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What is causing the intense heat in north, west, central and east India?

Published: 7th May, 2022


There is an ongoing heatwave in India that has forced millions of people indoors, with air conditioner sales nearly doubling when compared to the previous year. The driving up temperatures beyond normal in north, west, central and east India in the month of March and April is a cause of concern for India.

  • IMD has said that April was the hottestin northwest India in 122 years.


Present situation

  • The India Meteorological Department (IMD) has forecast a gradual rise in maximum temperatures by 2-4 degree Celsius over most parts of north-western and central India.
  • While heatwaves are common in India, especially in May and June, summer began early this year with high temperatures from March itself with average maximum temperatures in the month.
  • The Centre for Science and Environment, a think-tank, says that early heatwaves this year have affected around 15 states, including the northern state of Himachal Pradesh, known for its pleasant temperatures.


What are heatwaves?

  • The India Meteorological Department qualitatively describes heatwave as a condition of air temperature which becomes fatal to the human body when exposed.

The General Occurrence

  • Months: Heat waves usually occur in the months of March to June and in some rare cases even in July.
    • The peak month of the heat wave over India is May.
  • Regions:Heat waves generally occur over plains of northwest India, Central, East and north Peninsular India.
    • It covers Punjab, Haryana, Delhi, Uttar Pradesh, Bihar, Jharkhand, West Bengal, Odisha, Madhya Pradesh, Rajasthan, Gujarat, parts of Maharashtra and Karnataka, Andhra Pradesh and Telangana.
    • Sometimes it occurs over Tamil Nadu and Kerala also.
  • Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
  • Declaration
    • Heatwave:A heatwave is declared when an area logs a maximum temperature of 45 degree Celsius.
    • Severe heatwave:A severe heatwave is declared if the maximum temperature crosses 47 degrees.
  • For coastal regions, heat wave may be described provided actual maximum temperature is 37 degrees or more.

Different standards on heatwaves

Several countries have adopted their own standards on heatwaves:

  • United States: The US National Weather Service defines a heatwave as a spell of “abnormally and uncomfortably hot and unusually humid weather” over two days or more.
  • Denmark: A heatwave occurs when the mean of the highest recorded temperature measured over three consecutive days exceeds 28°C (82.4°F).
  • Australia: In Adelaide, Australia, a heatwave is defined as five straight days with temperatures at or above 35°C (95°F), or three consecutive days at or over 40°C (104°F).

Understanding the geographical aspect of heatwave

  • One of the causes of these extreme heat waves has been the unusual north-westerly winds.
  • These anomalous north-westerlies overpowered the moist southerly winds that typically come off the water and kept pre-monsoon showers offshore.
  • This deviation from normal wind trends allows hot air from desert areas to the northwest to spread over much of the country.
  • Heat waves form when high pressure aloft in around 3,000–7,600 metres above the region and remains over a region for several days up or
  • This is common in summer, both in Northern and Southern Hemisphere as the jet stream 'follows the sun'. On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.
  • During summer, weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly.
  • Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds.
  • Reduction of clouds increases shortwave radiation reaching the surface.
  • A low pressure at the surface leads to surface wind from lower latitudes that bring warm air, enhancing the warming.
  • Alternatively, the surface winds could blow from the hot continental interior towards the coastal zone, leading to heat waves there causing adiabatic warming.

What are the favourable conditions of heat wave formation?

  • Transportation / Prevalence of hot dry air over a region: There should be a region of warm dry air and appropriate flow pattern for transporting hot air over the region.
  • Absence of moisture in the upper atmosphere: As the presence of moisture restricts the temperature rise.
  • The sky should be practically cloudless: To allow maximum insulation over the region.
  • Large amplitude anti-cyclonic flow over the area.

Wet Bulb Temperature

  • This heatwave has also brought the wet bulb temperature concept into mainstream conversation.
  • In simpler terms, wet bulb temperature tells us at what level our bodies will be unable to cool themselves down by sweating.
  • In this case, the threat of a heat stroke rises dramatically.
  • Wet bulb temperature combines heat and humidity to indicate how much evaporation can be absorbed into the air.
  • It measures the lowest temperatures that our bodies can reach when we are in hotter environments, by sweating.

Intense heating of earth during summer

  • The Northern Hemisphereexperiences more direct sunlight during May, June, and July, as the hemisphere faces the Sun.
  • It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months, which increases the solar flux.
  • However, due to seasonal lag, June, July, and August are the warmest months in the Northern Hemisphere.
  • Cause of summers in Northern hemisphere:
  • Axial Tilt: The seasons result from the Earth's axis of rotationbeing tilted with respect to its orbital plane by an angle of approximately 23.4 degrees. This tilt is also known as "obliquity of the ecliptic".
  • Regardless of the time of year, the northernand southern hemispheres always experience opposite seasons. The effect of axial tilt is observable as the change in day length and altitude of the Sun at solar noon (the Sun's culmination) during the year.
  • Elliptical Earth orbit: Compared to axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth's distance to the Sun because of its elliptical orbit. Orbital eccentricity can influence temperatures.
  • Maritime and hemispheric: Seasonal weather fluctuations also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds.
  • Tropics: The tropicaland subtropical regions see little annual fluctuation of sunlight. However, seasonal shifts occur along a rainy, low-pressure belt called the Intertropical Convergence Zone (ICZ). As a result, the amount of precipitation tends to vary more dramatically than the average temperature.
  • Mid-latitude thermal lag: In meteorological terms, the solstices (the maximum and minimum insolation) do not fall in the middles of summer and winter. The heights of these seasons occur up to 7 weeks later because of seasonal lag. Seasons, though, are not always defined in meteorological terms.

What is Loo?

  • The heat waves are normal during summer in India, often called as Loo. This originates in the deserts of Rajasthan and western part of the continent.
  • It affects mostly the north and the central parts of India, and remains over the region till the onset of monsoon.
  • The Loo is a strong, dusty, gusty, hot and dry summer wind from the west which blows over the Indo-Gangetic Plain region of North India and Pakistan.
  • It is especially strong in the months of May and June.

Is climate change responsible for heatwaves in India?                                                                                          

  • The heat-trapping consequences of global warming imply that climate extremes such as heatwaves are expected to rise in frequency. Instances of extreme rainfall, as well as longer rainless spells are expected, according to assessments by the Intergovernmental Panel on Climate Change.
  • The main reason for the scorching heat in the northern parts of the country is lack of rainfall.
  • Usually, periods of high temperature are punctuated by periodic episodes of rain but this was largely absent during March and April.
  • Ironically, April also saw maximum instances of extreme rainfall since 2018 though it was concentrated in the south and north-eastern India.
  • The rain-bearing western disturbances originate because of temperature gradients between the northernmost parts of the globe and the latitudes passing through West Asia. Weaker gradients mean weaker rains.
  • This March and April, cooler than normal conditions in the Pacific Ocean failed to aid rainfall in north India.

What is heat Index?

  • The heat index is a measure of how hot it feels when relative humidity is factored with the actual air temperature. 
  • It is used to measure the intensity of heat waves in a region.
  • In certain countries it is defined in term of the heat index based on temperature and humidity or based on extreme percentile of the temperatures.

Impacts on India

  • Health effects:  Hyperthermia, also known as heat stroke, becomes common during periods of sustained high temperature and humidity.
  • Older adults, very young children, and those who are sick or overweight are at a higher risk for heat-related illness.
  • The chronically ill and elderly are often taking prescription medications that interfere with the body's ability to dissipate heat can face problems due to excessive heating.
  • Mortality: Heatwaves have killed more than 17,000 people in 50 years in India, according to a research study by IMD scientists.
  • Crop losses: heat waves and excessive heating impacts the standing crops of the regions, as they are not immune to more heat at that time of the year.
  • Psychological and sociological effects: In addition to physical stress, excessive heat causes psychological stress, to a degree which affects performance, and is also associated with an increase in violent crime.
  • High temperatures are associated with increased conflict both at the interpersonal level and at the societal level.
  • Increase in surface ozone: ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves. During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual.
  • Lead to wild fires: If a heat wave occurs during a drought, which dries out vegetation, it can contribute to bushfires and wildfires.
  • Power outages: Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, exacerbating the problem.

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