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Heatwave and Urban Heat Island – Causes and Effects

Heatwave, a silent disaster

  • A heat wave is a period of excessively hot weather.
  • IMD has prescribed the following criteria for heatwaves:
  • Maximum temperature of a station reaches 40 °C for the plains, 37 °C for coastal stations and 30 °C for hilly areas.
  • In case normal maximum temperature of a station is lower or equal to 40 °C, heat wave departure from normal is 5 to 6 °C and severe heat wave departure from normal is 7 °C or more.
  • In case normal maximum temperature of station is more than 40 °C, heat wave departure from normal is 4 to 5 °C and severe heat wave departure from normal is 6 °C or more.
  • When actual maximum temperature remains 45 °C or more regardless of normal maximum temperature, heat waves should be declared
  • The impact of heatwaves is not limited to cities, but cities aggravate this phenomenon (Urban Heat Island).

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  • India Meteorological Department and the Indian Institute of Tropical Meteorology (Pune) have established that the frequency and severity of heatwaves have risen significantly in India over the last three decades.
  • As emissions continue to rise, India will suffer from warming worse than the rest of the planet.
  • The global average temperature increase since 1900 is around 1.3 °C; in India, the average temperature increase in the same period has already crossed 2 °C.

Effects of heatwave

  • Sunstroke (results in a body temperature greater than 40 °C) and vital organs failure: In 2015, over 2,300 people died reportedly as a result of heatwaves.
  • Reduced human output due to adverse effects on mental health (the body works best in a narrow range of body temperature — 36-37.5 °C).
  • Economic cost: Increased expenditure on cooling appliances.
  • Ecological damage: reduced biological activity and carbon sequestration.
  • Forest fires.

Causes for Heat Wave

Anthropogenic causes
Natural Climatic Phenomenon: Shifting jet streams
  • In 2010, South-Western strip of Russia was devastated by a very severe heatwave.
  • A strong upper-atmospheric high located over European Russia diverted the meandering Sub-Tropical Jet Stream farther north than usual.
  • Desert heat from central Asia, the Arabian Peninsula and North Africa began to flow northward into Russia.

heat wave in Russia

El Nino, La Nina
  • They alter cloud cover, rainfall, wind patterns, jet stream location, etc.
Hot local winds like Loo
  • Loo is a hot wind that originates in the desert regions of Iran, Pakistan and Thar Desert and blows eastwards into the Indian plain region in the months of May and June, usually in the afternoons.
  • Its temperature invariably ranges between 45°C and 50°C and causes severe heatwaves in the plain region.

Measures required

  • Improve the overall ventilation of the cities. E.g. Stuttgart, which is located in a weak wind-flow region, has progressively implemented policies to improve ventilation since the 1980s.
  • Develop a strategy to incorporate the green belt concept in urban planning and implement building codes that entail passive cooling practices (use energy from the environment to dissipate heat).
  • Increasing tree and vegetation cover within cities.
  • Consider heat wave as a natural disaster.

Disaster Management: Heatwave is not considered a natural disaster

  • The National Disaster Management Act, 2005, and the National Policy on Disaster Management, 2009, do not consider heatwave to be a natural disaster (natural calamity).
  • It is not notified in the list of 12 disasters eligible for relief through national or state disaster relief funds.
  • The National Disaster Management Authority now has guidelines for preparing action plans.
  • In India, many cities and states have adopted such plans.
  • As part of its weather forecasting protocol, the IMD has been providing a seasonal temperature and heat-wave conditions outlook over the country at the subdivision level since 2016.
  • And since last year, it has been providing heat-wave ‘guidance’ for two weeks at a time.

Relation between heatwave and Cyclone Vayu, El Niño

  • Cyclone Vayu and El Niño are responsible for the heatwave spell that many parts of India are experiencing.
  • On June 11, temperature in parts of Delhi crossed 48 degrees Celsius.
  • The El Niño effect caused the westerlies blowing in from Pakistan and Rajasthan (local hot wind called Loo) to heat up, which led to a prolonged dry spell and a delayed monsoon.
  • Monsoon hit Kerala on June 8, but, at the same time offshore trough (a band of thunderstorm clouds) developed along the western coast of India and turned into very severe cyclonic system Vayu.
  • The cyclones that develop in the Arabian Sea impact the monsoon more because the monsoon winds enter the Indian Peninsula from the western coast along the Arabian Sea (cyclones suck up all the moisture).

Urban Heat Island

  • An urban heat island is an urban or an industrial area that has temperatures considerably higher than its surrounding rural areas (both share the same climate) primarily due to anthropogenic reasons.

UPSC Mains 2013: Bring out the causes for the formation of heat islands in the urban habitat of the world.

Urban Heat Island

Basics: Albedo

UPSC Mains 2013: Bring out the causes for the formation of heat islands in the urban habitat of the world.
  • Albedo of a surface is the proportion of sunlight that the surface can reflect back into space.
  • Fresh snow has the highest albedo (reflects back 80-90% of the incident sunlight).
  • Dark asphalt used to road construction and in roofing of buildings has very low albedo (~5%).

Albedo of different surfaces



Fresh Snow


Thick cloud 70-80%
Water near horizon 50-80%
Old Snow 45-50%



Light soil


Thin cloud 25-35%









Asphalt (used in road construction and roofing)


Numbers not important. Remember the descending order of Albedo.

Causes for Urban Heat Islands

  • Heat-trapping concrete and asphalt (have very low albedo or low reflectivity) replace natural vegetation and water (reduced evaporation and evapotranspiration).
  • High rise buildings that offer more surface area for heat absorption.
  • High vehicle density and a lot of heat released from their engines.
  • High pollution levels and high concentration of GHGs like CO2 (thermal power plants and vehicles).
  • GHGs, aerosols, particulate matter are good at absorbing outgoing infrared radiation (long wave radiation).
  • Cooling appliances like air conditioners that release the heat into the surroundings.
  • Bad monsoons: less evaporation of water from vegetation and soil.

Urban heat islands at night

  • Cities in northern plains used to cool down during night, but nowadays they are trapping this heat overnight.
  • ACs, pollution, close and dense network of buildings are the primary reason for the Urban heat islands at night.

Measures Required

  • Modifying the heat-generating and heat-retaining nature of cities: encourage public transport, electric vehicles, clean fuels, etc.
  • Use light asphalt instead of dark asphalt in road construction and roofing.
  • Cool pavements and rooftops designed to reflect more sunlight and absorb less heat.
  • Green roofing: roofs covered with green vegetation (both the above methods reduce air conditioning).
  • Decentralization of development and building green capitals and cities (e.g. Amaravati in AP).
  • Improve vegetation cover.

Measures Taken

  • Cool roof programmes, which are already a component of Indian cities’ heat action plans, involve lightening roof colours to reduce heat absorption.
  • Cool pavement programmes aim for a similar effect at street level.
  • Transition away from heat-absorbent materials, towards the use of alternative construction materials and technologies. The National Mission on Sustainable Habitat is aimed at such a transition.
  • For example, the Building Material and Technology Promotion Council (BMTPC) under the Union Ministry of Housing and Urban Affairs (MHUA) promotes 24 alternative materials and construction technologies.
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