According to the IMD, a region has a heat wave if its ambient temperature deviates by at least 4.5- 6.4°C from the long-term average. There is also a heat wave if the maximum temperature crosses 45°C (or 37°C at a hill station).
Heat waves are formed for one of two reasons: because warmer air is flowing in from elsewhere or because something is producing it locally. Air is warmed locally when the air is warmed by higher land surface temperature or because the air sinking down from above is compressed along the way, producing hot air near the surface.
A study published in Nature Geoscience offers explanations as to how different processes contribute to the formation of a heat wave. (The study’s findings have been adapted here to the Indian context):
The 2022 heat wave season was unusual because the heat waves extended much further south into peninsular India because of a north-south pressure pattern set up by the La Niña, a world-affecting weather phenomenon in which a band of cool water spreads east-west across the equatorial Pacific Ocean.
Heat wave events have now started to occur once every few years, and are also more intense. The area covered by these heat waves has been expanding.
In sum, heat waves have a sophisticated anatomy with important implications for how well we can predict them. Nonetheless, early-warning systems can take advantage of the processes, modes of formation, location, and age of the air mass to improve the quality of warnings and also increase how soon they can be issued.
The adverse impacts of heat waves indicate that effective disaster adaptation strategies and more robust disaster management policies are required in heatwave zones to lessen the impact of heatwaves.
Further improvements in forecast warnings coupled with city-wide graded heat action plans are required to protect the vulnerable.
Answer Credits: https://www.thehindu.com/sci-tech/science/explained-imd-is-already-sensing-heat-waves-what-are-they-and-why-do-they-happen/article66556185.ece#:~:text=Sophisticated%20anatomy&text=The%20north%2Dnorthwestern%20heatwaves%20are,are%20barely%20a%20day%20old.