Outlook Explains: What Is A Heat Dome? Why The US Is Facing Dangerous Temperatures

A severe heatwave has gripped large parts of the United States, with forecasters warning that triple-digit temperatures would spread across the Southwest and Great Plains and could persist for more than a week.

Meteorologists said a powerful dome of high pressure was trapping hot air over much of the country, allowing the heatwave to expand eastward. The system is expected to affect as much as two-thirds of the continental United States.

Earlier this month, the National Weather Service (NWS) placed nearly 120 million people under extreme heat warnings. Forecasts showed temperatures ranging between 32°C and 40°C, with high humidity pushing the heat index to around 46°C in some areas.

The NWS said temperatures in parts of North Dakota were expected to exceed 100°F (37°C) through Tuesday and urged residents to stay hydrated and seek cooler shelter, warning that temperatures in several regions would remain 8°C to 14°C above seasonal averages, even during the night.

The science behind a heat dome

A heat dome is a large area of high atmospheric pressure that forms when warm air moves northward and becomes trapped over a region. The high-pressure system acts like a lid, preventing the hot air from escaping and allowing temperatures at the surface to rise.

As the trapped air sinks, it compresses and warms further. At the same time, the high-pressure system blocks cooler air, cloud cover and rainfall from moving into the region. With little wind and abundant sunshine, the ground absorbs more heat, creating a self-reinforcing cycle that drives temperatures even higher.

Why it creates prolonged extreme heat

Heat domes are associated with persistent heatwaves because the stagnant high-pressure system can remain in place for days or even weeks. By preventing cooler air and storms from breaking the pattern, the dome allows heat to accumulate, resulting in prolonged periods of extreme temperatures.

The current heat dome is expected to strengthen over the coming days, expanding from the central United States towards the East Coast. According to meteorologists, the system is large enough to affect as much as two-thirds of the continental US, trapping sweltering temperatures while blocking cooling winds and rainfall. Although the East Coast is expected to avoid the worst conditions initially, forecasters say the heat dome could gradually shift eastward and persist for more than a week.

Why cities suffer more than rural areas

Cities are expected to bear the brunt of the heatwave as dense infrastructure and limited green spaces trap heat, causing temperatures to remain high even after sunset.

The National Weather Service has forecast record overnight temperatures across parts of Texas, Florida and North Carolina, with temperatures expected to remain above 80°F (27°C) in Fort Lauderdale, Miami, Tampa, Galveston and Charleston.

The most dangerous conditions are forecast along a broad corridor from the Great Lakes to the East Coast, where several cities could record their hottest day of the year so far. New York City, Philadelphia, Washington, Baltimore, Chicago, Indianapolis, Detroit and St. Louis are expected to see extreme temperatures, while cities farther south, including Dallas, Little Rock and Memphis, will also face intense heat.

Several of the affected cities are hosting FIFA World Cup events. In Philadelphia, organisers have already adjusted Fan Festival timings, delaying opening hours to reduce visitors' exposure to peak afternoon temperatures.

What governments can do to adapt

Authorities across the United States have rolled out emergency measures to help residents cope with the extreme heat.

Chicago said it would open cooling centres and deploy city workers to check on vulnerable residents.

In New York City, Mayor Zohran Mamdani's office announced what it described as an “unprecedented” response, including hydration vans and pop-up cooling stations equipped with misting fans and cooling towels.

In Washington, D.C., where temperatures were forecasted to exceed 38°C (100°F) from early in July, officials had been preparing for the heat to coincide with Fourth of July celebrations, including what organisers say will be the largest fireworks display ever held on the National Mall.

Earlier this month, PJM Interconnection, the largest US power grid operator, ordered electricity generators to operate at maximum capacity and directed idle power plants to come online as soaring temperatures placed increasing strain on the grid.

The emergency measures were aimed at maintaining the reliability of the power network serving around 67 million people across the Mid-Atlantic, the South and Washington, D.C., including the world's largest concentration of data centres.

How climate change is making heatwaves worse

AP reported meteorologists saying the recently formed El Niño is still too weak to have played a major role in the current heatwave. Instead, they said the primary driver of the extreme temperatures is long-term human-caused climate change resulting from the burning of coal, oil and natural gas.

“We know that heat waves are becoming more intense, they’re lasting longer, they’re covering larger areas than they used to because of human-caused climate change,” climate scientist Daniel Swain told AP. “And so when we see an event like this, we know there is at least a partial contribution by the long-term warming trend.”

Research group Climate Central reached a similar conclusion using its Climate Shift Index, which compares current weather conditions with a hypothetical world unaffected by greenhouse gas emissions. The analysis found that a 20,000-square-mile (52,000-square-kilometre) region stretching from Southern California to northern Minnesota, home to about 24 million people, was experiencing the highest level on the index, indicating the heat was at least five times more likely because of climate change.

Climate Central also reported similar findings for the heatwave that affected the US East Coast over the July 4 weekend and the recent extreme heat across the Southeast.

How heat affects economy

Heatwaves and persistent heat domes impose significant economic costs by damaging infrastructure, reducing agricultural productivity, disrupting labour, straining power grids and increasing healthcare expenditure, according to the Fifth National Climate Assessment. Rising temperatures also reduce worker productivity, particularly in outdoor sectors such as construction and agriculture, while increasing the risk of heat-related illnesses.

The assessment says these costs are not evenly distributed. Low-income communities, older adults and people with pre-existing health conditions are disproportionately affected because they are more exposed to extreme heat and have fewer resources to adapt. Beyond direct financial losses, prolonged heat also affects ecosystems, water resources, tourism and public health, creating wider economic burdens that are expected to intensify as climate change makes heatwaves more frequent, longer-lasting and more severe.

According to ABC News, research has found that rising temperatures are already taking a measurable toll on the global economy.

A study published last year by researchers from the University of Florida, the European Stability Mechanism and the International Monetary Fund, which analysed data from 203 countries over a 40-year period, found that a higher frequency of extreme heat and severe droughts was associated with a 0.2% decline in gross domestic product (GDP).

ABC News also cited a 2022 study by researchers at Dartmouth College, which estimated that the economic damage caused by human-induced extreme heat totalled as much as $50 trillion globally over a 30-year period, highlighting the enormous financial costs of increasingly frequent and intense heat events.

Health risks

According to the World Health Organization (WHO), extreme heat is a major environmental and occupational health hazard and one of the leading causes of weather-related deaths worldwide.

Heat stress can trigger life-threatening conditions such as heatstroke—a medical emergency with a high fatality rate—and worsen existing illnesses, including cardiovascular disease, diabetes, asthma and mental health disorders. It can also increase the risk of accidents and the spread of certain infectious diseases.

The WHO says the number of people exposed to extreme heat has risen sharply due to climate change. Heat-related deaths among people aged 65 and above increased by about 85% between 2000–2004 and 2017–2021.

Globally, an estimated 489,000 people die from heat-related causes each year, with 45% of those deaths occurring in Asia and 36% in Europe, according to studies cited by the WHO. Europe alone recorded an estimated 61,672 excess heat-related deaths during the summer of 2022.

The health impacts of extreme heat are influenced by factors such as age, pre-existing medical conditions, occupation and socio-economic status, making older adults, outdoor workers and low-income communities particularly vulnerable.

The WHO says many of the health risks posed by extreme heat are predictable and largely preventable through effective public health measures, early warning systems, climate adaptation and preparedness strategies.