A heat wave is generally defined as an extended period of hot and humid weather, although there is no universal definition of the term, and is measured relative to the normal weather in the area. Temperatures that people from a hot climate consider normal could be a heat wave in a cooler area if these are outside the normal climate pattern for that area. With climate change, the frequency and duration of heat waves have gone up, and the world is facing hot days, hot nights, and heat waves more frequently.

In recent years, frequent heat waves in developing economies have seriously impacted workers in exposed occupations, especially in urban areas where the work pattern has little seasonal variation.

This paper identifies the coping strategies and labour reallocation of very poor urban workers on a heat wave day compared to a normal summer day by surveying workers who work in the open. Findings show workers to work one or two hours less, spend less time at home, and rest one-and-a-half or two hours longer on a heat wave day than on a normal summer day. They resort to other measures like eating food with high water content, covering the roof of their living space with paddy straw, and using fans for longer hours to adapt to frequent heat waves. In the heat wave month, their routine expenditure increases by Rs 600 (approximately US$10) on average. This extra expenditure constitutes some 7–35 per cent of the monthly income of the workers as most of them earn between Rs 20,000 and Rs 100, 000 annually. Nearly 80 per cent of workers surveyed reported to have suffered some form of heat attack as these adaptations are inadequate and they cannot afford more. Seemingly Unrelated Regression Estimates show only natives (not migrants) and those who have changed occupation are not suffering work time loss or are adapting better to heat waves.