The Deadly Combination of Heat and Humidity in India and Pakistan in Summer 2015

Observations and Impacts. Andhra Pradesh, Telangana, and other southeastern Indian states suffered a deadly heat wave in late May and early June of 2015. Daily high temperatures exceeded 45°C in many places throughout India for several days in a row. In late June and early July, just a few weeks later, Pakistan also suffered from a deadly heat wave with similar daily high temperatures. Although the Pakistani heat wave occurred very soon after the Indian heat wave, they were distinct meteorological events. Ratnam et al. (2016) classify heat waves over India into two types, those that occur over north-central India and those that occur over coastal eastern India. The study finds that the former tend to be associated with anomalous blocking over the North Atlantic Ocean. Heat waves over coastal eastern India were found to be associated with westerly anomalies over the Indian landmass, thereby reducing the land–sea breeze along the coastal regions. Hence the Loo, a strong afternoon overland wind, brought hot and dry conditions to India (Fig. 16.1a). By late June, the Indian monsoon was well developed, curtailing these winds and terminating the heat wave (see www .tropmet.res.in/~lip/Publication/Scientific-Reports /RR-185.pdf). In Pakistan by this time, winds were onshore (Fig. 16.1b), and the unusually hot conditions were also unusually humid. The high numbers of deaths in both events are attributed not only to the weather conditions but also to institutional failures. Hospitals were overwhelmed with patients suffering from heat-related symptoms and at some point had to turn away patients (Salim et al. 2015). It is difficult to be precise about the ultimate number of fatalities associated with these heat waves, but upwards of 2500 excess deaths are estimated to have occurred in the Indian heat wave (Ratnam et al. 2016) and at least 700 alone in the Pakistani megacity of Karachi (Masood et al. 2015) with many more throughout the country. To further characterize these heat waves, we have analyzed 1973–2015 subdaily (hourly and 3-hourly) temperature and heat index (Steadman 1979a,b) calculated from the HadISD v1.0.4.2015p quality controlled weather station dataset (Dunn et al. 2012). Heat index, one of several methods to measure the combined effect of temperature and humidity on human health, is a bicubic function of both variables intended to fit a model of a fully clothed adult (see Supplemental Material for its definition). Figures 16.1c,d show the daily maximum instantaneous heat index (thick red lines) and the temperature (thick black lines) associated with it during the heat waves in Hyderabad (in the Indian state of Telangana, WMO station number 431280) and Karachi (the largest city in Pakistan, WMO station number 417800). The pentadal averages of these daily maxima are shown with thin lines. Climatological averages over 1974–2014 are shown with horizontal dotted lines for May in Hyderabad and June in Karachi to show the events’ relative severity. In Hyderabad, the daily maximum heat index was about 2°–4°C higher than temperature during the heat wave. In Karachi, this difference was about 7°–12°C, reflecting a much higher relative humidity. The first column of Table 16.1 shows representative values of temperature and heat index during the most severe periods of the 2015 heat waves drawn from Figs. 16.1c,d. Figures 16.2a,b are scatterplots of relative humidity against temperature at the time of the daily maximum heat index value over the observational record of 1973–2015. Colored dots show the official U.S. NOAA advisory heat index levels of caution, extreme caution, danger, and extreme danger. In such hot climates, the 1974–2014 average daily maxiWe find that the deadly heat waves in India and Pakistan in 2015 were exacerbated by anthropogenic climate change. Although the impacts of both events were severe, the events themselves were not connected to each other.