Heat Wave

A heat wave is a condition of air temperature that becomes fatal to the human body when exposed. 

It is a Qualitative definition. However, since it is not as visible as other disasters, we need a quantitative definition in order to identify a heat wave when it comes.

Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal. In certain countries, it is defined in terms of the heat index based on temperature and humidity or based on the extreme percentile of the temperatures.

Criteria to Declare Heat Waves

According to IMD, a Heatwave is considered if the maximum temperature of a station reaches: 

• At least 40°C or more for Plains, 
• At least 37°C or more for coastal stations and 
• At least 30°C or more for Hilly regions.

The following criteria are used to declare a heat wave:

1. Based on Departure from Normal:
   1. Heat Wave: Departure from normal is 4.5°C to 6.4°C
   2. Severe Heat Wave: Departure from normal is > 6.4°C
2. Based on Actual Maximum Temperature (for plains only):
   1. Heat Wave: When actual maximum temperature ≥ 45°C
   2. Severe Heat Wave: When actual maximum temperature ≥ 47°C
3. To declare a heat wave, the above criteria should be met at least in 2 stations in a Meteorological sub-division for at least two consecutive days and it will be declared on the second day.

Heat waves typically occur between March to June, and in some rare cases even extend till July. There have been 32 heat waves affecting 23 states in 2019, the 2nd-longest spell of high temperatures on record. 

Rajasthan, Madhya Pradesh and Maharashtra were facing “extreme” heat conditions having experienced the longest spell of dry weather this year.

What is the Heat Index?

The Heat Index is a parameter that considers both temperature and humidity to calculate the apparent temperature or “feel like” temperature for human beings. It helps in understanding the impact of humidity on high temperatures and how it contributes to human discomfort during hot weather.  The Heat Index has been launched on an experimental basis by the India Meteorological Department (IMD). It aims to provide general guidance for regions experiencing higher apparent temperatures causing discomfort to people. Indication of Heat Stress: High Heat Index values indicate a greater risk of heat-related stress and health issues. It serves as a warning for potential heat-related illnesses and dangers. Categorization of Heat Levels: The Heat Index categorizes the apparent temperature into different levels using colour codes: Green: Experimental heat Index less than 35°C. Yellow: Experimental heat Index in the range of 36-45°C. Orange: Experimental heat Index in the range 46-55°C. Red: Experimental heat Index greater than 55°C. Useful Tool for Public Health: By understanding the Heat Index, individuals and communities can take proactive steps to protect public health during heat waves. It assists in making informed decisions and formulating heat action plans to ensure the well-being of the population.

Reasons for Severe Heat Waves:

Heat waves can be caused by a variety of reasons.

1. The delayed arrival of the southwest monsoon in June creates a heatwave.
2. Changes in land use and land cover create heat waves.
3. Unplanned urban development, densely populated areas increasing urban sprawl and unique challenges associated with it such as the Urban Heat Island effect in cities are exacerbating the impact of heat waves.

Above all, Climate change is driving temperatures higher as well as increasing the frequency and severity of heat waves in India.

Non-recognition as a disaster:

Heat Wave has not been notified as a Disaster as defined under DM Act, 2005 by the government yet. 

• Heatwave is not even notified in the list of 12 disasters eligible for relief under National/ State Disaster Response Fund norms.
• Recently in 2024, the Tamil Nadu government made a gazette notification that heatwave is a state disaster, clubbing it with 13 other eventualities such as deaths caused by electrocution, thunderstorms and lightning, floods and snakebites. It is now the only government that recognises heat waves as a disaster.

It has been proposed by the experts, that other states must follow this precedence. 

Response to Heat Waves: 

National Guidelines for Preparation of Action Plan – Prevention and Management of Heat Wave: Released by NDMA.

• Government Engagement: Mandating participation from State and district government leaders, municipal health agencies, disaster management authorities and local partners.
• Appointing State Nodal Agency and Officer to conduct tabletop exercises, simulations and drills before the summer season as well as to ensure coordination among various stakeholders.
• Vulnerability assessment and establishing Heat-Health threshold temperatures: The state should coordinate with IMD to develop threshold temperatures.
• Drafting and developing Heat Action Plan: The State Nodal Officer can coordinate with the local IMD office to start receiving summer season forecasts and release early warning and daily alert system with colour codes.
• Implementation and monitoring: Do’s and Don’ts during a heat wave should be available in the local language and disseminated through media including social media.
• Evaluating and Updating the plan: After every heat season, the city or state must assess the efficacy of the heat action plan, including processes, outcomes and impacts.
• Strategies for reducing extreme heat exposures and adapting to climate change (Long Term): States should consider mitigation strategies, such as increasing green cover in a city to red.

Urban Heat island 

We have already seen that the global average temperature has risen by around 1.65oC above the pre-industrial level. However, it has been seen in recent times that urban areas have been experiencing severe heat waves more frequently than rural areas. 

In general, the urban landscape is hotter than the rural landscape. This phenomenon is known as the Urban Heat Island (UHI) Effect.

This happens due to 3 reasons:

• Greater Heat absorption: The surface is darker in the urban region which absorbs more heat. Further, the reflective design of buildings channelizes the sun’s rays towards the darker regions.

• Greater concentration of Greenhouse gases: Due to vehicular pollution, industrial pollution, burning of fuel and leakage of greenhouse gases, the urban environment traps more heat.

• Greater generation of heat: Due to higher rates of consumption of electricity, and cooling systems pumping out heat into the environment, the urban areas produce more heat. This again gets trapped by the greenhouse gases.

• Humidity: Humidity traps heat in the atmosphere, preventing cooling through evaporation. It creates discomfort for the residents, as sweat doesn’t evaporate efficiently. This leads to higher perceived temperatures and intensifies the overall heat retention in urban environments.

In the case of Delhi recently the urban heat island was physically visible in terms of a Fog hole.

Fog Hole in Delhi

In Physical Geography, we have learned about the conditions that lead to the creation of fog. In north India when the moist air cools in the winter, it creates fog; In other words, the moisture in the air condenses. However, in recent years, we have started seeing a new phenomenon. When the whole of North India is under dense fog, the sky in Delhi is clear!! A similar situation is visible in other cities too, as visible in this image. How is this possible? Explanation: In rural areas, temperatures remain cooler along with sufficient moisture availability from higher green cover. Due to intense urbanisation, cities are generally warmer than rural areas, especially during winter months.  There is also a relative lack of vegetation cover (for example trees, croplands, grass cover)  within city premises, which leads to low relative humidity inside cities compared to rural areas, which can lead to sharp differences in fog formation across urban-rural landscapes. “The fog hole phenomenon, especially over Delhi, is so strong that it causes nearly 50% reduction in fog frequency compared to Delhi’s surroundings, seen over the 17 years  (2000-2016)”.

Preventing Urban Heat Island

To mitigate UHI, several strategies can be implemented:

1. Green Patches: Planting trees, creating green roofs, and establishing community gardens and parks can provide shade and reduce surface temperatures, cooling urban areas.
2. Green Transport: Promoting walking, cycling, and the use of electric public transportation reduces vehicle emissions and traffic heat, lowering the overall temperature.
3. Green Cover: Expanding urban forests, implementing green infrastructure (e.g., permeable pavements), and adopting sustainable landscaping techniques like xeriscaping can help absorb heat and enhance cooling.
4. Particulate Matter Control: Reducing emissions from vehicles and industries, using air purification systems, and enhancing green spaces can decrease particulate matter in the air, which also contributes to heat retention.

Effective UHI mitigation requires integrated urban planning, incorporating green spaces, sustainable transport, and pollution reduction. Public awareness and strong policies are essential for driving widespread change and creating healthier, cooler cities.

What is the quantitative definition of a Heat Wave according to the IMD?

Heat Wave is declared by the India Meteorological Department (IMD) when the maximum temperature reaches:
40°C or more for plains
37°C or more for coastal areas
30°C or more for hilly regions
Further classification includes:
Heat Wave: Departure from normal is 4.5°C to 6.4°C or actual temperature is ≥ 45°C
Severe Heat Wave: Departure from normal is > 6.4°C or actual temperature is ≥ 47°C
The condition must persist for two consecutive days in at least two stations in a meteorological sub-division.

What is the Heat Index, and why is it important?

The Heat Index combines temperature and humidity to calculate the “apparent temperature” or the “feels-like” temperature.
It helps understand human discomfort and potential health risks during hot weather.
IMD has launched it experimentally, using color codes:
Green: <35°C
Yellow: 36–45°C
Orange: 46–55°C
Red: >55°C
It is a crucial tool for public health preparedness and formulating heat action plans.

Why has the impact of heat waves increased in urban areas?

Due to the Urban Heat Island (UHI) effect, urban areas are hotter than surrounding rural areas because of:
Darker surfaces that absorb more heat
High greenhouse gas emissions from vehicles and industries
Excess heat generation from cooling systems and appliances
Low vegetation and moisture, which reduces natural cooling
This results in higher temperatures, discomfort, and reduced fog formation in cities like Delhi (observed as a “fog hole”).

Is a heat wave officially recognized as a disaster in India?

No, Heat Waves are not notified as disasters under the Disaster Management Act, 2005, and are not included in the list of 12 disasters eligible for central or state relief funds.
However, Tamil Nadu in 2024 became the first state to notify heatwaves as a state disaster, grouping them with other hazards like lightning and snakebites.

What are the key components of the NDMA’s Heat Wave Action Plan?

The National Disaster Management Authority (NDMA) outlines these steps:
Appoint State Nodal Officers and conduct drills
Identify vulnerable populations and assess threshold temperatures
Develop Heat Action Plans with early warnings and public advisories
Ensure wide dissemination of Do’s and Don’ts in local languages
Promote long-term strategies like green cover expansion and UHI mitigation
It emphasizes coordination across departments and regular evaluation of the plan post-heatwave season.