Heat Waves and Cold Waves: When Temperature Becomes Life-Threatening

Extreme temperature events - both heat waves and cold waves - are among the most deadly natural hazards in the United States, yet they often receive less attention than more visually dramatic events like tornadoes or hurricanes. Together, they cause more deaths annually than any other weather-related hazard in most years.

Heat Waves: Rising Threat in a Warming World

A heat wave is defined by NOAA as a period of unusually hot weather that typically lasts two or more days, with temperatures substantially above the historical averages for a given area. Because the human body's ability to regulate temperature depends heavily on the difference between body temperature and the surrounding environment, the threshold for dangerous heat is relative - what constitutes a life-threatening heat wave in Seattle may be routine summer weather in Phoenix.

How Heat Kills: The Physiology of Heat Illness

The human body maintains its core temperature through sweating and increasing blood flow to the skin. During a heat wave, both of these mechanisms can be overwhelmed, particularly when nighttime temperatures remain elevated (preventing the body from recovering overnight) or when humidity is high (limiting sweat evaporation). The progression of heat illness follows a spectrum:

• Heat cramps: Painful muscle spasms caused by electrolyte loss through sweating.
• Heat exhaustion: Heavy sweating, cold/pale/clammy skin, fast/weak pulse, nausea, and weakness. The body is still compensating but under severe stress.
• Heat stroke: Core body temperature exceeds 103°F (39.4°C). Hot/red/dry or damp skin, fast/strong pulse, and possible unconsciousness. Heat stroke is a medical emergency that can cause permanent organ damage or death within minutes without treatment.

Public Health Reality: Extreme heat is the leading cause of weather-related mortality in the United States, killing an average of 700 people per year according to the CDC - a number considered a significant undercount by many researchers who study heat-attributable mortality.

Vulnerable Populations

Heat vulnerability is not uniformly distributed across populations. Those at greatest risk include the elderly (diminished thermoregulatory capacity), very young children, outdoor workers, individuals with chronic cardiovascular or kidney disease, those without access to air conditioning, residents of urban heat islands, and people taking certain medications that impair heat tolerance. Socioeconomic factors - including housing quality, neighborhood green space, and access to cooling resources - are major determinants of heat-related illness and death.

Urban Heat Islands

Urban areas experience substantially higher temperatures than surrounding rural areas due to the urban heat island (UHI) effect. Dark, impervious surfaces such as asphalt and concrete absorb and re-emit solar radiation as heat. Reduced tree canopy limits evapotranspiration cooling. Waste heat from vehicles, HVAC systems, and industrial processes adds to the thermal load. During heat waves, UHI effects can add 10 to 20°F to nighttime temperatures, dramatically increasing health risks for urban residents.

Cold Waves: The Underappreciated Hazard

A cold wave is a rapid and significant decrease in temperature over a 24-hour period, or a sustained period of below-normal temperatures that can stress infrastructure and threaten human health. While warming temperatures are reducing the frequency of some extreme cold events nationally, cold waves remain a serious hazard capable of causing widespread power grid failures, transportation disruptions, and hypothermia deaths.

Hypothermia (dangerously low core body temperature) and frostbite are the primary cold-related health risks. Paradoxically, most hypothermia deaths occur at temperatures above freezing, when wet and windy conditions accelerate heat loss from the body. Infants, the elderly, unhoused individuals, and those without adequate heating are most at risk.

Climate Change and Temperature Extremes

The scientific literature is unequivocal that human-caused climate change is increasing the frequency, intensity, and duration of heat extremes. Research published in Nature Climate Change shows that heat waves that previously occurred once every 50 years now occur every 10 years globally. Attribution science - which quantifies the degree to which climate change alters the probability of specific extreme events - has established clear human fingerprints on many recent deadly heat waves.

Staying Safe During Extreme Temperature Events

During heat waves:

• Seek air-conditioned environments during the hottest part of the day (typically 10 a.m. to 4 p.m.).
• Drink water consistently, even if not thirsty. Avoid alcohol and caffeine.
• Check on elderly neighbors and family members, who may not recognize their own heat-related symptoms.
• Never leave children or pets in parked vehicles.

During cold waves:

• Dress in moisture-wicking base layers, insulating middle layers, and a windproof outer shell.
• Recognize hypothermia warning signs: uncontrolled shivering, slurred speech, drowsiness, and confusion.
• Winterize your home before cold season and know how to prevent and thaw frozen pipes safely.