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Study Highlights Worsening Heat Stress in India’s Megacities


A study by the Delhi-based Centre for Science and Environment reveals that India’s major cities—Delhi, Mumbai, Chennai, Bengaluru, Kolkata, and Hyderabad—are facing increasing heat stress. This is attributed to rising relative humidity over the past two decades. Additionally, the study indicates that these cities are experiencing warmer nights due to the urban heat island effect.


GS III: Environment and Ecology

Dimensions of the Article:

  1. Key Highlights of the Study
  2. What is Heat Stress?
  3. What is an urban heat island?

Key Highlights of the Study

Increasing Heat Stress in Megacities:

  • Cities like Delhi, Mumbai, Chennai, Bengaluru, Kolkata, and Hyderabad are facing worsening heat stress.
  • This is attributed to a trend of rising relative humidity over the past two decades.
  • Except for Bengaluru, all other metros have experienced a 5-10% increase in average relative humidity during summer.

Impact of High Heat and Humidity:

  • High heat and humidity compromise the body’s cooling mechanism, sweating.
  • Sweat evaporation cools the body, but higher humidity levels inhibit this natural process.
  • This combination can make people sick and, in severe cases, be fatal, even at lower ambient temperatures.

Warmer Nights and Urban Heat Island Effect:

  • These cities are experiencing warmer nights due to land surface temperatures not cooling as much as a decade ago.
  • This is blamed on the urban heat island effect.
  • Hot nights are as dangerous as peak daytime temperatures, with little chance for recovery if nighttime temperatures remain high.

Rising Heat Index:

  • The combination of rising air and land surface temperatures and high humidity increases the heat index and heat stress in these cities.
  • The heat index measures discomfort due to high heat and humidity.

Monsoon Temperature Changes:

  • The monsoon period has become hotter in Delhi, Mumbai, and Kolkata.
  • In Chennai, the marginal cooling effect during monsoon has disappeared.
  • Bengaluru and Hyderabad experienced slightly cooler monsoon temperatures compared to the pre-monsoon period.
Importance of the Study

Comprehensive Heat Management:

  • Assessing heat trends is crucial for developing a comprehensive heat management plan for urban centers.

Emergency Measures:

  • This study will aid in implementing emergency measures during heatwaves to protect public health.

Long-Term Strategies:

  • It will also help develop long-term strategies to mitigate heat by:
  • Increasing green areas and waterbodies.
  • Improving thermal comfort in buildings.
  • Reducing waste heat from vehicles, air conditioners, and industries.

What is Heat Stress?


  • Heat stress occurs when the body cannot dissipate excess heat, leading to an increase in core temperature and heart rate. It represents the physiological strain experienced in high-temperature environments.
  • High ambient temperatures
  • High humidity levels reducing the body’s cooling efficiency through sweating
  • Physical exertion, particularly in hot conditions
  • Inadequate hydration
  • Poor ventilation in workspaces or living environments
  • Initial signs include difficulty concentrating, irritability, sickness, and loss of the desire to drink.
  • If the body continues to store heat, it can lead to fainting and, in severe cases, death if not addressed.

What is an urban heat island?

  • An urban heat island is a local and temporary phenomenon experienced when certain pockets within a city experience higher heat load than surrounding or neighbouring areas on the same day.
  • The variations are mainly due to heat remaining trapped within locations that often resemble concrete jungles.
  • The temperature variation can range between 3 to 5 degrees Celsius.
Why are cities hotter than rural areas?
  • Larger green cover: Rural areas have relatively larger green cover in the form of plantations, farmlands, forests and trees as compared to urban spaces. This green cover plays a major role in regulating heat in its surroundings.
  • Transpiration : It is a natural way of heat regulation. This is the scientific process of roots absorbing water from the soil, storing it in the leaves and stems of plants, before processing it and releasing it in the form of water vapour.
  • Highrise buildings, roads in Urban areas: Urban areas lack sufficient green cover or gardens and are often developed with highrise buildings, roads, parking spaces, pavements and transit routes for public transport. As a result, heat regulation is either completely absent or man-made.
  • Heat absorption: Cities usually have buildings constructed with glass, bricks, cement and concrete — all of which are dark-coloured materials, meaning they attract and absorb higher heat content.
  • This forms temporary islands within cities where the heat remains trapped.
How can urban heat islands be reduced?
  • The main way to cut heat load within urban areas is increasing the green cover; filling open spaces with trees and plants.
  • Other ways of heat mitigation include appropriate choice of construction materials, promoting terrace and kitchen gardens, and painting white or light colours on terraces wherever possible to reflect heat.

-Source: The Hindu

June 2024