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Heatwaves & Climate: Essential Concepts



  Apr 08, 2024

Heatwaves, Wet Bulb Temperature,Heat Island and Heat Index



The India Meteorological Department (IMD) recently declared the first heatwave of 2024 in isolated regions of west Rajasthan on March 27, 2024. Despite this, many areas likely experienced humid heatwaves, a phenomenon not fully captured in the IMD's current heatwave data due to the exclusion of relative humidity in their criteria. Here's an organized overview of the situation, highlighting the complexities of measuring and responding to heatwaves in India.

IMD's Heatwave Criteria:

● IMD declares a heatwave when temperatures exceed 40°C in plains, 37°C in coastal areas, and 30°C in the hills.
● A heatwave is announced if the temperature is above normal by at least 4.5°C for two consecutive days.
● Temperatures crossing 45°C prompt an immediate heatwave declaration, irrespective of the deviation from normal temperatures.

Challenges with Current Criteria:

● The criteria do not account for relative humidity, which significantly affects the human experience of heat.
● Humid heatwaves, where high moisture levels make lower temperatures feel much hotter, are increasingly common.
●The wet bulb temperature, a measure that considers both temperature and humidity, better represents the true impact of heat on the human body.
● Excessive humidity impedes the body's cooling mechanisms, potentially leading to heatstroke and death.

Case Studies and New Terminologies:

● In Maharashtra, areas like Sholapur and Jalgaon recorded wet bulb temperatures exceeding the safe limit, indicating severe heat stress.
● IMD introduced terms like "warm night conditions" and "hot humid weather" to describe heat stress, though these terms lack clear definitions.

Need for Revised Thresholds:

● The universally recognized safe wet bulb temperature limit is below 30°C, with 35°C being critically dangerous.
● Research suggests the need for lower thresholds in tropical climates, challenging the applicability of the current 35°C benchmark.
●This adjustment is crucial for accurately monitoring humid heatwaves and informing affected populations.

Conclusion:

As India faces more frequent and severe heatwaves, it's evident that IMD's heatwave criteria need to evolve to include relative humidity and wet bulb temperature measurements. This would ensure a more accurate assessment of heat stress and better prepare populations for the dangers of humid heatwaves. Continuous monitoring and updated alerts tailored to different climatic conditions are essential for safeguarding public health in the face of changing global weather patterns.

WWWWet  Bulb Temperature

The wet bulb temperature (WBT) is a measure of heat stress in direct sunlight, taking into account temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation). It represents the lowest temperature that can be achieved through the evaporation of water only. WBT is crucial in understanding human heat stress since it affects the body's ability to cool down through sweating. High WBT conditions, especially above 35°C, can be lethal as the human body struggles to regulate its temperature, leading to heatstroke or death. This measure is particularly relevant in assessing the risk of humid heatwaves, which can have severe impacts even at lower temperatures due to high moisture content in the air.

Heat Island Effects

Urban Heat Island (UHI) effect describes how urban regions experience significantly warmer temperatures than their rural surroundings, primarily due to human activities and alterations to the natural environment. This phenomenon results from the absorption and retention of heat by buildings, roads, and other infrastructure, combined with the lack of vegetation. UHI effects can exacerbate the impact of heatwaves, increasing energy consumption (due to higher air conditioning use), elevating emission of pollutants and greenhouse gases, and posing significant health risks including increased mortality rates during heatwaves.

Heat Index of the IMD

The Heat Index (HI), often referred to as the "felt air temperature," is a measure used by the India Meteorological Department (IMD) to indicate how hot it feels to the human body when relative humidity is combined with the air temperature. This index is important in heatwave forecasts as it provides a more accurate representation of the perceived outdoor temperature, thereby helping in assessing the risk of heat-related illnesses. The IMD utilizes the heat index in its advisories to inform the public about potential heat stress conditions, urging precautions. Unlike the wet bulb temperature, the heat index does not necessarily provide a threshold for human survivability but rather indicates discomfort levels and potential health risks due to high temperatures combined with humidity.



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