit so fast that they can’t be forecast using existing machineries and communications systems.
Arjumand Habib, Deputy Director of Bangladesh’s Department of Meteorology