Many regions in India suffer from moist heatwaves during the southwest monsoon (SWM) season but the phenomenon is not well understood. A new research paper published in the journal Climate Dynamics showed that timings and locations of the moist heatwaves are controlled by the active and break periods of the SWM and can be predicted weeks in advance for better adaptation by people, especially to the adverse health impacts of the events.
Moist heatwaves are triggered by a combined impact of high temperatures and elevated humidity levels in the atmosphere during the SWM and sometimes even before that. “When air is already humid, sweat cannot evaporate from skin, leaving the body unable to cool itself. This can lead to heat exhaustion and fatal heatstroke within hours,” according to a press statement from the University of Reading, whose scientists published the study.
“This is because high humidity impairs evaporation, limiting the human body’s ability to dissipate metabolic heat and regulate core temperature. In such circumstances, the physiological strain on the human body intensifies, exacerbating the risks of cardiovascular and respiratory illnesses. A thermoregulatory failure can lead to hyperthermia, heat exhaustion, and fatal heatstroke in extreme cases,” wrote the authors of the study.
During such moist heatwaves humidity plays a much more significant role, different from dry heatwaves of the summer season when high temperatures mostly due to high pressure areas in the atmosphere are the trigger for heat events that can lead to deaths.
“We often find people being more aware of dry heatwaves in India given the scorching summer season, but moist heat remains less known and is therefore more dangerous. Outdoor public gatherings are notorious for causing heat exhaustion,” said Akshay Deoras, the lead author of the paper and research scientist at the University of Reading in the statement.
The best way to measure the combined effect of humidity and temperature is the ‘wet bulb temperature’. Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at constant pressure. This is a process happening on human skin when people sweat. The globally accepted level for wet bulb temperature that forms the limit of human survivability is 35°C. But these limits are being revised with latest research.
A research paper published in the journal Nature Communications in November 2023 found that wet bulb limits of human survivability vary widely with recent values between 25.8°C to 34.1°C for younger adults and 21.9°C to 33.7°C for older adults. “Notably, older female adults exhibit the lowest tolerance, with survivability thresholds in dry conditions estimated to be up to 7°C to 13°C below the conventional 35°C benchmark,” according to the study.
Deoras and his colleagues studied weather data from 1940 to 2023 to determine that depending on whether the SWM is active or in a break phase, the regions where moist heatwaves could occur changes from one part of the country to another.
Generally, when a break in the SWM season occurs the main monsoon trough or an extended low-pressure region that brings rain, it moves north from its normal position over the Indo-Gangetic plains to the foothills of the Himalayas.
This reduces the rainfall over central India and much of south India. The rainfall over the Himalayan foothills and Northeast of the country increases during such break phases. When the monsoon trough is active over northern plains of the country, the SWM is in active phase.
In an earlier study, scientists from University of Reading and United Kingdom’s Met Office had found that there were 188 monsoon break periods and 261 active phases between 1940 and 2023. They also found that the occurrence of monsoon breaks was influenced by the flow of dry desert winds from the west.
In the current study, the researchers found that when the SWM is in active phase and the northern plains and central India are receiving rainfall, the same regions face the risk of moist heatwaves. During the break phases, the risk shifts towards southern and eastern parts of the country.
“This happens because northern India is normally dry, so monsoon moisture pushes humidity to dangerous levels. In the south and east, humidity is already high, so dry spells and clear sky conditions trap heat instead,” according to the press release.
The research paper also informed that the moist heatwaves are connected to a large-scale weather pattern that can be forecast two to four weeks in advance. “During one specific phase of this large-scale weather pattern, heatwave occurrence over northern India rises by 125% above normal,” according to the statement.
The large-scale pattern is known as the Boreal Summer Intraseasonal Oscillation which is a 30-90 day pattern of the movement of active convection or stormy weather from the Indian Ocean to the western Pacific Ocean between June and September.
“Advance warning of a moist heatwave could allow organisers of public events, such as cricket matches and other large gatherings, to reschedule activities, extend drinks breaks, and strengthen on-site medical provision to protect both participants and spectators,” said Deoras.
“Advance warnings could also allow hospitals to increase staffing before moist heat-related admissions rise, enable city authorities to open cooling centres and adjust school hours, and help power grid operators manage infrastructure strain,” he added.