Wet-Bulb Temperatures Threaten the Eastern U.S.

An intense and oppressive heat dome has engulfed the eastern regions of the United States this week, leading the National Weather Service (NWS) to issue urgent heat warnings affecting nearly 170 million Americans. The situation is exacerbated by severe levels of humidity, which makes the high temperatures feel even more unbearable and dangerous. The combination of oppressive heat and high humidity creates a perilous situation for public health, necessitating increased awareness and urgent precautions to protect vulnerable populations.

The deadly interplay of extreme heat and humidity poses significant risks. The human body primarily regulates its temperature through the process of perspiration. When sweat evaporates from the skin’s surface, it cools the body down. However, high humidity levels impede this evaporation process, intensifying the risk of heat-related health issues. Meteorologists utilize the wet-bulb temperature to measure the cumulative physiological effects of heat and humidity on the human body. This critical metric provides valuable insights into human survivability, especially in the context of a rapidly changing climate.

David Romps, a distinguished professor of Earth and planetary science at the University of California-Berkeley, explained the concept of wet-bulb temperature to Gizmodo, noting that it reflects the temperature of a wet thermometer’s bulb. This measurement is achieved by placing a damp cloth over the bulb of a thermometer, allowing it to cool itself through evaporation, akin to how a person sweats to cool down. However, Romps emphasizes that the wet-bulb thermometer operates under different conditions than a human body, making it an essential tool for understanding heat stress.

Humans generate body heat that must dissipate into the surrounding air. Romps pointed out that, all else being equal, a person who is sweating will generally be warmer than the wet bulb reading. When wet-bulb temperatures rise close to 98.6 degrees Fahrenheit (37 degrees Celsius)—the normal human body temperature—it becomes exceedingly challenging to maintain a safe internal body temperature. Prolonged exposure to such conditions can lead to severe heat-related illnesses or even fatalities, highlighting the urgent need for awareness and action.

Historically, experts have considered a wet-bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit at 100% humidity or 115 degrees Fahrenheit at 50% humidity) to be the critical point beyond which the human body struggles to cool itself effectively. However, contemporary research suggests that this threshold may actually be significantly lower, indicating a more pressing concern for public health.

According to Kat Fisher, a PhD candidate from the human thermoregulatory lab at Penn State University, research indicates that a wet-bulb temperature of around 87 degrees Fahrenheit (30.6 degrees Celsius) at 100% humidity serves as the critical threshold. Beyond this point, humans are unable to maintain a stable core temperature if subjected to those conditions for extended periods. This finding underscores the growing urgency to address the impact of extreme heat and humidity on human health.

Meteorologists combine wet-bulb temperature measurements with air temperature, wind speed, cloud cover, and the angle of the sun to calculate the wet-bulb globe temperature (WBGT), which offers a comprehensive assessment of heat stress in direct sunlight. On July 29, the NWS reported WBGT values reaching the high 80s to low 90s Fahrenheit (upper 20s to mid-30s Celsius) across much of the eastern U.S., particularly impacting regions in the Southeast and Midwest, where heat stress levels are alarmingly high.

WBGT readings exceeding 90 degrees Fahrenheit (32 degrees Celsius) are classified as extreme and can lead to significant heat stress within just 15 minutes of physical activity in direct sunlight, as per NWS guidelines. Meteorological officials anticipate that these oppressive conditions will continue through Wednesday, July 29, before the heat dome gradually dissipates later in the week, but the risk remains acute.

Looking ahead, the prevalence of dangerous wet-bulb temperature events is expected to persist. Romps cautioned that human-induced global warming is driving up wet-bulb temperatures, pushing even healthy individuals closer to their physiological limits. This limit is a serious concern, as the human body struggles to endure wet-bulb temperatures that approach or exceed its internal temperature.

As the atmosphere continues to warm, its capacity to retain moisture increases, which in turn elevates the frequency and severity of extreme wet-bulb temperatures. Climate models predict that specific areas of the world could experience wet-bulb temperatures regularly exceeding 95 degrees Fahrenheit (35 degrees Celsius) within the next 30 to 50 years, according to NASA. In the United States, states in the Midwest—such as Arkansas, Missouri, and Iowa—are likely to reach critical wet-bulb temperature limits within the next half-century.

Throughout the 300,000-year history of our species, there has been no necessity to endure such extreme wet-bulb temperatures, as these conditions likely did not occur regularly in the past. Romps emphasizes that global warming is rapidly altering our climate patterns, making these extreme conditions a grim reality.

Extreme heat has already emerged as the deadliest weather hazard in the United States. Data from the Centers for Disease Control and Prevention (CDC) indicate that approximately 2,000 Americans succumb to heat-related causes annually, although some experts argue that this figure is significantly underestimated. Understanding the limits of human survivability in an increasingly warmer world becomes a critical issue, one that could determine life or death for many. There is a pressing need to adapt infrastructure, public health systems, and emergency response strategies to effectively manage the challenges posed by extreme heat.