The remnants of Tropical Storm Nora from the Pacific, which had lasted for two days in the South China Sea, moved west over the Malay Peninsula on November 5. The remnants of this system contributed to the development of a new depression in the central Bay of Bengal on the morning of November 8. The depression intensified as it moved slowly northward, and the India Meteorological Department upgraded it to a cyclonic storm the next day. No country in the region had ever named tropical cyclones during this time, so no new identity was given. The storm became nearly stationary that evening near 14.5° N, 87° E, but began to accelerate to the north on November 10.
The storm further intensified into a severe cyclonic storm on November 11 and began to turn towards the northeast as it approached the head of the bay. It developed a clear eye, and reached its peak later that day with 3-minute sustained winds of 185 km/h (115 mph) and a central pressure of 966 hPa. The cyclone made landfall on the East Pakistan coastline during the evening of November 12, around the same time as the local high tide. Once over land, the system began to weaken; it degraded to a cyclonic storm on November 13 when it was about 100 km (62 mi) south-southeast of Agartala. The storm then rapidly weakened into a remnant low over southern Assam that evening.
The Indian government received many ship reports from the Bay of Bengal that were giving meteorological information on the cyclone, but as Indo-Pakistani relations were generally hostile, the information was not passed on to the Pakistani government. A large part of the population were reportedly taken by surprise by the storm. There were indications that the storm warning system that existed in East Pakistan was not used properly, which may have cost tens of thousands of lives. The Pakistan Meteorological Department issued a report calling for "danger preparedness" in the coastal regions in danger during the day on November 12. As the storm neared the coast, a "great danger signal" was broadcast on Pakistan Radio. Survivors later said that this meant little to them, but that they had recognised a No. 1 warning signal as representing the greatest possible threat.
Following two previously destructive cyclones in October 1960 which killed at least 16,000 people in East Pakistan, the Pakistani government contacted the American government for assistance in developing a system to avert future disasters. Gordon Dunn, the director of the National Hurricane Center at the time, carried out a detailed study and submitted his report in 1961. However, the government did not carry out all of the recommendations Dunn had listed.
Although the North Indian Ocean is the least active of the tropical cyclone basins, the coast of the Bay of Bengal is particularly vulnerable to the effects of tropical cyclones. While it is unsure how many people were killed, it is estimated to be 300,000 to 500,000 people in total. The 1970 Bhola cyclone was not the most powerful of these, however; the 1991 Bangladesh cyclone was significantly stronger when it made landfall in the same general area as a Category 5-equivalent cyclone with 260 km/h (160 mph) winds.
The 1970 cyclone is nonetheless the deadliest tropical cyclone on record and is one of the deadliest natural disasters in recent history. The exact death toll will never be known, but it is estimated that between 300,000 and 500,000 people lost their lives. A comparable number of people died as a result of the 1976 Tangshan earthquake and the 2004 Indian Ocean earthquake, but because of uncertainty in the number of deaths in all three disasters, it may never be known which one was the deadliest.
The meteorological station in Chittagong, 95 km (59 mi) to the east of where the storm made landfall, recorded winds of 144 km/h (89 mph) before its anemometer was blown off at about 2200 UTC. A ship anchored in the port in the same area recorded a peak gust of 222 km/h (138 mph) about 45 minutes later. As the storm made landfall, it caused a 10-metre (33 ft) high storm surge at the Ganges Delta. In the port at Chittagong, the storm tide peaked at about 4 m (13 ft) above the average sea level, 1.2 m (3.9 ft) of which was the storm surge.
Pakistani radio reported that there were no survivors on the 13 islands near Chittagong. A flight over the area showed the devastation was complete throughout the southern half of Bhola Island, and the rice crops of Bhola Island, Hatia Island and the nearby mainland coastline were destroyed. Several seagoing vessels in the ports of Chittagong and Mongla were reported damaged, and the airports at Chittagong and Cox's Bazar were under 1 m (3.3 ft) of water for several hours.
Over 3.6 million people were directly affected by the cyclone, and the total damage from the storm was estimated at $86.4 million (1970 USD, $450 Million 2006 USD). The survivors claimed that approximately 85% of homes in the area were destroyed or severely damaged, with the greatest destruction occurring along the coast. Ninety percent of marine fishermen in the region suffered heavy losses, including the destruction of 9,000 offshore fishing boats. Of the 77,000 onshore fishermen, 46,000 were killed by the cyclone, and 40% of the survivors were affected severely. In total, approximately 65% of the fishing capacity of the coastal region was destroyed by the storm, in a region where about 80% of the protein consumed comes from fish. Agricultural damage was similarly severe with the loss of $63 million worth of crops and 280,000 cattle. Three months after the storm, 75% of the population was receiving food from relief workers, and over 150,000 relied upon aid for half of their food.
The cyclone brought widespread rain to the Andaman and Nicobar Islands, with very heavy rain falling in places on November 8 and 9. Port Blair recorded 130 mm (5.1 in) of rain on November 8, and there were a number of floods on the islands. The MV Mahajagmitra, a 5,500-ton freighter en route from Calcutta to Kuwait, was sunk by the storm on November 12, with the loss of all 50 people on board. The ship sent out a distress signal and reported experiencing hurricane-force winds before it sank. There was also widespread rain in West Bengal and southern Assam. The rain caused damage to housing and crops in both Indian states, with the worst damage occurring in the southernmost districts.
Two medical relief surveys were carried out by the Pakistan-SEATO Cholera Research Laboratory: the first in November and the second in February and March. The purpose of the first survey was to establish the immediate medical needs in the affected regions, and the second, more detailed, survey was designed as the basis for long-term relief and recovery planning. In the second survey, approximately 1.4% of the area's population was studied.
The first survey concluded that the surface water in most of the affected regions had a comparable salt content to that drawn from wells, except in Sudharam, where the water was almost undrinkable with a salt content of up to 0.5%. The mortality was estimated at 14.2%—equivalent to a death toll of 240,000. Cyclone-related morbidity was generally restricted to minor injuries, but a phenomenon dubbed "cyclone syndrome" was observed. This consisted of severe abrasions on the limbs and chest caused by survivors clinging to trees to withstand the storm surge. Initially, there were fears of an outbreak of cholera and typhoid fever in the weeks following the storm, but the survey found no evidence of an epidemic of cholera, smallpox or any other disease in the region affected by the storm.
The totals from the second survey were likely a considerable underestimate as several groups were not included. The 100,000 migrant workers who were collecting the rice harvest, families who were completely wiped out by the storm and those who had migrated out of the region in the three months were not included, and by excluding these groups, the risk of hearsay and exaggeration was reduced. The survey concluded that the overall death toll was, at minimum, 224,000. The worst effects were felt in Tazumuddin, where the mortality was 46.3%, corresponding to approximately 77,000 deaths in that Thana alone. The mean mortality throughout the affected region was 16.5%.
The results showed that the highest survival rate was for adult males aged 15–49, while more than half the deaths were children under ten, who only formed a third of the pre-cyclone population. This suggests that the young, old and sick were at highest risk of perishing in the cyclone and its surge. In the months after the storm, the mortality of the middle-aged was lower in the cyclone area than in the control region, near Dhaka. This reflected the storm's toll on the less healthy individuals.
The day after the storm struck the coast, three Pakistani gunboats and a hospital ship carrying medical personnel and supplies left Chittagong for the islands of Hatia, Sandwip and Kutubdia. Teams from the Pakistani army reached many of the stricken areas in the two days following the landfall of the cyclone. Pakistani President Yahya Khan returned from a state visit to China and overflew the disaster area on November 16. The president ordered "no effort to be spared" to relieve the victims. He also ordered that all flags should be flown at half-mast and announced a day of national mourning on November 21, a week after the cyclone struck land.
In the ten days following the cyclone, one military transport aircraft and three crop-dusting aircraft were assigned to relief work by the Pakistani government. The Pakistani government said it was unable to transfer military helicopters from West Pakistan as the Indian government did not grant clearance to cross the intervening Indian territory, a charge the Indian government denied. By November 24, the Pakistani government had allocated a further $116 million to finance relief operations in the disaster area. Yahya Khan arrived in Dhaka to take charge of the relief operations on November 24. The governor of East Pakistan, Vice Admiral S. M. Ahsan, denied charges that the armed forces had not acted quickly enough and said supplies were reaching all parts of the disaster area except for some small pockets.
A week after the cyclone's landfall, President Khan conceded that his government had made "slips" and "mistakes" in its handling of the relief efforts. He said there was a lack of understanding of the magnitude of the disaster. He also said that the general election slated for December 7 would take place on time, although eight or nine of the worst affected districts might experience delays, denying rumours that the election would be postponed.
As the conflict between East and West Pakistan developed in March, the Dhaka offices of the two government organisations directly involved in relief efforts were closed for at least two weeks, first by a general strike and then by a ban on government work in East Pakistan by the Awami League. Relief work continued in the field, but the long-term planning was curtailed.
Political leaders in East Pakistan were deeply critical of the central government's initial response to the disaster. A statement released by eleven political leaders in East Pakistan ten days after the cyclone hit charged the government with 'gross neglect, callous indifference and utter indifference'. They also accused the president of playing down the news coverage. On November 19, students held a march in Dhaka in protest of the speed of the government response, and Maulana Abdul Hamid Khan Bhashani addressed a rally of 50,000 people on November 24, when he accused the president of inefficiency and demanded his resignation. The president's political opponents accused him of bungling the efforts and some demanded his resignation.
The Pakistan Red Crescent began to operate independently of the government as the result of a dispute that arose after the Red Crescent took possession of 20 rafts donated by the British Red Cross. A pesticide company had to wait two days before it received permission for two of its crop dusters, which were already in the country, to carry out supply drops in the affected regions. The Pakistani government only deployed a single helicopter to relief operations, with Yahya Khan later stating that there was no point deploying any helicopters from West Pakistan as they were unable to carry supplies.
A reporter for the Pakistan Observer spent a week in the worst hit areas in early January and saw none of the tents supplied by relief agencies being used to house survivors and commented that the grants for building new houses were insufficient. The Pakistan Observer regularly carried front page stories with headlines like "No Relief Coordination", whilst publishing government statements saying "Relief operations are going smoothly." In January, the coldest period of the year in East Pakistan, the National Relief and Rehabilitation Committee, headed by the editor of Ittefaq, said thousands of survivors from the storm were "passing their days under [the] open sky". A spokesman said families who were made homeless by the cyclone were receiving up to 250 rupees (US$55 in 1971 dollars; $325 in 2017) to rebuild, but that resources were scarce and he feared the survivors would "eat the cash".
The Awami League, headed by Sheikh Mujibur Rahman, swept to a landslide victory in the national elections in December 1970, in part because of dissatisfaction over failure of the relief efforts by the national government. The elections for nine national assembly and eighteen provincial assembly seats had to be postponed until January 18 as a result of the storm.
The government's handling of the relief efforts helped exacerbate the bitterness felt in East Pakistan, swelling the resistance movement there. Funds only slowly got through, and transport was slow in bringing supplies to the devastated regions. As tensions increased in March, foreign personnel evacuated because of fears of violence. The situation deteriorated further and developed into the Bangladesh Liberation War in March. This conflict widened into the Indo-Pakistani War of 1971 in December and concluded with the creation of Bangladesh. This was one of the first times that a natural event helped to trigger a civil war.
India became one of the first nations to offer aid to Pakistan, despite the generally poor relations between the two countries, and by the end of November had pledged $1.3 million (1970 USD, $6.9 million 2007 USD) of assistance for the relief efforts. The Pakistani government refused to allow the Indians to send supplies into East Pakistan by air, forcing them to be transported slowly by road instead. The Indian government also said that the Pakistanis refused an offer of military aircraft, helicopters and boats from West Bengal to assist in the relief operation.
US President Richard Nixon allocated a $10 million ($62 million in 2017) grant to provide food and other essential relief to the survivors of the storm, and the U.S. ambassador to Pakistan pledged that he would "assist the East Pakistan government in every way feasible." The American government also sent a number of blankets, tents and other supplies. Six helicopters, two helicopters at an aid mission in Nepal and four from the United States, were sent to East Pakistan. Some 200,000 tons of wheat were shipped from the United States to the stricken region. By the end of November, there were 38 helicopters operating in the disaster area, ten of which were British and ten American. The Americans had provided about 50 small boats and the British 70 for supply distribution.
CARE halted aid shipments to the country the week after the cyclone hit, because of unwillingness to let the Pakistani government handle distribution. However, by January, they had reached an agreement to construct 24,000 cement brick houses at a cost of about $1.2 million ($7 million in 2017). American concerns about delays by the Pakistani government in determining how the relief should be used meant that $7.5 million ($46 million in 2017) of relief granted by the US Congress had not been handed over in March. Much of the money was earmarked to be spent on constructing cyclone shelters and rebuilding housing. The American Peace Corps offered to send volunteers but were rebuffed by the Pakistani government.
A Royal Navy task force, centred on HMS Intrepid and HMS Triumph, left Singapore for the Bay of Bengal to assist with the relief efforts. They carried eight helicopters and eight landing craft, as well as rescue teams and supplies. Fifty soldiers and two helicopters were flown in ahead of the ships to survey the disaster area and bring relief work. The British task force arrived off the Pakistan coast on November 24, and the 650 troops aboard the ships immediately began using landing craft to deliver supplies to offshore islands. An appeal by the British Disasters Emergency Committee raised about £1.5 million (£21 million in 2017) for disaster relief in East Pakistan.
The Canadian government pledged $2 million of assistance. France and West Germany both sent helicopters and various supplies worth $1.3 million. Pope Paul VI announced that he would visit Dhaka during a visit to the Far East and urged people to pray for the victims of the disaster. The Vatican later contributed $100,000 to the relief efforts. By the start of 1971, four Soviet helicopters were still operating in the region transporting essential supplies to hard-hit areas. The Soviet aircraft, which had drawn criticism from Bengalis, replaced the British and American helicopters that had operated immediately after the cyclone.
The government of Singapore sent a military medical mission to East Pakistan which arrived at Chittagong on December 1, 1971. They were then deployed to Sandwip where they treated nearly 27,000 people and carried out a smallpox vaccination effort. The mission returned to Singapore on December 22, after bringing about $50,000 worth of medical supplies and 15 tons of food for the victims of the storm. The Japanese cabinet approved a total of $1.65 million of relief funds in December. The Japanese government had previously drawn criticism for only donating a small amount to relief work. The first shipment of Chinese supplies to East Pakistan was a planeload of 500,000 doses of cholera vaccine, which was not necessary as the country had adequate stocks. The Chinese government sent $1.2 million in cash to Pakistan. Mohammad Reza Pahlavi declared that the disaster was also an Iranian one and responded by sending two planeloads of supplies within a few days of the cyclone striking. Many smaller, poorer Asian nations sent nominal amounts of aid.
The United Nations donated $2.1 million in food and cash, whilst UNICEF began a drive to raise a further million. UNICEF helped to re-establish water supplies in the wake of the storm, repairing over 11,000 wells in the months following the storm. UN Secretary-General U Thant made appeals for aid for the victims of the cyclone and the civil war in August, in two separate relief programs. He said only about $4 million had been contributed towards immediate needs, well short of the target of $29.2 million. By the end of November, the League of Red Cross Societies had collected $3.5 million to supply aid to the victims of the disaster.
The World Bank estimated that it would cost $185 million to reconstruct the area devastated by the storm. The bank drew up a comprehensive recovery plan for the Pakistani government. The plan included restoring housing, water supplies and infrastructure to their pre-storm state. It was designed to combine with a much larger ongoing flood-control and development program. The Bank provided $25 million of credit to help rebuild the East Pakistan economy and to construct protective shelters in the region. This was the first time that the IDA had provided credit for reconstruction. By the start of December, nearly $40 million had been raised for the relief efforts by the governments of 41 countries, organisations and private groups.
In 1971, ex-Beatle George Harrison was inspired to organize The Concert for Bangladesh, in part from the 1970 Bhola Cyclone, and from the 1971 Bangladesh Genocide and Bangladesh Liberation War. Although it was the first benefit concert of its type, it was extremely successful in raising money, aid and awareness for the region's plight.
In December, the League of Red Cross Societies drafted a plan for immediate use should a comparable event to the cyclone hit other "disaster prone countries". A Red Cross official stated some of the relief workers sent to East Pakistan were poorly trained, and the organisation would compile a list of specialists. The UN General Assembly adopted a proposal to improve its ability to provide aid to disaster-stricken countries. In 1966, the Red Crescent had begun to support the development of a cyclone warning system, which developed into a Cyclone Preparedness Programme in 1972, today run by the Government of Bangladesh and the Bangladesh Red Crescent Society. The programme's objectives are to raise public awareness of the risks of cyclones and to provide training to emergency personnel in the coastal regions of Bangladesh.
In the 30 years after the 1970 cyclone, over 200 cyclone shelters were constructed in the coastal regions of Bangladesh. When the next destructive cyclone approached the country in 1991, volunteers from the Cyclone Preparedness Programme warned people of the cyclone two to three days before it struck land. Over 350,000 people fled their homes to shelters and other brick structures, whilst others sought high ground. While the 1991 cyclone killed over 138,000 people, this was significantly less than the 1970 storm, partly because of the warnings sent out by the Cyclone Preparedness Programme. However, the 1991 storm was significantly more destructive, causing 1.5 billion dollars in damage (2 billion inflation-adjusted) compared to the 1970 storm's 86.4 million dollars in damage.