Data: Significant Increase in the Average Number of Heat Wave Days in the Last two Decades
Sai Krishna Muthyanolla
April 29, 2024
In India, over the past two decades, many parts of the country have experienced record-breaking temperatures during summer months. A study has indicated a 68% likelihood of temperatures and humidity levels breaking records in India and other tropical nations in this ongoing 2024 summer season.
Since the advent of the industrial era, global temperatures have been on the rise, leading to a noticeable increase in heat waves worldwide. These heat waves characterized by prolonged periods of excessively high temperatures, are becoming more frequent, intense, and prolonged, impacting ecosystems, human health, and infrastructure. Scientists attribute this trend to climate change, driven primarily by burning fossil fuels, deforestation, and other human activities that have contributed to the build-up of greenhouse gases in the atmosphere, trapping heat and causing the planet’s average temperature to rise. As temperatures continue to climb, the risk of heat-related illnesses, heat stress, and heat-related deaths escalates, particularly in vulnerable populations and regions with limited access to cooling infrastructure. Apart from public health, heat waves also pose a significant challenge to agriculture and ecosystems.
Even in India, over the past two decades, many parts of the country have experienced record-breaking temperatures during summer months. For instance, in May 2016, Rajasthan faced an exceptionally high temperature reaching approximately 51°C, among the highest recorded globally. A study has indicated a 68% likelihood of temperatures and humidity levels breaking records in India and other tropical nations in the 2024 summer season. To further understand these trends, we analyse the heat wave days reported by Indian states since 1967.
As per the Indian Meteorological Department (IMD), a heatwave occurs if the maximum station temperature reaches at least 40°C in the plains and at least 30°C in hilly areas. The IMD further states that in coastal areas, heat waves occur when the maximum temperature departure is 4.5°C or more than normal. In India, heat waves typically occur during the summers from March to June, and in some rare cases, even extend till July. The average heat wave is calculated by totalling the average number of heat wave days in states during the summer. This data has been captured in Envistats. Data on state-wise reported number of heat wave days since 1967 as per Envistats is available in Dataful. This dataset has been used for this story and by the number of heat wave days, we refer to the average number of heat wave days. The states have been categorized into regions as in the table below, for analysis.
Number of heat wave days reported by states has risen steadily over the years
In the 55 years for which data is available, the highest number of heat wave days of all states together were reported in 2010 (279 days). The number of days crossed 200 only on three occasions during this period – in 2010, 1972 (202 days) and 2012 (201 days). The number of days touched/crossed the 150 mark once each in the 1970s and 1980s, twice in the 1990s and 2000s, and four times in the 2010s. If the average number of heat wave days reported in each decade is considered, in each decade since 1970, the annual average number of heat wave days in India of all states together dropped from 100 days in 1970-1979 to 93 days in 1990-1999. This rose to 139 days during 2010-2019. Meanwhile, the lowest number of heat wave days were reported in 1990 with 17 days, 1971 with 20 days and 2020 with 29 days.
Uttarakhand reported highest number of days in a single year in 2010
Haryana reported the highest number of heat wave days in the 55 years considered. More than 520 heatwave days have been reported by the state during this period. On average, the state has reported about 10 heatwave days every year. 2010 followed by 2022 was when the state reported the highest number of heat wave days in a single year. Punjab also reported the highest number of days in 2022. Uttarakhand reported 32 days in 2010, the highest for any state since 1967. Across all these northern states, there has been an increase in the average number of heat wave days reported over the decades. Himachal Pradesh did not report any heat wave days during this period.
AP reported highest number of heat wave days in South while Karnataka reported least
Among the southern states, Andhra Pradesh reported the most frequent occurrences of heat waves with more than 500 days reported since 1967. The decadal average number of heat wave days in the state was 11 in the 2010s as compared to 8 in the 1990s. Since the 1990s, the average has been on the rise. The state also reported the highest number of heat wave days in 2012 and 2014, with 16 days each year.
Meanwhile, in Tamil Nadu, the average number of days has gone up from 3 in the 1960s to 6 in the 2000s and has declined since then, whereas in Telangana, the average number rose from 5 in the 1970s to 7 in the 2010s. In Karnataka, on average, only one day has been reported throughout the decades. Kerala did not report any heat wave days during this period.
Significant rise in heat wave days reported in Rajasthan
While the average number of heat waves reported has been fluctuating between 2 and 3 in Gujarat, there has been a significant rise in its neighbouring state of Rajasthan, as evident in the chart below. Not only the average but there is an increasing trend in even the annually reported figures since 2011. The state also had the highest number of heat wave days reported in 2022, with 26 days and has reported more than 500 days since 1967.
Chhattisgarh and Madhya Pradesh also saw an increase in the average number of heat wave days reported. However, the rise is not as sharp as in Rajasthan. In Maharashtra, the highest number of days for a single year was reported in 2019 with 15 days.
Even though all the Eastern states reported an increase in the average number of heat wave days, the average number of days reported during the 2010s was more than 10 in Jharkhand and Odisha. Both these states saw a stark increase in frequency in recent years.
Many states reported highest number of heat wave days in 2022
Data indicates that there has been an overall increase in the number of heat waves being recorded across almost all the states in general. Many states reported the highest number of such days in recent years and some of them, in 2022. Rajasthan, Andhra Pradesh, and Haryana are the ones that reported the highest number of heat wave days since 1967. These states, which are heavily reliant on agriculture, are the most susceptible to the dire impact of extreme weather events.
As per the Indian Meteorological Department (IMD), it has taken significant steps to enhance monitoring and provide timely early warnings for heatwaves, thereby minimizing loss of life and property. Collaborating with the National Disaster Management Authority (NDMA) and local health departments, IMD has initiated Heat Action Plans (HAPs) in many regions across the country. These plans aim to forecast and advise on actions to be taken during heatwave conditions.  IMD issues a daily heatwave bulletin every day, including a 5-day forecast with heatwave warnings categorized by colour codes (Green, Yellow, Orange, and Red) and detailed impact descriptions following NDMA guidelines. These bulletins are disseminated at district levels through Meteorological Centres/Regional Meteorological Centres. To improve outreach, IMD has also introduced new platforms like daily and weekly video messages for stakeholders’ easy understanding. Social media channels such as YouTube, Facebook, WhatsApp, Instagram, and X are also being extensively used for warning message dissemination.