What is a Heat Dome and why is it causing extreme heatwaves in the USA?

Cities across southern and western states in the United States issued warnings and opened cooling centres to deal with the ongoing heatwave, with temperatures soaring above 110 F or 43.3 degrees Celsius (°C) in some places.

From Northern California to Texas, excessive heat warnings and advisories blanket the region, urging residents to take precautions against the blistering heat. In Las Vegas, temperatures soared to a staggering 112 degrees Fahrenheit or 44.4°C, shattering records and turning the iconic Strip into an oven.

Even in Death Valley in California, known for its extreme heat, the mercury climbs to a sweltering 121°F or 49.4°C, making it one of the hottest places on Earth.

In Phoenix, where last year’s summer saw a staggering 55 straight days of temperatures exceeding 110°F or 43.3°C, officials are scrambling to provide relief with cooling centres and closed hiking trails. With heat-related deaths on the rise, Phoenix firefighters are using ice immersion to treat heatstroke victims, a desperate measure in the face of escalating temperatures.

The National Weather Service (NWS) has said the heat dome that’s currently putting a hot lid on the western US will bring high temperatures that are 20 to 30 degrees hotter than normal for early June. The NWS also released a HeatRisk map that shows colour-coded risk categories for people confronted by heat that can pose health problems.

Areas around California, Las Vegas, and some areas in Texas are at risk of long-duration extreme heat conditions with little to no overnight relief.

The reason behind this is a condition known as a ‘heat dome’. But what is it?

A heat dome is a weather phenomenon where a ridge of high pressure gets stuck in the atmosphere.

It traps hot ocean air like a lid or cap on a boiling pot. Hot air expands vertically into the atmosphere and high pressure pushes it toward the ground. This air then becomes compressed and heats up even more, leading to a dome-shaped area of hot air that can persist for several days or even weeks.

Children, the elderly, and people with chronic conditions can be especially vulnerable. The NWS has forecast mild cooling over the region this weekend, but only by a few degrees.