Directions: Read the given passage carefully and answer the question that follows.

When scientists evaluate the danger posed by extreme heat events, they typically focus on frequency and severity. Yet the true measure of harm depends significantly on how well communities can protect themselves. Access to air conditioning, reliable electricity grids, and public cooling infrastructure all play a decisive role in determining whether a heat event becomes a catastrophe. Research consistently shows that wealthier nations are far better positioned to invest quickly in heat-mitigation strategies, whereas the least affluent quarter of the global population — regions where adaptation tends to be slowest — will bear a disproportionately growing burden of heat-related harm.

Extreme heat events rank among the most lethal of all climate-related hazards, causing widespread damage to agricultural systems, animal husbandry, and critical infrastructure. Current estimates suggest that nearly a third of people worldwide already live in regions where combined heat and humidity conditions can be life-threatening on more than 20 days annually, and the trend shows no sign of reversing.

Protective strategies — including the establishment of community cooling shelters, retrofitting homes with passive cooling features, redesigning urban landscapes to reduce heat absorption, and improving early-warning systems — can meaningfully reduce a population's exposure to dangerous temperatures. Nevertheless, a nation's capacity to deploy such strategies is tightly linked to its economic strength, institutional quality, cultural readiness, and access to technical knowledge. For many lower-income countries, meeting everyday public-service demands already stretches resources thin, leaving little margin to prepare for intensifying climate hazards.

Analysts estimate that the world's poorest quarter trails the wealthiest quarter by roughly 15 years in terms of readiness to cope with rising temperatures. This gap is calculated from the rate at which countries are developing and financing adaptation programmes, as reported in leading international environmental assessments. In practice, the actual disparity will differ by location due to variations in economic inequality and local governance capacity, but the 15-year figure offers a useful approximation of the widening divide. Globally, heat risk has climbed, but the steepest increases have occurred in lower-income regions. Analysis of the past four decades reveals a 60% rise in the number of extreme heat days recorded in the 2010s relative to the 1980s. For the purpose of this analysis, an extreme heat event is defined as a period during which daily temperatures exceed the 97th percentile for that specific location, persisting without interruption for a minimum of three consecutive days.

Further findings indicate that the duration of extreme heat seasons has lengthened, with more heat events occurring at the margins of the traditional warm season — early spring and late autumn — periods when bodies are less acclimatised and therefore more vulnerable. During the 2010s, people living in the world's poorest quarter experienced on average more than 40% greater exposure to extreme heat events than those in the wealthiest quarter, amounting to roughly 2.4 billion person-days of exposure per year compared to approximately 1.7 billion in richer nations. This disparity frequently goes unacknowledged in global discourse, partly because mortality data linked to heat are inconsistently recorded across many lower-income countries, making the true scale of the crisis difficult to quantify.

According to the passage, how is an extreme heat event defined?