About Heat waves

• A Heat Wave is a period of abnormally high temperatures, more than the normal maximum temperature that occurs during the summer season in the North-Western parts of India. 
• Qualitatively, a heat wave is a condition of air temperature which becomes fatal to the human body when exposed. 
• Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
• Heat Waves typically occur between March and June, and in some rare cases even extend till July. 

Criteria for Heat waves 

• The Indian Meteorological Department (IMD) has given the following criteria for Heat Waves :
  • Heat wave is considered if the maximum temperature of a station reaches at least 40°C or more for Plains and at least 30°C or more for Hilly regions;
  • Based on Departure from Normal
    • Heat Wave: Departure from normal is 4.5°C to 6.4°C
    • Severe Heat Wave: Departure from normal is >6.4°C
  • Based on Actual Maximum Temperature
    • Heat Wave: When actual maximum temperature ≥ 45°C
    • Severe Heat Wave: When actual maximum temperature ≥47°C
  • For coastal regions, when maximum temperature departure is 4.5°C or more from normal, Heat Wave may be described provided the actual maximum temperature is 37°C or more.

• Higher daily peak temperatures and longer, more intense heat waves are becoming increasingly frequent globally due to climate change. 
• India too is feeling the impact of climate change in terms of increased instances of heat waves which are more intense in nature with each passing year, and have a devastating impact on human health thereby increasing the number of heat wave casualties.

Vulnerable Regions

• Heat waves are common over the Core heatwave zones – Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana, as categorized by India Meteorological Department.
•  The regions in the extreme north, northeast, and southwestern India are less prone to heat waves.

Causes for increasing heatwave events in the country

•  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
  • This is because the presence of moisture restricts the temperature rise
• The sky should be practically cloudless 
  • This is to allow maximum insulation over the region.
• Large amplitude anti-cyclonic flow over the area.

Impacts

• The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion, and heat stroke. 
• The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death.
• Heatwaves can also increase strain on water, energy, and transportation resulting in power shortages or even blackouts. 
• Food and livelihood security may also be strained if people lose their crops or livestock due to extreme heat. 
• Multiple areas of the economic sector experience reduced worker productivity during heatwaves, especially agriculture and construction.
• Heatwaves can lead to water shortages and increased stress for plants, particularly in arid regions. This has the effect of reducing plant growth, the basis of energy production and the food chain, with an overall drying-out of the landscape.

Why in news?

• In Gujarat more than 12 persons have died due to intense heat waves.