Extreme Heat

The 10 warmest years since 1850 have all occurred since 2013. In 2023, the average annual temperature for the contiguous U.S. was 54.4°F, 2.4°F above the 20th-century average, which made 2023 the fifth warmest year in the 129-year record for the country as a whole. Approximately 65 million people were impacted by record heat during 2023 and 35 states experienced a top-10 warmest year for the state. 

Heat waves turning into heat domes have become an annual occurrence over the past decade, with the frequency of high heat days and the peak temperature on those days both increasing year over year across the United States. A heat dome occurs when the atmosphere traps hot ocean air like a lid or cap. This happens when strong, high-pressure atmospheric conditions combine with influences from La Nina, creating vast areas of sweltering heat that gets trapped under the high-pressure dome.

Extreme Heat is defined as either:

• A period of high heat and humidity with temperatures above 90 degrees for at least two to three days (Department of Homeland Security).
• A summertime temperature that is much hotter and/or humid than average. Because some places are hotter than others, this depends on what’s considered average for a particular location at that time of year. 

Heat Wave: a period of two or more consecutive days when the daily minimum apparent temperature (the actual temperature adjusted for humidity) in a particular city exceeds the 85th percentile of historical July and August temperatures (1981–2010) for that city.

Heat Dome: an exceptionally hot air mass that develops when high pressure aloft prevents warm air below from rising, thus trapping the warm air as if it were in a dome. 

Wet-Bulb Temperature (WBT): is the temperature of adiabatic saturation which combines dry air temperature with humidity and is considered to be the threshold for human survivability (National Weather Service).

• Heat waves turning into heat domes have become an annual occurrence over the past decade, with the frequency of high heat days and the peak temperature on those days both increasing year over year across the United States.
  • A heat dome occurs when the atmosphere traps hot air like a lid or cap over a region with minimal wind flow as storm systems are pushed over the region, this also increases the humidity during the event, further amplifying the extreme conditions.
• When heat and humidity are too high for sweat to evaporate from the skin—the body's natural cooling mechanism—a condition known as the wet bulb limit is approached.
  • If the temperature exceeds 95°F and humidity persists at 95% or higher, wet bulb conditions are considered “critical” and can be fatal for even young, physically fit individuals.
  • Climate forecasts indicate critical wet bulb threats will expand rapidly across the US by 2053.

CISA’s Role in Increasing Extreme Heat Resiliency

CISA is a partner in the NIHHIS Extreme Heat Interagency Working Group to collaborate on Heat.gov and applicable policy initiatives by providing critical infrastructure impact insights.

CISA and Lawrence Livermore National Lab (LLNL) produced the Drought and Extreme Heat Impacts to Datacenters in Northern California, bridging topics of climate and information technology systems. 

CISA provides extreme weather and extreme heat threats scenarios to emergency managers and first responders in exercises with interagency partners and public-private sector partnerships to assist in identifying resiliency best practices.

Critical Infrastructure Impacts

Extreme heat threatens critical infrastructure across the country. High heat events can impact infrastructure sectors such as transportation when roadways, runways, and railways begin to buckle, with secondary impacts across all sectors from heat-related power outages and health threats to the general population. The cost of U.S. Road maintenance and repairs due to rising temperatures could reach $26 billion by 2040. The cost to the U.S. economy of extreme heat is estimated to be $100 billion annually. 

Additional impacts include:

• Power lines become less efficient from overheating effects like drooping and load shedding capacity can be overwhelmed as transformers degrade or become damaged.
• Concrete degradation from direct sunlight and heat can cause cracks, dimpling of the material, expansion within the structure which allows moisture to infiltrate and break down the stability of the foundation/support.
• Overheating of materials can permeate buildings and cause greater energy needs for cooling, dangerous internal heat levels during power outages, and degraded refrigerant abilities.
• Cast iron cannot bend or stretch and will crack to relieve stresses of a weld if there is no preheat event.
• The loss of cooling abilities at datacenters and end offices could result in power loss as sites will have to shut down or reduce operations to prevent overheating with threats to server bays and equipment storage.
• Crop decay and increased livestock mortality rates are expected due to worsening heat. 
• Snowpacks have begun to melt earlier and form later as winter shortens causing earlier blooms and out of cycle harvest periods along with reduced water storage capabilities. 
• Local dams, levees, and other water retention structures could see loss in efficiency as cracks, breaks, and elevation shifts from subsidence expand across regions from the stressors of heat and overpulling.