India’s deadliest heat may no longer come from crossing 45°C alone. As humidity rises and summers grow harsher, the IMD is changing how heatwaves are defined — a shift that could save thousands of lives.
India is facing an exceptionally punishing summer. With an El Niño event threatening to bring below-normal rainfall and warmer temperatures through July, the India Meteorological Department (IMD) has announced a major upgrade to its heatwave warning system.
For decades, heatwaves in India were measured using fixed temperature limits. Now, the IMD is introducing a percentile-based threshold system — a historic shift aimed at tracking extreme weather more accurately, especially the suffocating humid heat experienced across India’s coastal regions.
The Big Problem with Fixed Numbers
Under the old framework, a heatwave was declared in the plains when temperatures reached 45°C or rose 4.5°C to 6.4°C above the seasonal average. In coastal regions, the fixed benchmark stood at 37°C.
While these thresholds worked reasonably well for dry northern and western regions such as Rajasthan or Delhi, they often failed to reflect the dangerous reality of humid coastal states. In places like Kerala or coastal Karnataka, temperatures may not cross 44°C, yet intense humidity can make conditions physically unbearable and medically dangerous.
High humidity prevents sweat from evaporating efficiently, trapping heat inside the body. Because the earlier system focused largely on air temperature alone, many dangerous humid-heat days went officially unrecognised.
What Is the Percentile-Based System?
The new framework issues heat alerts when temperatures cross the 95th percentile of historical weather data for a specific location.
In simple terms, the 95th percentile refers to temperatures recorded only during the hottest 5% of days in that region’s weather history.
This means warnings will now be tailored to local climate realities instead of relying on a single national temperature benchmark. According to IMD Director General M. Mohapatra, forecasters will no longer wait for temperatures to rise 5°C above normal levels. If local weather models indicate that a region is approaching its historical extreme range, heat alerts can be issued much earlier.
Comparing the Two Systems
Feature
Old Framework
New Percentile Framework
Primary Metric
Fixed temperatures such as 45°C for plains
Historical 95th percentile based on local climate data
Coastal Inclusion
Often missed humid heat conditions
Better captures humidity-driven heat stress
Main Focus
Absolute temperature rise
Localised extreme weather frequency
Warning Timing
Triggered after large deviations
Allows earlier and more region-specific alerts
The Hidden Danger of Wet-Bulb Temperatures
A major scientific reason behind this shift is growing concern over wet-bulb temperature — a measure that combines heat and humidity to determine how effectively the human body can cool itself.
When humidity becomes extremely high, sweat stops evaporating properly. Once this natural cooling process fails, the body struggles to regulate internal temperature, increasing the risk of heat exhaustion, organ stress, and fatal heatstroke.
For years, scientists believed the upper survival threshold for humans was a wet-bulb temperature of 35°C. However, newer research conducted by Harvard University in collaboration with India’s Union Environment Ministry suggests the danger zone may begin much earlier — closer to 31°C.
That finding is especially alarming for densely populated and humid regions across South Asia.
Why the Timing Matters
The IMD’s policy shift comes at a critical moment. The World Meteorological Organization (WMO) has warned of a strong likelihood that El Niño conditions will intensify between June and August, potentially pushing global temperatures to new highs while weakening or delaying monsoon rainfall across parts of India.
Climate scientists and disaster-management experts have broadly welcomed the IMD’s updated approach. By using local historical extremes instead of rigid national temperature thresholds, authorities can identify dangerous heat conditions earlier and activate emergency heat action plans more effectively.
As Indian summers become longer, hotter, and increasingly humid, the country’s battle against heatwaves is no longer just about crossing 45°C. It is about understanding how heat is experienced on the ground — and how quickly it can turn deadly.
India is facing an exceptionally punishing summer. With an El Niño event threatening to bring below-normal rainfall and warmer temperatures through July, the India Meteorological Department (IMD) has announced a major upgrade to its heatwave warning system.
For decades, heatwaves in India were measured using fixed temperature limits. Now, the IMD is introducing a percentile-based threshold system — a historic shift aimed at tracking extreme weather more accurately, especially the suffocating humid heat experienced across India’s coastal regions.
The Big Problem with Fixed Numbers
Under the old framework, a heatwave was declared in the plains when temperatures reached 45°C or rose 4.5°C to 6.4°C above the seasonal average. In coastal regions, the fixed benchmark stood at 37°C.
While these thresholds worked reasonably well for dry northern and western regions such as Rajasthan or Delhi, they often failed to reflect the dangerous reality of humid coastal states. In places like Kerala or coastal Karnataka, temperatures may not cross 44°C, yet intense humidity can make conditions physically unbearable and medically dangerous.
High humidity prevents sweat from evaporating efficiently, trapping heat inside the body. Because the earlier system focused largely on air temperature alone, many dangerous humid-heat days went officially unrecognised.
What Is the Percentile-Based System?
The new framework issues heat alerts when temperatures cross the 95th percentile of historical weather data for a specific location.
In simple terms, the 95th percentile refers to temperatures recorded only during the hottest 5% of days in that region’s weather history.
This means warnings will now be tailored to local climate realities instead of relying on a single national temperature benchmark. According to IMD Director General M. Mohapatra, forecasters will no longer wait for temperatures to rise 5°C above normal levels. If local weather models indicate that a region is approaching its historical extreme range, heat alerts can be issued much earlier.
Comparing the Two Systems
|
Feature |
Old Framework |
New Percentile Framework |
|
Primary Metric |
Fixed temperatures such as 45°C for plains |
Historical 95th percentile based on local climate data |
|
Coastal Inclusion |
Often missed humid heat conditions |
Better captures humidity-driven heat stress |
|
Main Focus |
Absolute temperature rise |
Localised extreme weather frequency |
|
Warning Timing |
Triggered after large deviations |
Allows earlier and more region-specific alerts |
The Hidden Danger of Wet-Bulb Temperatures
A major scientific reason behind this shift is growing concern over wet-bulb temperature — a measure that combines heat and humidity to determine how effectively the human body can cool itself.
When humidity becomes extremely high, sweat stops evaporating properly. Once this natural cooling process fails, the body struggles to regulate internal temperature, increasing the risk of heat exhaustion, organ stress, and fatal heatstroke.
For years, scientists believed the upper survival threshold for humans was a wet-bulb temperature of 35°C. However, newer research conducted by Harvard University in collaboration with India’s Union Environment Ministry suggests the danger zone may begin much earlier — closer to 31°C.
That finding is especially alarming for densely populated and humid regions across South Asia.
Why the Timing Matters
The IMD’s policy shift comes at a critical moment. The World Meteorological Organization (WMO) has warned of a strong likelihood that El Niño conditions will intensify between June and August, potentially pushing global temperatures to new highs while weakening or delaying monsoon rainfall across parts of India.
Climate scientists and disaster-management experts have broadly welcomed the IMD’s updated approach. By using local historical extremes instead of rigid national temperature thresholds, authorities can identify dangerous heat conditions earlier and activate emergency heat action plans more effectively.
As Indian summers become longer, hotter, and increasingly humid, the country’s battle against heatwaves is no longer just about crossing 45°C. It is about understanding how heat is experienced on the ground — and how quickly it can turn deadly.
