What Kind of Conditions Can Lead to Wet Bulb Events in the United States?

When most people think about extreme heat, they imagine soaring air temperatures. But a critical factor that determines how dangerous heat can become is humidity. Wet-bulb temperature is a climate metric that combines air temperature and humidity to indicate how well the human body can cool itself through sweating. When humidity is high and temperatures are elevated, the wet-bulb temperature rises. In extreme combinations, it can reach levels that push the human body to its limits of survival. This blog explores the weather and environmental conditions that can trigger dangerous wet-bulb events in the United States, why these events are increasingly relevant, and what drives their development.

What Is a Wet Bulb Event?

A wet-bulb event is a period when the wet-bulb temperature climbs into dangerous levels. Wet bulb temperature measures how cool an object (or human skin) can become when moisture evaporates from it. When the air is saturated with moisture, sweat does not evaporate efficiently, and the body’s primary cooling system fails. Heat stress escalates quickly, leading to dehydration, heat exhaustion, heat stroke, and even death if people are exposed without proper cooling or shade. Scientists often note that wet bulb values above about 30 degrees Celsius (86 degrees Fahrenheit) are hazardous. Values above 32 degrees Celsius (90 degrees Fahrenheit) can be life-threatening if exposure is prolonged.

High Air Temperature and Humidity Together

The single most important condition for a dangerous wet bulb event is the combination of high air temperature and high humidity. In isolation, each is uncomfortable. Together, they can create oppressive atmospheric conditions.

High air temperature increases the heat load on the human body. When humidity is low, sweat evaporates easily, helping to maintain a safe body temperature. But when humidity is high, the air cannot accept more moisture, so evaporation slows. This makes the same temperature far more dangerous because sweat cannot cool the body effectively.

Regions of the United States with consistently high humidity, such as the Gulf Coast and the Southeast, are most prone to these conditions during heat waves. The presence of abundant moisture from the Gulf of Mexico and the Atlantic Ocean increases dew points, elevating wet-bulb readings when paired with heat.

Heat Domes and Prolonged High Pressure

One of the primary drivers of extreme heat and humidity combinations is a weather pattern commonly called a heat dome. A heat dome occurs when a persistent area of high atmospheric pressure traps warm air at the surface. This layer of hot air acts like a lid, blocking cooler air and prolonging heat. Heat domes are associated with stagnant atmospheric conditions, which, in turn, allow humidity to build and linger, thereby enhancing wet-bulb values.

During heat dome events, daytime heating is amplified, and nights remain warm because heat cannot escape efficiently. This reduces overnight cooling, keeps humidity high, and increases the likelihood of dangerously high wet bulb temperatures year after year.

Heat domes have been linked to several historically severe heat waves in the United States, including widespread events in the Midwest and Eastern states. These conditions can persist for days or weeks, making tropical conditions more than occasional discomfort and turning them into public health emergencies.

Moisture Sources and Evapotranspiration

Another contributor to humidity and wet bulb events in the United States is moisture from the land itself. In agricultural regions, especially the Corn Belt, crops release significant moisture into the atmosphere through a process called evapotranspiration. During the peak growing season, this natural release of moisture can substantially increase localized humidity. In very hot conditions, such as during a heat dome, this added moisture can push humidity and dew points high enough to dangerously elevate wet-bulb temperatures.

This process is especially pronounced in states like Iowa, Illinois, and Indiana, where expansive fields of crops act like giant moisture sources. The phenomenon, sometimes referred to as “corn sweat,” adds to the moisture load in the air as temperatures climb, setting up conditions that favor high wet-bulb values.

Stalled Weather Patterns and Lack of Cooling Fronts

Weather fronts that move through a region and bring cooler, drier air are an important natural check on heat buildup. When these cooling fronts fail to arrive due to stalled weather patterns, heat and humidity accumulate over several days. Many historic U.S. heat waves were exacerbated by persistent high pressure that prevented cooler air from moving in. In such block patterns, heat and moisture intensify, often without respite overnight, which contributes to elevated wet bulb readings.

Lack of wind under these stagnant conditions also contributes. Wind helps mix the air and promote evaporation, which in turn helps lower humidity in certain circumstances. When air is stagnant, humidity remains elevated, and all the moisture released into the air stays near the surface rather than dispersing. This can maintain wet-bulb temperatures at high levels for longer periods.

Marine and Tropical Influences

The presence of warm water bodies can also contribute to conditions conducive to wet-bulb events. In coastal areas, ocean and gulf currents can deliver warm, moist air inland. Marine heat waves, periods when sea surface temperatures remain well above average, add significant moisture to the air that can later move over land. Warm oceans effectively act as moisture factories during the summer months.

For example, the Gulf of Mexico and the Atlantic along the Southeast and Gulf Coast provide a steady source of humid air as summer progresses. These conditions, combined with high summer temperatures, can elevate wet-bulb temperatures and sustain heat stress inland.

Monsoon Influences in the Southwest

In the Southwest United States, including Arizona and New Mexico, the North American Monsoon season brings moisture from the Pacific and the Gulf of Mexico northward into desert regions. This seasonal transfer of humidity into normally dry air increases dew points and can raise wet-bulb values during summer months. This influx of moisture, combined with already hot desert temperatures, creates a mix that can push wet-bulb temperatures higher than they would in dry heat alone.

Urban Heat Island Effects and Local Climate Amplification

Urban centers can also create conditions that favor higher wet-bulb temperatures. Concrete, asphalt, and buildings absorb and reemit heat, often keeping urban areas warmer at night than surrounding rural areas. This urban heat island effect can prevent cooling overnight. When combined with humidity from surrounding regions or localized moisture sources, such as bodies of water, this effect can keep wet-bulb temperatures elevated for extended periods. Cities like Chicago, St. Louis, and Atlanta have experienced record warm nights during heat waves, which can suppress overnight relief and elevate wet-bulb temperatures for vulnerable populations.

Climate Change and More Frequent Extreme Events

Scientific research indicates that dangerous wet-bulb events are becoming more frequent due to climate change. In a study of global weather station data, NOAA researchers found that extreme wet-bulb temperatures have already increased in frequency and have been detected across the southeastern United States, particularly along the Gulf Coast and into the central South. While rare relative to other heat indicators, these events are not confined to tropical zones alone.

As global temperatures rise, the atmosphere holds more moisture. A warmer atmosphere has a greater capacity to retain water vapor. This means that even without record-high air temperatures, elevated moisture levels can push a wet-bulb temperature higher than would have been possible a few decades ago. The result is more frequent periods when wet-bulb values approach thresholds that can cause severe heat stress.

Recent summers have seen unprecedented dew points in cities like Philadelphia, Washington, D.C., and Baltimore, where frequent humidity spikes have created days of “muggy” conditions that were previously rare. These conditions contribute to elevated wet-bulb temperatures and heat-stress risks across broad areas of the country.

Human Impacts of Wet Bulb Events

The conditions that produce high wet-bulb temperatures, high heat, high humidity, stagnant air, and abundant moisture, directly affect human health and safety. During a wet-bulb event, the body can no longer rely on evaporative cooling to regulate its internal temperature. This increases the risk of heat exhaustion, heat stroke, and other heat-related illnesses. Vulnerable populations such as outdoor workers, older adults, athletes, and people without access to cooling are especially at risk.

To address these risks, workplace safety programs often monitor more than just air temperature. Metrics such as Wet-Bulb Globe Temperature (WBGT), which accounts for humidity and radiant heat, improve risk assessment and help guide decisions around hydration, rest breaks, shading, and work scheduling.

Preparing for Wet Bulb Conditions

Understanding the atmospheric and environmental conditions that lead to wet bulb events helps communities and employers prepare more effectively. This preparation includes:

• Using real-time monitoring tools such as WBGT meters
• Anticipating periods of high humidity and heat during forecasts
• Planning work schedules to minimize exposure during peak conditions
• Providing shaded or cooled rest areas and frequent hydration
• Educating workers and public populations about signs of heat stress

Preparing for a Hotter, More Humid Future