The Core Heatwave Zone (CHZ) is a geographical concept, not an act or institution, designated by the India Meteorological Department (IMD) to identify the regions in India most prone to heatwave conditions. It was created to focus research and disaster management efforts on the areas that experience the highest frequency and duration of heatwaves, which typically occur from March to June.
The CHZ was defined based on a long-term IMD study covering the period 1961–2020 to understand the climatology and trends of heatwaves. This study found that heatwave frequency in the CHZ was rising by 0.1 days per decade and duration by 0.44 days per decade. The zone encompasses a large swathe of central, north, and peninsular India, including the states of Punjab, Haryana, Delhi, Uttar Pradesh, Rajasthan, Gujarat, Madhya Pradesh, Chhattisgarh, Bihar, Jharkhand, West Bengal, Odisha, and Telangana, along with the meteorological subdivisions of Vidarbha, Marathwada, Madhya Maharashtra, and coastal Andhra Pradesh.
The concept connects directly to the IMD's criteria for declaring a heatwave, which is a mechanism based on maximum temperature thresholds and departure from normal temperature. For instance, a heatwave is declared in the plains when the maximum temperature reaches at least 40°C or the departure from normal is between 4.5°C to 6.4°C. The CHZ is a key area of focus for the development of Heat Action Plans (HAPs), which are state-level strategies for mitigating the health and socio-economic impacts of extreme heat. While the CHZ itself remains a fixed geographical classification, the IMD is currently considering revising the parameters for declaring a heatwave to better account for factors like humidity and changing climatic realities.