Humidity is a measure of the water vapor suspended in the air. While the common term “relative humidity” changes with temperature, the dew point is a more accurate measurement of the actual moisture content. The dew point is the temperature at which the air becomes completely saturated. When the dew point rises, the air feels increasingly uncomfortable and muggy, which is the experience New York residents know well. This region experiences high humidity levels, particularly between early June and late September.
The Influence of Coastal Geography
New York’s humid climate is due to its coastal geography, providing a nearly inexhaustible supply of water vapor. The region is situated directly on the Atlantic Ocean, which acts as a vast reservoir continuously feeding moisture into the atmosphere through evaporation. The ocean’s enormous surface area ensures that any air mass moving toward the coast is constantly being refreshed with water molecules.
This baseline of moisture is amplified by the numerous surrounding water bodies integral to the New York landscape. Long Island Sound, the New York/New Jersey Harbor, the Hudson River Estuary, and various bays contribute a significant volume of local evaporation. These bodies of water ensure that local air circulation patterns are saturated, setting a consistently high floor for the region’s humidity levels.
Atmospheric Systems Transporting Moisture
While local water bodies provide a constant source of moisture, the primary driver of New York’s oppressive summer humidity is a powerful, large-scale weather phenomenon. The semi-permanent Bermuda High-Pressure System, a massive area of high pressure anchored in the western Atlantic Ocean, sets the stage for the region’s summer weather. This high-pressure system operates with a slow, clockwise circulation that acts like an atmospheric pump.
The circulation around the western edge of the Bermuda High directs a continuous flow of air from the south and southwest straight up the East Coast. This air mass originates over the tropical Atlantic and the warm waters of the Gulf of Mexico, picking up tremendous amounts of moisture as it travels. The warm, tropical maritime air is then shunted northward, directly into the New York region, resulting in the characteristic “hazy, hot, and humid” conditions.
The strength and position of the Bermuda High directly correlate with the severity of the humidity in any given summer. When the system is strong and expands further north and west, it establishes long-lasting periods of southerly surface winds that lock in the heat and moisture. This continuous flow of tropical air is the dynamic mechanism responsible for the most uncomfortable stretches of muggy weather, often pushing dew points into the truly oppressive range.
How Local Conditions Intensify the Feeling
The feeling of stifling humidity is amplified within New York City itself by the Urban Heat Island (UHI) effect. The built environment of the city—a dense landscape of concrete, asphalt, and tall buildings—absorbs and retains solar heat throughout the day more effectively than natural landscapes. This stored thermal energy is then slowly released back into the atmosphere, especially at night, preventing the air from cooling down significantly.
This trapped heat means the air has a greater capacity to hold moisture, but it also inhibits the body’s natural cooling process. Human comfort depends on sweat evaporating from the skin, a process that becomes difficult in air already saturated with moisture. The city’s lack of widespread green space reduces the cooling effect that comes from evapotranspiration, where plants release water vapor. This combination of trapped heat and suppressed evaporation can make the air feel several degrees hotter than in surrounding rural areas.