The sticky, oppressive feeling of summer is directly related to the amount of water vapor suspended in the air, known as humidity. The more accurate measure for human discomfort is the dew point—the temperature at which the air becomes completely saturated and water vapor condenses into liquid. When the dew point is high, typically above 65°F, the atmosphere holds a significant volume of moisture, leading to the muggy conditions we experience. This current spell of high moisture results from a combination of immediate large-scale weather patterns, localized environmental factors, and long-term atmospheric changes.
Immediate Meteorological Causes
The movement of large air masses is the primary reason for transporting tropical moisture into a region. A major factor influencing summer humidity across the eastern and central United States is the position and strength of the semi-permanent high-pressure system known as the Bermuda High. This massive atmospheric feature, which rotates clockwise, acts like a powerful pump on its western edge, drawing air from the warm waters of the Atlantic Ocean and the Gulf of Mexico northward or inland.
This sustained southerly flow continuously funnels warm, moisture-laden air deep into the continent. If this high-pressure ridge remains anchored in place for an extended period, it establishes a persistent weather pattern that prevents cooler, drier air from moving in to offer relief. This constant advection of highly humid air establishes a high baseline of atmospheric moisture over the region.
A second atmospheric factor involves the jet stream, the ribbon of fast-moving air high in the atmosphere. When the jet stream develops an unusually wavy or meandering pattern, often referred to as an “atmospheric block,” it can become stuck or stalled. This immobility effectively locks the moisture-laden air masses in place over a region, preventing the normal progression of weather fronts that would typically sweep the humidity away.
How Local Conditions Exacerbate Humidity
While large-scale weather systems deliver the initial moisture, local surface conditions intensify the humidity at ground level. Recent periods of heavy rainfall can saturate the ground, which then contributes a substantial amount of water vapor back into the lower atmosphere through evaporation. This process is particularly efficient because the moisture is readily available in the topsoil and is quickly drawn out by solar energy.
Proximity to large bodies of water, such as major rivers or lakes, naturally increases the local moisture content of the air compared to more arid, inland areas. This constant surface evaporation provides a local source of water vapor that feeds directly into the atmosphere above.
Vegetation and Transpiration
In addition, the presence of lush vegetation, especially during peak growing season, significantly contributes to atmospheric moisture. Plants absorb water from the soil and release it into the air as vapor through a process called transpiration. Extensive fields of crops, dense forests, and even urban green spaces are constantly moving water from the ground into the air, adding to the total atmospheric water content.
Long-Term Trends and Warming Air
The ability of the atmosphere to hold water vapor is fundamentally linked to temperature through a physical principle known as the Clausius-Clapeyron relation. This relation dictates that for every 1°C (1.8°F) rise in air temperature, the atmosphere’s capacity to hold water vapor increases by approximately 7%.
As global temperatures and sea surface temperatures continue to rise, the baseline moisture content of the atmosphere also increases. This means that the air masses they are transporting now start with a significantly higher volume of water vapor than they did decades ago. This higher starting point leads to more extreme humidity events when the right weather patterns occur.
The warming of the Arctic, which is occurring at a rate faster than the rest of the planet, also plays a role in the persistence of these events. This rapid warming reduces the temperature difference between the pole and the equator, which is the driving force of the jet stream. A less energetic jet stream becomes slower and more prone to stalling, increasing the likelihood that a humid air mass will linger over a single region.
Practical Effects on Comfort and Health
The human body cools itself primarily through the evaporation of sweat from the skin. When the air is highly saturated with moisture, the rate at which sweat can evaporate slows dramatically because the air simply cannot absorb much more water vapor. This impairment of the body’s natural cooling mechanism is what makes high humidity feel so uncomfortable.
The Heat Index was created to quantify this effect, representing how hot the air actually feels when humidity is factored in with the air temperature. When the body cannot cool itself effectively, its internal temperature can rise, placing significant strain on the cardiovascular system as the heart works harder to circulate blood to the skin. This can lead to serious health risks, including heat exhaustion and the potentially dangerous condition of heat stroke.
Beyond the immediate heat-related risks, high humidity exacerbates several indoor and respiratory issues. Moist air promotes the rapid growth of biological allergens, such as mold, mildew, and dust mites, leading to poorer indoor air quality. For individuals with existing conditions like asthma or chronic obstructive pulmonary disease, high moisture can cause airways to constrict, leading to breathing difficulties and the worsening of symptoms.