Redefining Heatwaves: The Role of Humidity and Wet Bulb Temperature in India

Introduction

The India Meteorological Department (IMD) announced the first heatwave of 2024 in certain areas of west Rajasthan on March 27, 2024. While this declaration was made, numerous regions likely faced humid heatwaves, a situation not entirely reflected in IMD's data due to the absence of relative humidity in their assessment criteria. This article presents a detailed overview of the challenges in measuring and responding to heatwaves in India.

IMD’s Heatwave Criteria

• Temperature Thresholds: The IMD defines a heatwave when temperatures exceed 40°C in plains, 37°C in coastal regions, and 30°C in hilly areas.
• Consecutive Days: A heatwave is declared if temperatures remain above normal by at least 4.5°C for two consecutive days.
• Immediate Declaration: If temperatures surpass 45°C, a heatwave is declared without considering the normal temperature deviation.

Challenges with Current Criteria

• The current criteria overlook relative humidity, which plays a crucial role in how heat is felt by individuals.
• Humid heatwaves, characterized by high moisture levels, make lower temperatures feel significantly hotter.
• The wet bulb temperature is a more accurate measure that considers both temperature and humidity, better representing heat's impact on human health.
• High humidity can hinder the body's natural cooling processes, increasing the risk of heat-related illnesses like heatstroke.

Case Studies and New Terminologies

• In Maharashtra, regions such as Sholapur and Jalgaon recorded wet bulb temperatures above safe limits, indicating severe heat stress.
• IMD has introduced terms such as "warm night conditions" and "hot humid weather" to describe heat stress, although these terms lack precise definitions.

Need for Revised Thresholds

• The globally accepted safe wet bulb temperature limit is below 30°C, with levels above 35°C considered critically dangerous.
• Research suggests that lower thresholds are necessary for tropical climates, challenging the applicability of the current 35°C benchmark.
• Adjusting these thresholds is vital for effectively monitoring humid heatwaves and informing communities at risk.

Conclusion

As India grapples with increasingly frequent and severe heatwaves, it is clear that the IMD's criteria must adapt to incorporate relative humidity and wet bulb temperature measurements. This evolution is essential for accurately assessing heat stress and adequately preparing populations for the hazards posed by humid heatwaves. Continuous monitoring and tailored alerts are crucial in protecting public health amid shifting global weather patterns.

Understanding Wet Bulb Temperature

The wet bulb temperature (WBT) measures heat stress in direct sunlight, accounting for temperature, humidity, wind speed, sun angle, and cloud cover. It represents the lowest temperature achievable through water evaporation alone. WBT is critical for understanding human heat stress, as it influences the body's ability to cool down through sweating. Dangerous conditions arise when WBT exceeds 35°C, leading to serious health risks like heatstroke.

Urban Heat Island Effects

The Urban Heat Island (UHI) effect describes how urban areas experience significantly warmer temperatures than surrounding rural regions, primarily due to human activities and environmental changes. This phenomenon exacerbates the impact of heatwaves, increasing energy consumption, elevating pollution levels, and posing severe health risks, including higher mortality rates during extreme heat events.

IMD's Heat Index

The Heat Index (HI), often referred to as the "felt air temperature," is a measure used by the IMD to convey how hot it feels to the human body when humidity is factored in with air temperature. This index is crucial in heatwave forecasts, providing a more accurate reflection of perceived outdoor temperatures and helping assess the risk of heat-related illnesses. The IMD integrates the heat index into its advisories to inform the public about potential heat stress conditions, advocating for necessary precautions.