Europe's deadly heatwave and the 'Omega Block': why dangerous humid-heat days are rising, including in India

Western Europe has been hit by another severe heatwave, with countries from the Netherlands and the United Kingdom to France and Italy affected. Around fifty people are reported to have died, schools and public transport were disrupted, and France recorded its hottest day since record-keeping began in 1947, with the temperature reaching 44.3 degrees Celsius. The crisis has put a spotlight on a weather pattern called the Omega Block and on the wider rise of dangerous humid heat driven by climate change.

An Omega Block, also called a heat dome, is named after the Greek letter omega because of its shape: a large zone of high pressure sitting between two zones of low pressure. Under the high-pressure zone, air sinks and is squeezed, which warms it, suppresses cloud formation and allows long spells of strong sunshine. Warm air rising from the ground cannot escape upward and gets trapped near the surface, so temperatures climb day after day. Because the pattern is slow-moving and stable, the heat lingers for a long time over the same region.

A second, related idea is 'dangerous humid heat', measured using the wet-bulb temperature. Wet-bulb temperature combines heat and humidity to capture how well sweating can cool the body. When the air is very humid, sweat does not evaporate easily, so the body cannot cool down even at lower air temperatures. A new global study by a US-based research group found that dangerous humid-heat days, those with a daily maximum wet-bulb temperature of 25 degrees Celsius or higher, have risen sharply worldwide, from about 10 a year in the 1970s to 23 a year in the last decade, and the increase was attributed mainly to human-caused climate change.

India is among the worst affected. The study found dangerous humid-heat days in India rose from an average of 101 a year in the 1970s to 141 a year during 2016-2025. City numbers are striking: Delhi rose from 96 to 135 days, Mumbai from 136 to 206, Chennai from 205 to 257, and Tirunelveli in Tamil Nadu, the worst hit, from 119 to 273. Because much of India is both hot and humid, rising wet-bulb temperatures threaten outdoor workers, farmers and the elderly, and strain health systems and power grids.

For aspirants, this brings together physical geography and climate change. Remember the Omega Block (a blocking high-pressure ridge that traps heat), the concept of wet-bulb temperature and why high humidity makes heat deadly, and the data showing a sharp rise in dangerous humid-heat days in India. These ideas appear in prelims and in mains answers on heat stress, climate adaptation and disaster management.