Heatwaves

As per the World Meteorological Organisation, Heatwaves are more intense and are starting earlier than in the past in India.

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. Heat Waves typically occur between March and June, and in some rare cases even extend till July.
The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death.
A heatwave occurs when a system of high atmospheric pressure moves into an area and lasts two or more days. In such a high-pressure system, air from upper levels of our atmosphere is pulled toward the ground, where it becomes compressed and increases in temperature.

A heatwave is declared when the maximum temperature is over 40 degree Celsius and at least 4.5 notches above normal.
A severe heatwave is declared if the departure from normal temperature is more than 6.4 degrees, according to the IMD. Based on absolute recorded temperatures, a heatwave is declared when an area logs a maximum temperature of 45 degree Celsius. A severe heatwave is declared if the maximum temperature crosses 47 degrees.

IMD has given the following criteria for Heat Waves:

Heat Wave need not be considered till maximum temperature of a station reaches atleast 40oC for Plains and atleast 30oC for Hilly regions
When normal maximum temperature of a station is less than or equal to 40oC Heat Wave Departure from normal is 5oC to 6oC Severe Heat Wave Departure from normal is 7*C or more
When normal maximum temperature of a station is more than 40oC Heat Wave Departure from normal is 4oC to 5oC Severe Heat Wave Departure from normal is 6oC or more
When actual maximum temperature remains 45oC or more irrespective of normal maximum temperature, heat waves should be declared. Higher daily peak temperatures and longer, more intense heat waves are becomingly 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.
Higher daily peak temperatures and longer, more intense heat waves are becomingly 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.

Heatwaves can burden health and emergency services and 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.
Social impacts: Extreme heat can lead to heat-related illness and death, particularly in elderly populations, the poor, outdoor workers, and in urban areas. Heatwaves have killed more than 17,000 people in 50 years in India, according to a research study by IMD scientists.
Economic impacts: Multiple areas of the economic sector experience reduced worker productivity during heatwaves, especially agriculture and construction. In parts of India, changes to weather patterns and the resulting impacts on agriculture and other sectors are expected to translate into a 9% decrease in standards of living by 2050 assuming no adaption
Ecological impacts: Heatwaves, without concomitant increases in precipitation, 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.

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