A heat wave is a period of abnormally hot weather lasting multiple days, defined relative to a region's usual climate and seasonal norms. Since the 1950s, these events have become more frequent and intense across nearly all land areas due to climate change, with the World Meteorological Organization reporting record-breaking temperatures in 2024 across Europe, North America, and China, including consecutive days above 45°C. They typically form when a high-pressure area in the upper atmosphere strengthens and traps heat near the surface, and they are usually forecastable in advance, allowing authorities to issue warnings. The impacts are severe, ranging from reduced labor productivity and catastrophic crop failures to thousands of deaths from hyperthermia, increased wildfire risk, and stress on electrical grids that can cause blackouts. Definitions vary by region—for instance, the IPCC defines a heat wave as lasting from two days to months, while Denmark requires three consecutive days above 28°C, and Sweden requires five days above 25°C. Additionally, marine heatwaves are defined as prolonged periods of unusually warm sea surface temperatures, often using a 90th percentile threshold, allowing for global comparisons.