INTRODUCTION:

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.

BODY PARAGRAPH:

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):

1. 1. In spring, India typically has air flowing in from the west-northwest. The Middle East is warming faster than other regions in latitudes similarly close to the equator and serves as a source of the warm air that blows into India.
2. 2. Air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan, so some of the compression also happens on the leeward side of these mountains, entering India with a bristling warmth.
4. 3. The air flowing over the oceans is also expected to bring cooler air, but the Arabian Sea is warming faster than most other ocean regions.
6. 4. The strong upper atmospheric westerly winds that come in from the Atlantic Ocean over to India during spring control the near-surface winds. The energy to run past the earth near the surface, against the surface friction, can only come from above. This descending air compresses and warms up to generate some heat waves.
8. 5. Lapse rate – the rate at which temperatures cool from the surface to the upper atmosphere – is declining under global warming. Global warming also tends to warm the upper atmosphere faster than the air near the surface. This in turn means that the sinking air is warmer due to global warming and thus produces heat waves as it sinks and compresses.
10. 6. The other factors that affect the formation of heat waves are the age of the air mass and how far it has travelled. The north-northwestern heatwaves are typically formed with air masses that come from 800-1,600 km away and are around two days old. Heat waves over peninsular India on the other hand arrive from the oceans, which are closer (around 200-400 km) and are barely a day old. As a result, they are on average less intense.

CONCLUSION:

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.