A comparative analysis of accelerating humid and dry heat stress in India

Extreme humid heat stress poses distinct challenges to human health and productivity that cannot be mitigated solely by heat action plans designed for dry heat stress. This study investigates the trends in extreme heat stress, differentiating between dry and moist heat, in India from 1943 to 2022 using the high-resolution hourly ERA5 reanalysis data. The analysis utilizes the Heat Index (HI) as a key parameter to identify instances of extreme humid heat stress, characterized by HI values exceeding 41 degrees Celsius and relative humidity more than 50%. The findings indicate that certain eastern coastal regions in peninsular India experience extreme humid heat stress from May to June due to persistent high humidity levels. In addition, northwest, northcentral and inland eastern coastal regions encounter extreme dry heat stress preceding the monsoon season, followed by a transition to humid heat stress immediately after the onset of the monsoon. The results also show that there has been a significant increase ofsix times (on average) in the extreme humid heat stress hours per grid over the past 80 years compared to a threefold increase in dry heat stress. Our research underscores the need to shift from exclusively addressing dry heat stress to adopting a comprehensive approach that considers the impacts of humid heat stress.


Introduction
In the context of our changing climate, heat stress has emerged as a critical concern due to the unprecedented increase in temperatures [1][2][3].Heat stress occurs when the body core temperature, regulated by the process of thermo-homeostasis, exceeds the thermal comfort limits.Thermo-homeostasis involves a continuous exchange of heat between the human body and its surroundings.External factors like temperature, humidity, radiative heat, wind speed, and clothing, as well as internal metabolic processes, muscular activity, and the extent of adaptation, contribute significantly to determining heat stress [4,5].Hence the elderly, children, individuals with health conditions, and outdoor workers are the most vulnerable populations.Extreme heat stress has wideranging consequences including discomfort, heat-related illness, mortality, decreased labour productivity, increased energy utilization and economic losses [6][7][8][9].
Heat stress is categorised into dry heat stress and humid heat stress based on the level of ambient humidity.Dry heat stress occurs in conditions of extreme temperatures and low humidity.While it can lead to dehydration and other severe effects, proper hydration and heat action plans can address these issues.On the other hand, humid heat stress arises from the combination of high temperatures and high humidity resulting in reduced heat dissipation in the body and ultimately, posing severe health risks [10,11].The impact of high temperatures is intensified by humidity [12,13].Moreover, increasing dry-bulb temperatures contribute to a 7% rise in the air's capacity to hold water vapour for every 1 °C increase, as per the Clausius-Clapeyron relation.This implies a growing risk of humid heat stress amid ongoing climate change [12].However, traditional mitigation measures are inadequate to prevent humid heat stress.South Asian cities, particularly Bangladesh, India, and Pakistan, will continue to experience the highest levels of heat stress-related risks by the end of the century [14,15].India experiences both dry and humid heat stress because its climate varies significantly, ranging from arid desert regions to humid coastal areas.Extreme humid heat conditions prevail due to the high surface temperatures and high humidity levels from the presence of water bodies, vegetation, and/or irrigation [16][17][18].Existing efforts in managing heat stress-induced mortalities in India have been geared towards dry heat stress prevailing during the summer season [19].However, studies have found an increase in humid heat in India [1,[20][21][22][23].Managing humid heat scenarios presents greater challenges because of the difference in underlying thermoregulatory coping mechanisms [24].Currently, no efficient strategies exist beyond relocating to cooler areas (or air conditioning) to cope with humid heat conditions.Nonetheless, there is a critical need to broaden the scope to include fatalities arising from extreme humid heat stress incidents, even in the absence of extreme temperatures.As a result, it becomes imperative to distinguish and study both types of heat stress independently.
Heat stress is assessed primarily using heat metrics that consider the complex relationship between temperature and other factors, such as the individual and environmental elements.Previous studies have introduced approximately 170 heat stress metrics to assess the impact of extreme heat on humans [25,26].These are primarily categorised into three groups: (i) empirical indices developed through laboratory-based studies that consider environmental parameters, (ii) analytical indices derived from heat balance models, and (iii) direct indices based on a single-sensor measurement of climate variables [13,25,27].A few of these techniques have been widely adopted but many of them have limited practical implementation.Among these, only a handful consider humidity for climate impact studies, namely, Heat Index (HI) [11,20,[28][29][30], Discomfort Index (DI) [31,32], Universal Thermal Climate Index (UTCI) [2,33], Wet-Bulb Temperature (TWB) [1,23], Wet-Bulb Globe Temperature (WBGT) [9,34].The Pennsylvania State University's Heat Environmental Age Thresholds (PSU HEAT) project validated the impact of index-defined heat conditions on human health [35].They have found the Heat Index (HI) to be accurate in predicting heat stress compensability during light activities in a group of young, and healthy subjects.The HI exhibited a significant correlation with subjects' perception of temperature and humidity, as well as their body core temperature, establishing a link between the perception of the ambient environment and physiological responses in these conditions.When applied judiciously, these heat metrics enable a comprehensive understanding of heat stress patterns and contribute significantly to effective mitigation and adaptation strategies.
Recent studies have analysed heat metrics in India, using both mean and extreme heat metrics [11,16].Calculating the monthly mean is insufficient to capture heat stress exceeding the limits of human physiological tolerance.This is because such humid heat typically exists for only a couple of hours [23].Therefore, heat stress is not limited to heatwave events.In this study, we employ hourly Heat Index (HI) with clearly defined human threshold levels to investigate extreme heat stress, differentiating between dry and moist heat specifically in the Indian context.Our primary objective is to analyse the spatial and temporal patterns of dry and humid heat stress in India over the past 80 years using the reanalysis data from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) [36].Additionally, we compare the occurrence of extremely humid heat stress hours in 2022 in Bikaner and Kolkata.These regions exhibit distinct climatic conditions during the same season.Our findings emphasize the importance of distinguishing between humid and dry heat stress.

Study area
We examine the trends in the hourly occurrence of extreme heat stress during May-July for the past eight decades in India.The study area is characterized by a diverse range of climatic features due to its vast geographical expanse.It encompasses the snow-capped peaks of the Himalayas to the north, arid deserts in the west, tropical rainforests in the south, and a varied landscape in between.With a population of over 1.4 billion, India is one of the most vulnerable countries [37], making it an ideal region for studying the impacts of climate change, particularly concerning heat stress.The complex topography and high demographic density in India contribute to unique heat stress challenges and necessitate tailored adaptation strategies.Two regions are examined in detail for analysing Heat Stress in 2022: Kolkata and Bikaner.Kolkata is a coastal city in northeast peninsular India with year-round high humidity levels whereas Bikaner is a location known for experiencing frequent dry heat waves in Rajasthan (northwest India).

Climate data
We used hourly records of air and dewpoint temperature at 2 m at a spatial resolution of 0.25°× 0.25°for the years 1943 to 2022 for grids over India from the European Centre for Medium-Range Weather Forecast Reanalysis version 5 (ERA5) reanalysis dataset provided by the Copernicus Climate Data Store (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=form.last accessed on 07/09/23) [36].ERA5 is a state-of-the-art data archive characterising the climate on a global scale by merging modelgenerated data with observations from all over the world, providing an integrated and uniform dataset [38].
Several studies have systematically compared ERA5's performance with other datasets, consistently recognising it as a standout choice among reanalysis products for robust studies on hydrological processes and dynamics in India [39][40][41][42].While acknowledging the dataset's strengths, it's crucial to be mindful of certain limitations, such as the potential underestimation of extreme temperature trends [43] and heavy rainfall [44] in some parts of India.Many recent papers have used ERA5 datasets with continuous high-resolution data for heatwave studies [9,16,22,45,46].In this study, we have used 4650 grids lying within the Indian land boundary.

Heat Index (HI)
The 'Heat Index (HI)' also known as apparent temperature, is an energy balance strain index (empirical index) that considers both ambient temperature and relative humidity [47].It allows for determining the thermal comfort of a person undertaking minimal work under shade.Lans P. Rothfusz improved the findings by conducting multiple regression analyses and developed the widely used HI equation published by the U.S. National Weather Service (NWS) [48,49].The final equation and adjustments (equations ( 1 87 where HI is the Heat Index (°F), T is the 2m dry bulb temperature (°F), and RH is the relative humidity in %.When <  HI F 79 , another simplified formula (equation ( 2)) is used as below: HI is best suitable for temperatures more than 26.7 °C and more than 40% relative humidity and thus can be applied to the Indian subcontinent [8] National Oceanic and Atmospheric Administration (NOAA) has published a well-defined and widely used severity scale concerning indoor heat stress employing the NWS HI, affording policymakers a valuable instrument for informed decision-making and early warning systems (Supplementary material table S1).
We used 2 m air temperature and corresponding relative humidity to compute the hourly HI at 0.25-degree grid intervals over India from 1943 to 2022 using equation (1).As per the categorical threshold given by NWS (Supplementary material table S1), HI greater than 41 degrees Celsius (hereafter abbreviated as HI41) is under the danger and extremely dangerous category.It's important to note that these HI values assume that individuals are either in the shade or indoors.Hence direct exposure to sunlight will increase the risks.In this context, we define extreme dry heat stress hours as HI41 and extreme humid heat stress hours are characterized by an hourly relative humidity exceeding 50% in conjunction with HI41 conditions.We use this methodology to quantify the hourly occurrences of dry and humid stress from April to July for the last eight decades.

Total heat stress hours
The total number of hours of HI41 per month for the past 80 years (1943-2022) for the whole of India was calculated.We restrict the analysis to May, June, and July as these months show the highest occurrences of heat stress hours of HI41 (figure 1).For brevity, figures for other months are provided in the Supplementary material (figure S1).
Results illustrate that many parts of India experienced high heat stress in May and June.During the premonsoon month of May, the southeastern coastal areas experienced the highest number of heat stress hours as depicted in figure 1(a).This phenomenon is the result of a combination of factors, including high summer temperatures, elevated humidity levels, high sea surface temperatures (SSTs), and reduced ocean circulation within the semi-enclosed Bay of Bengal [23].Most parts of South India get cooled by the dominance of the southwest monsoon in June (figure 1(b)).However, the northwest and northcentral regions, still under premonsoon conditions, have maximum heat stress hours of HI41 in June.This is because of low cloud cover, high insolation, and daily maximum temperatures that usually exceed 40 °C in this region during pre-monsoon.In such circumstances, the potent solar and thermal radiative forcings will intensify heat stress, leading to extremely dangerous conditions.Starting in July, the advent of the monsoon introduces more cloud cover and lower temperatures, both of which mitigate radiative forcing effects and less heat stress conditions [18] except for certain parts of northwest India (figure 1(c)).Hence, the findings align with recent studies indicating that the regions along the Northwest, central, and eastern coasts of India have witnessed the highest heat stress and related fatal heat events in the country [23,50].

Spatial variation of dry and humid heat stress in India
Next, we compute the total number of hours of dry and humid heat stress per month over the past 80 years during May, June, and July.The spatial analysis identified regions within India that have experienced the highest instances of these extreme conditions each month.
Dry heat stress occurs in most parts of the country during May-July, especially the northwest, east and north central India as shown in figure 2. This can be attributed to extremely high summer temperatures and moderate relative humidity due to a lack of proximity to large water bodies.The southeast inland region also experienced significant instances of extremely dry heat stress during May (pre-monsoon) due to high temperatures and moderate humidity (RH<50) (figure 2(a)).This condition changes by June (figure 2(b)) due to the advent of the monsoon in Southeast peninsular regions which brings about a significant transformation in the temperature and relative humidity dynamics [23].
The total number of hours of humid heat stress per month is concentrated on the east coast during May (figure 3(a)) and by June the most occurring region becomes the northeast coastal region (figure 3(b)) and finally moves to the northwest and north central India by July (figure 3(c)).This can be related to the path of monsoon onset bringing in moisture combined with moderate or high temperatures [23].

Temporal variation of dry and humid heat stress in India
To understand the temporal trend, 5-year moving averages are used, considering the rapid variation in the annual time series of dry and humid heat stress hours per grid from 1943 to 2022. Figure 4(a) shows the change in the 5-year moving average of the dry heat stress hours (HI41) per grid over the whole of India from May to July for the last 80 years.In comparison to the 1940s, there is an increase in dry heat stress hours/grid by a factor of 3.75 in May.Other months exhibit changes of less than twice the initial values.This increase in May can be related to the increased summertime temperatures observed throughout the country.By June, the advent of the Southwest monsoon reduces the dry bulb temperatures.However, the 5-year moving average of humid heat stress hours per grid from May to July (MJJ), spanning from 1943 to 2022, reveals a striking trend (figure 4(b)).Currently, the humid heat stress hours have increased by a factor in the range of 5.5-6.5 when compared to the levels recorded in the 1940s.The results agree with previous studies that show humid heat stress has increased in India especially after 1967, when the green revolution started resulting in enhanced irrigation [16].The regions experiencing year-round high relative humidity levels cross the critical threshold of HI41 even with low temperatures [50].
Figure 5 indicates a rightward shift in the probability distribution curves for both dry and humid HI41 hours/grids over the last two decades compared to 1943-1963.In the initial twenty years of the study period, humid heat stress hours were lower, averaging 37.5 h grid −1 , compared to dry heat stress, which averaged 65.7 h grid −1 .Subsequently, the mean humid heat stress hours/grid experienced a fourfold increase from 1943-1963 (37.5 h grid −1 ) to 2002-2022 (137 h grid −1 ) while the mean dry heat stress hours/grid only doubled (65.7 h grid −1 to 137.8 h grid −1 ).Consequently, recent years have seen an overall increase in heat stress occurrences, particularly in the humid heat stress hours.

Case study: 2022 heat stress in Bikaner and Kolkata regions
The HI classification by NOAA, 2009 has been visually represented as a contour plot for better analysis (figure 6).The orange and red shade satisfying the HI41 condition is taken as the extreme threshold for this study, and it falls under the danger and extreme danger categories, respectively.The yellow shade is where HI is between 32 °Celsius and 41 °Celsius.
The total number of hours per day with HI41 condition is computed for two distinct regions in India, namely Bikaner and Kolkata in June 2022 (figure 6).The extreme dry heat stress observed in Bikaner during June can be attributed to the combination of high temperatures and low humidity levels ranging from 30%-40% within the region except for four days in June when RH was more than 70%.The spontaneous rise in RH was due to rainfall during those days (Supplementary figure S2).However, these days do not experience humid HI because the extent of the reduction in temperature was more than the increase in RH.On the other hand, Kolkata's high humid heat stress in June can be linked to the consistently high ambient humidity levels (RH > 70%) even though the average temperature lies below 35 °C.Kolkata experiences year-round high humidity levels due to its proximity to the sea.
Notably, Bikaner displayed high HI levels that entered the dangerous range in both June and July (Supplementary figures S2 and S3).On the other hand, Kolkata encountered hazardous HI conditions solely in June.Despite May experiencing elevated mean daily temperatures reaching around 40 degrees Celsius in Bikaner, it managed to avoid crossing the threshold into dangerous HI41 conditions.This was primarily attributed to the low relative humidity levels, which were approximately around 20% during that period.In July and August of 2022, there were days when the relative humidity in Bikaner exceeded 50% and HI41, resulting in periods of humid or moist heat stress days (Supplementary figure S4).This phenomenon can be attributed to the influence of the monsoon onset in this region increasing humidity levels.

Discussion
The findings of this study highlight the spatial variation of heat stress across India and the significance of humid heat stress in the context of changing climate.Spatial analysis of the total number of hours of HI41 for May-July over the past 80 years agrees with a recent study that shows summertime heat metrics are highest in the northwestern parts of India [2].This can be attributed to high summer temperatures and moderate relative humidity due to a lack of proximity to large water bodies [51].This region experiencing high dry heat stress during May and June undergoes a transition to high humid heat stress by July with the onset of the monsoon.As the monsoon advances, most parts of India witness a decline in heat stress as the amount of cooling due to a strong monsoon dominates the amount of moistening.Consequently, the heat action plans designed and implemented for an area may be effective in addressing the impacts of dry heat stress but fail to tackle the humid heat stress during pre-monsoon season.
During May and June, the eastern coastal areas experience extreme humid heat stress caused by a combination of high summer temperatures, increased humidity levels, high sea surface temperatures, and reduced ocean circulation.In contrast, adjacent inland regions stretching to north-central India experience dry heat stress due to extreme temperatures.To effectively address these distinct forms of heat stress, it is important to develop regional heat action plans that consider the specific challenges faced by each area.Analysing temporal trends using 5-year moving averages reveals a significant rise in humid heat stress compared to its dry counterpart.Over the past 80 years, there has been a substantial historical increase by a factor of 5 to 7 times the levels recorded in the 1940s.This noteworthy rise is consistent with earlier studies linking humid heat stress to factors like the Green Revolution and enhanced irrigation [16].According to the Clausius-Clapeyron (CC) equation in thermodynamics, for every 1 °C rise in temperature, the atmospheric water vapour can increase by around 7%.Thus, as air temperature increases under changing climate, the humid heat stress increases more rapidly than dry heat stress.This was evident from the probability distribution curves of humid heat stress hours/grid (figure 5) which demonstrates a fourfold increase in mean heat stress hours/grid over the past two decades.While humid heat stress occurrences were lower in the past, they are now similar to the increased dry heat stress occurrences.This accelerating rate of increase in humid heat stress is beyond the degree of human adaptability to heat and thus poses a significant risk to millions of people [51].If these trends persist, there is a high likelihood of increased mortality and morbidity in the coming years.

Conclusion
India experiences both dry and humid heat stress conditions due to its diverse climate across the land.Managing dry heat stress can be effectively achieved by implementing heat action plans, as seen in Ahmedabad [19] but will not be effective in reducing humid heat stress.Consequently, there is a pressing need to analyse and study humid heat stress separately to implement distinct adaptation methods.In this study, HI was used to identify extreme humid heat stress hours from 1943 to 2022, defined as conditions when HI exceeds 41 °C (dangerous category as per US NWS) and relative humidity more than 50% using high-resolution ERA5 reanalysis data.Dry heat stress occurs when HI exceeds 41 °C and relative humidity is below 50%.The spatial and temporal analysis of extremely humid and dry heat stress hours reveals distinct impact zones from May to July.The eastern peninsular and north central India emerged as the most humid heat-stressed regions in India.The transition from dry to humid heat stress as the monsoon season progresses highlights the dynamic nature of heat stress patterns in these regions.Thus, the same place will experience dry and humid heat stress depending on the prevailing weather conditions.Moreover, humid heat stress has become a major concern in this era because of its accelerated sixfold increase over time, its adverse impact on human health and its exclusion in heat action plans.As India grapples with both dry and humid heat stress, it becomes imperative to broaden the scope of adaptation methods, considering the distinct challenges posed by each type of heat stress.The findings emphasize the urgency of adopting region-specific and season-specific strategies tailored to the specific heat stress type.This study can be extended to investigate future projections of humid heat stress and the development of early warning systems.

Figure 1 .
Figure 1.Inter-monthly spatial variation of the total number of hours Heat Index (HI) that exceeds the threshold of 41 °C for the duration of 80 years from 1943 to 2022 over India in (a) May, (b) June, and (c) July.

Figure 2 .
Figure 2. Spatial variation of the number of hours of dry heat stress (HI > 41 C and RH < 50%) in (a) May, (b) June, and (c) July during 1943-2022 over India.

Figure 3 .
Figure 3. Spatial variation of the number of hours of humid heat stress (HI > 41C and RH > 50%) in (a) May, (b) June, and (c) July 1943-2022 over India.

Figure 4 .
Figure 4. Change in 5-year moving average of (a) dry heat stress hours (HI > 41C and RH < 50%) per grid, and (b) humid heat stress hours (HI > 41C and RH > 50%) per grid for May, June and July 1943-2022 over India.

Figure 6 .
Figure 6.Contour plot of Heat Index (HI) with mean daily temperature and mean daily relative humidity in June 2022 for Bikaner and Kolkata regions.
)-(2)) used by NWS and the equivalent algorithm illustrated in Brooke Anderson et al 2013 are used in this study to calculate HI.