Arya News - As climate change continues to disrupt historical weather patterns, rainfall is increasingly surpassing traditional benchmarks, leaving Bangladesh`s half-century-old meteorological terminology struggling to keep pace.
DHAKA – In an era of escalating climate extremes, the definition of a “rainy day” is no longer universal. What constitutes a torrential downpour in a typically dry region may be an unremarkable afternoon in a monsoon climate like Bangladesh.
But as climate change continues to disrupt historical weather patterns, rainfall is increasingly surpassing traditional benchmarks, leaving Bangladesh’s half-century-old meteorological terminology struggling to keep pace.
When the Bangladesh Meteorological Department (BMD) issued a forecast of “heavy to very heavy” rainfall in its regular bulletin on July 15, the terminology carried a precise technical meaning. Under the department’s classification system, which has remained unchanged since the country’s independence, daily rainfall is divided into five categories.
Rainfall between 1 and 10 millimetres is classified as light, 11 to 22 millimetres as moderate, 23 to 43 millimetres as moderately heavy, and 44 to 88 millimetres as heavy. Any rainfall exceeding 88 millimetres in a day is categorised simply as very heavy.
However, climate change is increasingly rendering those thresholds inadequate.
A resident or urban planner preparing for “very heavy” rainfall of 89 millimetres may be entirely unprepared for a 200-millimetre deluge, such as those experienced in parts of the country last week.
Bangladesh’s historical records underscore the growing disparity.
Sandwip recorded 501 millimetres of rainfall in a single day in 2001, while Dhaka recorded 341 millimetres in 2004 and 206 millimetres as recently as October 1, 2025. When 200 millimetres of rain is grouped under the same warning label as 89 millimetres, the public and policymakers lose the ability to gauge the true level of risk.
The limitations of the existing terminology have had serious consequences.
A week before the recent devastating landslides in the Chattogram region, the BMD forecast consecutive days of heavy to very heavy rainfall and explicitly warned of landslide risks.
However, because the terminology lacked the urgency conveyed by more modern, impact-based warning systems, the forecast failed to trigger an adequate emergency response from the relevant government agencies.
More than 50 people were killed across Chattogram, while Dhaka experienced severe waterlogging and widespread disruption.
By comparison, the India Meteorological Department (IMD) issued a “Red Alert” for Mumbai on July 6, urging residents to avoid non-essential travel.
Such impact-based forecasting, which combines weather predictions with explicit public guidance, has become standard practice in many countries but is still evolving in Bangladesh.
Because the meaning of “heavy” rainfall varies from one climate to another, meteorological agencies around the world regularly revise their classification systems to reflect changing weather patterns.
India’s IMD uses a more detailed rainfall scale. It classifies rainfall of up to 15 millimetres as light, 16 to 65 millimetres as moderate, and 66 to 111 millimetres as heavy. Rainfall between 115 and 204 millimetres is classified as very heavy, while rainfall of 205 millimetres or more is categorised as extremely heavy.
China follows a similarly expanded classification, defining rainfall between 100 and 249 millimetres as heavy torrential rain and reserving the category of extremely torrential rain for rainfall exceeding 250 millimetres.
Several Western countries also assess rainfall intensity on an hourly rather than daily basis.
In the United Kingdom, 8 millimetres of rainfall in an hour is considered very heavy, while the United States classifies rainfall of 7.6 millimetres per hour as heavy.
Recognising the shortcomings of the current system, the BMD has begun reviewing its rainfall classifications.
Meteorologists said internal discussions are under way to substantially revise the thresholds. One proposal under consideration would raise the lower limit for very heavy rainfall from 89 millimetres to 150 millimetres, requiring a broader recalibration of the entire rainfall classification system.
The BMD has also introduced several technological improvements.
Its newly launched website features a Common Alert Protocol-based, colour-coded risk matrix, replacing the previous platform. It also provides upazila-specific forecasts and sector-specific warnings for aviation, agriculture and maritime operations.
According to the BMD, the upgraded forecasting system is now capable of predicting rainfall and heatwaves five to seven days in advance.
Momenul Islam, acting director of the BMD, said coordinated action by government agencies based on the Met Office’s early warnings could significantly reduce disaster-related losses.
He said the department’s forecasting capability had improved substantially in recent years and that it can now provide rainfall forecasts five days in advance with about 95 percent accuracy.
Referring to the recent spell of torrential rain, he said the Met Office had been warning of heavy rainfall since July 4 and issued a landslide warning on July 5.
“We saw landslides in Cox’s Bazar on July 6. We warned about very heavy rainfall and also about landslides,” he said.
Regarding the current definitions of heavy and very heavy rainfall, Momenul said the department was considering revising the rainfall thresholds.
“But it needs discussion with the stakeholders before implementation to ensure proper communication,” he said.
The bigger challenge, however, lies not in forecasting but in translating forecasts into coordinated action across government agencies.
Bangladesh has become a global leader in disaster risk management, reducing cyclone-related deaths from hundreds of thousands to single digits over the past five decades. That success, experts say, has been possible because cyclone forecasts issued by the BMD are followed by swift, coordinated and well-communicated government action.
Rashed Chowdhury, an eminent climatologist, said Bangladesh already benefits from satellite technology and support from its international partners.
However, he said the country must invest further in strengthening its forecasting capacity by expanding the observation network, upgrading computing infrastructure, improving data assimilation, enhancing high-resolution weather models, strengthening impact-based forecasting, training forecasters, and ensuring timely communication of warnings to communities and policymakers.
He noted that operational forecasting currently focuses primarily on the next three to seven days.
While this remains essential, he said Bangladesh also needs to strengthen seasonal and interannual forecasting through regular, year-round monitoring of the El Niño-Southern Oscillation (ENSO) and other large-scale climate drivers on at least a monthly basis, rather than only when hazards appear imminent.
Such climate monitoring, he said, would provide the foundation for early preparedness in agriculture, water resources, public health and disaster risk reduction.
“Currently, there is limited institutional initiative and investment in this area,” he said.