Millions of Americans along the US East Coast are enduring a dangerous combination of extreme heat and stifling humidity, with daytime temperatures expected to remain near 38°C (100°F) for several days.
The US National Weather Service (NWS) has warned that the heat index, a measure of how hot it feels when humidity is combined with air temperature, could climb to 46°C (115°F) in major cities such as Washington, DC, Philadelphia and New York during the Fourth of July holiday weekend.
The heatwave comes on the heels of an unprecedented spell of early summer heat across Europewhere several countries have recorded all-time temperature records and widespread heat warnings.
The sweltering conditions also coincide with one of the busiest holiday weekends in the United States. Millions are expected to take part in celebrations marking America’s 250th Independence Day, with President Donald Trump leading commemorative events, while several FIFA World Cup matches are set to be played outdoors, heightening concerns about heat-related illnesses among players, fans and event staff.
The danger of this heatwave lies not just in the thermometer reading, but in how the human body experiences it. In New York, thermometers touched 35°C this week, yet residents said it felt closer to 46°C. They were not exaggerating.
They were describing the heat index, a measure that captures the combined effect of temperature and humidity, and one that explains why some heatwaves become deadly even when the air temperature alone does not appear exceptionally high.
WHAT IS A HEAT INDEX, AND WHY DOES IT FEEL HOTTER THAN THE ACTUAL TEMPERATURE?
The human body cools itself mainly by sweating. Sweat draws heat away from the skin as it evaporates. That trick only works if the surrounding air has room to absorb more moisture.
When humidity is high, the air is already saturated, so sweat sits on the skin instead of evaporating, and the body keeps overheating.
The heat index, sometimes called the feels like temperature, accounts for this by combining air temperature with humidity. America’s National Weather Service calculates it using a regression formula built on decades of physiology research. The result is not a guess. It is closer to a medical warning dressed up as a weather statistic.
WHAT IS A HEAT DOME, AND HOW DOES IT TRAP HEAT?
A heat dome forms when a large zone of high pressure settles over a region and refuses to move. Air beneath it slowly sinks, and sinking air compresses and warms, the same way a bicycle pump grows hot when you push it.
The high pressure also blocks clouds from forming, so sunlight reaches the ground almost unfiltered, day after day. Meteorologists call this a blocking pattern, because it blocks the normal, restless movement of weather systems that would otherwise bring relief.
WHAT IS A HUMIDITY WAVE, AND WHY DID IT MAKE THINGS WORSE?
Alongside the heat dome, moist air moved north from the Gulf of Mexico, an event some forecasters have informally called a humidity wave. This moisture prevented the usual overnight cooling.
Water vapour traps outgoing heat the way a blanket does, so nights stay warm, denying bodies and buildings the recovery time they need before the next scorching day.
CAN AIR CONDITIONING ALONE SOLVE EXTREME HEAT?
Air conditioning saves lives during events like this, and public health advice rightly urges people to use it. But it is not a complete answer. As millions of units switch on at once, demand on the power grid surges.
Hotter conductors also carry electricity less efficiently, since electrical resistance rises with temperature, meaning more energy is lost as waste heat exactly when the grid can least afford it. Transformers designed for ordinary summers can overheat and fail.
Air conditioning buys time. It does not fix the underlying problem of a warming atmosphere that can hold roughly seven per cent more moisture for every extra degree Celsius, a relationship physicists call the Clausius-Clapeyron equation.
The lesson from New York’s punishing week is not that summers are unbearable now. It is that heat and humidity, working together, can turn an ordinary hot day into a medical emergency, quietly and without much warning on the thermometer.
– Ends
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