Many parts of India recorded the hottest-ever summer months of March and April in 122 years. As reported by the Weather Channel (WC), the heat wave season, which began earlier than normal in March this year, turned rather unbearable for most parts of India throughout April. Though there seemed to be a respite in the first week of May, it was short-lived. Daytime temperatures remained over 45°C in many parts of Gujarat, Rajasthan, Haryana, Madhya Pradesh and Uttar Pradesh on Thursday, May 12.

Mercury levels have spiked sharply across northwest, west, and central India, including Delhi. These regions also logged two consecutive months – March and April – of hottest weather on record since 1901! The temperatures during summers are typically high across these regions. Still, the magnitude of the spike in mercury levels has been causing concerns around the long-term health and safety of the residents, especially those most vulnerable to such prolonged heat waves.  The scorching heat waves have already claimed many lives. A WC report states that experts claim the reported data is just the tip of the iceberg due to the ambiguity in determining the accurate cause of death. The death toll from heat waves in India could be many times more than the reported 300 to 400 per year.

The India Meteorological Department (IMD) declares a heat wave if the actual temperatures over a region stay above 40°C and cross temperature thresholds of 4.5°C above average. If they stay over 6.4°C above average, it’s classified as a severe heat wave. In North India, the summer rains are dependent on the Western Disturbances. These extra-tropical systems originate in the Caspian and the Red Sea and traverse over Iran, Afghanistan and Pakistan to reach India. Usually, while flowing, these systems carry moisture from the sea and are associated with rainfall, snowfall and fog in Northern India.

According to IMD officials, the absence of this periodic rainfall could have led to a dire situation this year! Although were six WDs in April, most of them were feeble and dry and moved across the higher ridges of the Himalayas. Due to these dry WDs, this April has become the hottest month for Northwest and central India in the last 122 years.

During such times, monitoring Wet-Bulb Temperatures (WBTs) is crucial. The WBT simply tries to provide more context on the effects of the temperature on our human bodies. It tells us what the weather feels like in a particular region rather than what it is. As a WC report explains, take, for example, an ambient temperature of 30°C. Most of us know that this will feel extremely different in Chennai and Mumbai versus non-coastal cities like Bangalore or Pune. The reason for this lies in the humidity of the two regions – which encapsulates the underlying concept of a WBT.

The WBT is calculated from the dry temperature and is a function of the humidity of the region. It represents the air temperature at complete air saturation (which means 100% relative humidity) and is usually much lower than the area’s ambient temperature. It also provides a reference to determine health and safety standards. To elaborate, a Wet-Bulb Temperature of 32-35°C – corresponding to a dry temperature of around 55°C – is usually considered the maximum limit for human functioning outside. Heat strokes become much more likely from this point onwards. Our body’s in-built process to regulate its internal temperature when it gets too high is sweating. However, when the particular region’s atmosphere is already full of moisture represented by high humidity levels, it becomes harder for it to absorb more moisture. Therefore, heatwaves are potentially deadlier in high humidity areas like coastal cities. Dry heat can also be fatal in the form of the loo, but heatwave + humidity is potentially more dangerous. Heatwaves affect vital organs in our body, such as the brain, heart, kidneys and lungs.

Other factors which may have led to unbearably hot weather conditions throughout the country are the prolonged La Niña phenomenon and climate change. As the WC report points out, according to recent IPCC reports and statements given by experts, the number of heatwaves this year could be a direct response to global warming. Estimates suggest that such intense heatwaves would have happened only once in every 50 years. However, due to climate change, they are now expected to occur once every four years. The World Meteorological Organization, on the other hand, says that “it is premature to attribute the extreme heat in India and Pakistan solely to climate change.”