Every summer, as Central and Northwest India temperatures rise above 45°C, the India Meteorological Department (IMD) issues heatwave alerts. Assam, in the northeast, rarely appears on the national heat map, despite high temperatures recorded across the state.
Across Assam, several stations have recorded maximum temperatures above 37°C and, in some instances, reached or exceeded 40°C in recent years. In September 2023, Guwahati recorded 38.2°C, 6.4°C above normal, while Silchar recorded 39.4°C, 7.4°C above normal. On May 25, 2024, Guwahati recorded 40.1°C, at least 8°C above normal. Later that year, the city recorded 39.3°C in September, breaking its all-time temperature record for the month. More recently, in July, maximum temperatures in Guwahati, Dibrugarh, Tezpur and Dhubri were around 4-5°C above normal for which the IMD issued a rare yellow alert.
These recordings highlight a challenge Assam continues to face: temperatures that do not always meet India’s conventional heatwave criteria can still produce significant heat stress.
Urban heat assessments of the state’s three major cities between 2020 and 2025 by the Directorate of Town and Country Planning show that land-use change, shrinking water bodies, loss of greenery and unplanned development are altering the cities’ thermal landscapes. Guwahati’s heat islands are expanding into larger heat corridors. Dibrugarh is seeing intensifying heat in its densely built core, while Silchar is emerging as the fastest-warming urban centre, with a sharp rise in critical heat-risk zones.
Recent district-level precautionary advisories have included school closures or revised timings and restrictions on outdoor activities.
The Assam State Action Plan on Climate Change 2.0 (ASAPCC 2.0) records a consistent warming between 1990 and 2019. Annual average maximum temperature increased by 0.049°C per year, while annual average minimum temperature rose by 0.013°C per year; with increases recorded across all districts
However, by the letter of India’s official heatwave definition, the state rarely qualifies for a heatwave.
Under the IMD criteria, a heatwave is declared only when a station’s actual maximum temperature reaches 40°C in the plains (37°C on the coast, 30°C in the hills) and departs from the long-term normal by at least 4.5°C to 6.4°C for two consecutive days or hits 45°C outright, which triggers an automatic declaration; 47°C or above constitutes a severe heatwave.
“This isn’t a contradiction; it’s a definitional blind spot,” said Partha Jyoti Das, environmental scientist and Professor of Practice at Gauhati University.
Since IMD had not declared heat waves in Assam, the state had not considered heat as a risk. Experts flag this as perhaps one possible reason Assam, so far, has no Heat Action Plan (HAP), while more than 250 cities and districts in 23 heat-prone states have operational plans.
High humidity not recorded
The IMD heatwave criteria use maximum air temperature and its departure from normal. Relative humidity is not part of the criteria for declaring a heatwave.
In Assam, however, the lower atmosphere generally has high relative humidity during the pre-monsoon months of March, April and May, and also the monsoon months of June to September, explained Das. “During the monsoon, relative humidity in the state can range between 75% and 95%. Because of this high humidity, people in Assam can experience greater heat stress at the same air temperature than those living in drier climates. This is reflected in the apparent or feels-like temperature,” he added.
On May 25, 2024, Guwahati recorded a maximum temperature of 40.1°C, with archived observations showing relative humidity ranging between 40% and 100% through the day and the apparent, or ‘feels-like’, temperature peaking at around 52°C. On September 23, 2024, when Guwahati recorded its highest September temperature, relative humidity ranged between 57% and 94%, while the apparent temperature reached around 53°C. Similarly, on July 4, 2026, Guwahati recorded a maximum temperature of 36°C, while relative humidity ranged between 60% and 100%. The apparent temperature went up to around 52°C.
Together, these episodes illustrate how high humidity can substantially amplify experienced heat.
The IMD separately calculates a Heat Index for general guidance on apparent or feels-like temperature, taking into account the combined effect of air temperature and humidity, and the potential for human discomfort. However, the Heat Index is not incorporated into the criteria used to declare a heatwave.
Accounting for humidity in heatwave thresholds
Alongside the India Meteorological Department, the Council on Energy, Environment and Water’s (CEEW) 2025 district-level heat risk report notes that high humidity can worsen heat stress by slowing sweat evaporation and making it harder for the body to release heat.
A 2023 review of 37 Heat Action Plans found that 67% defined heat hazards solely through daytime ambient-temperature extremes, without accounting for humidity or warm nights.
For a humid state such as Assam, this raises the question of whether temperature-only thresholds adequately capture the heat stress people actually experience.
Das argues that wet-bulb temperature (WBT) could be more relevant to understanding physiological heat stress in Assam than dry-bulb temperature (DBT), which is the conventional measure of air temperature.
Wet-bulb temperature measures the lowest temperature achievable through evaporative cooling and, therefore, indicates how effectively evaporation, including sweat from the human body, can provide cooling under given temperature and humidity conditions.
“Standard dry-bulb temperatures in Assam rarely cross the 40°C threshold,” Das said. “However, temperatures of around 35°C can occur alongside relative humidity exceeding 80%. Under such conditions, a DBT of 35°C can correspond to a wet-bulb temperature of around 31-32°C. The combination of high temperature and humidity can therefore create significant physiological heat stress even when the conventional heatwave threshold is not crossed.”
He added that although WBT is normally lower than DBT, a high wet-bulb temperature means evaporation is becoming more difficult.
A wet-bulb temperature of 35°C has long been cited as a theoretical upper limit for human tolerance to sustained heat. But research has shown that the physiological limits of heat tolerance can be reached at substantially lower wet-bulb temperatures, depending on environmental conditions and the individual.
An October 2026 report by IMD scientists adds evidence that high humidity intensifies thermal discomfort in Guwahati. It analysed the city’s wet-bulb temperature, relative humidity, maximum and minimum temperatures, and rainfall, comparing 2022–2024 with the 1991–2020 climatological period. The calculated Heat Index values exceeded 40°C from May to October during 2022–2024, peaking at 55.5°C in September. July, August and September fell within the authors’ “Extreme Danger” category, associated with a high likelihood of heatstroke under prolonged exposure or strenuous activity; no month fell within this category in their corresponding 1991–2020 analysis. Although the study does not propose changes to heatwave declaration criteria, its findings reinforce concerns that air temperature alone may inadequately convey the humid-heat burden, particularly during the monsoon.
Another 2026 study further underscores the importance of humidity in assessing heat stress. Using WBT, the researchers found that human-induced climate change has already substantially increased exposure to heat-stress days, with the largest increases occurring in tropical regions.
The study found that children under the age of 10 on average, experienced nine to 10 additional heat-stress days each year, attributable to anthropogenic climate change, compared with people aged 60 to 69. South Asia, Southeast Asia and West Africa recorded the highest numbers of children exposed to dangerous heat stress. The study estimated that up to 560 million children, around 43% of children under the age of 10 globally, experience at least one additional month of heat stress every year due to human-induced climate change.
Madhulika Jonathan, UNICEF Chief of Field Office for Assam and Northeast States, said that children are physiologically less able to regulate body temperature than adults.
“Heat exposure affects them through several channels at once, including health, nutrition, water access, school attendance and learning,” she said. “In Assam, this vulnerability exists alongside recurring floods and other hazards, with nearly half of the state’s districts among India’s most climate-vulnerable. Heat is now emerging as an additional risk.”
Jonathan added that while existing definitions and thresholds may not always classify these conditions as heatwaves, extreme heat is increasingly affecting lives, livelihoods and overall well-being.
Das pointed to another dimension of this risk. “We often see situations where flood-affected people, whether they are staying in their own homes or in relief camps, are simultaneously exposed to very hot days and nights, often without adequate indoor conditions to cope with the heat.”
He added that it is important to promote awareness and measurement of wet-bulb temperature among the public. “For humid states such as Assam, IMD should estimate and publicly disseminate heat indices that account for humidity and ‘feels-like’ temperatures, alongside conventional air-temperature readings,” Das said. “This could help people understand that seemingly normal air temperatures may still pose a high risk of heat stress when humidity is taken into account.”
Building Assam’s heat response
UNICEF, the Assam State Disaster Management Authority (ASDMA), Assam Climate Change Management Society (ACCMS), National Programme on Climate Change and Human Health (NPCCHH) under the National Health Mission (NHM), and technical and civil society organisations are exploring the establishment of a government-anchored Heat Action Alliance.
The proposed alliance aims to bring together evidence-informed and experience-driven action through coordination, knowledge sharing and collective planning.<
“A state-level Heat Action Plan is necessary, but may not be sufficient because heat risk is not uniform across the state,” Jonathan said. “Its impacts depend not only on heat exposure, but also on underlying vulnerabilities, including the proportion of young children, pregnant women and older persons, the prevalence of outdoor and informal workers, and access to health facilities and safe drinking water.”
The proposed alliance would emphasise enabling districts and urban local bodies to develop localised heat plans, vulnerability assessments and early-warning communication, rather than relying on a uniform protocol. “The approach would prioritise health-system preparedness, school and anganwadi safety, risk communication, and the inclusion of children and other vulnerable groups in vulnerability assessments,” she added.
Rizwan Uz Zaman, Technical Consultant at ACCMS, said that findings from the Assam State Action Plan on Climate Change have underscored the need for a Heat Action Plan.
“It will be developed through stakeholder consultation, including an assessment of whether different temperature thresholds are appropriate for Assam, given its distinct geo-physiographic and climatic conditions.”
Zaman said preparedness should also account for variations within the state. “Assam has six agro-climatic zones with differing climatic conditions, so a multi-criteria index could be developed to assess heat risk across these zones,” he added. “Heat action planning could then be tailored according to the risk profile and specific conditions of each zone.”
Jonathan said that a Heat Action Plan enables a shift from post-event relief towards preparedness, anticipatory action and mitigation investment, while providing an institutional framework with clearly defined responsibilities and triggers. “However, eligibility for relief assistance is governed by the State and National Disaster Response Funds (SDRF and NDRF) norms and does not depend on a state having a notified Heat Action Plan,” she added.
Heatwaves have now been added to India’s national list of notified natural calamities, following a recommendation of the Sixteenth Finance Commission. The policy shift creates a formal financing route for heat-related relief, while heat-mitigation projects can also be supported through the corresponding state and national disaster mitigation funds.
Zaman said relief assistance could be provided through the disaster-management framework when heatwave conditions arise. He acknowledged, however, that accessing such relief would first require the conditions to be recognised under the applicable norms, a potential limitation in Assam.
“Even in such situations, the disaster-management authorities issue advisories through district administrations during periods of excessive heat,” he added. “Districts have taken precautionary measures such as changing school timings or closing schools to reduce exposure,” he pointed.
This article is republished from Mongabay under a Creative Commons license. Read the original article here.