The summer experience in Nebraska often includes a sense of oppressive mugginess that seems out of place for a landlocked state. Humidity, the amount of water vapor present in the air, is responsible for this discomfort. The measurement most relevant to this sensation is the dew point, which indicates the temperature at which the air becomes saturated with water vapor. High summer dew points in Nebraska result from a unique meteorological and geographical alignment that constantly pumps massive amounts of moisture into the region. This phenomenon is a combination of a distant water source, a powerful atmospheric delivery system, and specific local factors that trap and add to the moisture content.
The Primary Source of Nebraska’s Moisture
Despite being far from any ocean, the primary source of Nebraska’s summer moisture is the warm, expansive body of water known as the Gulf of Mexico. The Gulf waters are consistently warm during the summer months, which encourages a high rate of evaporation into the atmosphere above it. This continuous evaporation creates a massive reservoir of water vapor available for transport northward across the central United States.
The warm sea surface temperatures of the Gulf of Mexico ensure that the air mass originating there is exceptionally moist. This air, rich with water vapor, is constantly being fed into the atmospheric circulation patterns that dominate the central continent. This vast moisture source dictates the maximum potential humidity that can be delivered to the Great Plains. The Gulf’s influence is so significant that it has been identified as the key moisture source for the Great Plains Low-Level Jet, the system that carries this moisture inland.
The Atmospheric Delivery System
The mechanism for transporting this Gulf moisture all the way to Nebraska is the Great Plains Low-Level Jet (LLJ), a rapid, low-altitude wind current that functions like a meteorological conveyor belt. This jet is characterized by a strong southerly flow that accelerates toward the north, typically operating within the lowest kilometer of the atmosphere. The LLJ is especially active at night, often reaching its maximum intensity in the early morning hours, which explains why the humidity often feels highest just before sunrise.
The Great Plains Low-Level Jet can rapidly transport warm, humid air masses from the Gulf of Mexico across the flat terrain of the central U.S. at speeds exceeding 25 to 50 knots. This powerful wind current is responsible for 70% to 80% of the moisture transport occurring in the southern Great Plains during a jet event. The flat topography of the plains offers little resistance, allowing the humid air to be carried far inland. This efficient, nocturnal delivery system allows Nebraska to experience dew points comparable to coastal regions.
Local Factors Amplifying Humidity
Once the moisture-laden air arrives, local environmental conditions specific to Nebraska amplify and trap the imported humidity. A significant contributor is the widespread practice of agricultural irrigation, particularly for corn crops, making Nebraska the leading state in the nation for irrigated acreage. This extensive irrigation leads to a process called evapotranspiration, where water is added to the local boundary layer through evaporation from the soil and transpiration from the plants.
The scale of this human-driven moisture addition is substantial, with irrigated fields generating significantly more evapotranspiration than non-irrigated land. In counties with the largest expansion of irrigation, relative humidity has been observed to increase at a rate three times faster than in less irrigated areas. This local moisture traps the imported Gulf air, leading to higher dew point temperatures and a muggier feel.
Furthermore, the lack of significant mountain barriers, such as the Rocky Mountains to the west, means there are no geographical features to force the humid air to rise and condense into precipitation. The relatively flat landscape allows the humid air mass to settle and remain stagnant. Persistent summer high-pressure systems further exacerbate the issue by suppressing vertical air movement. This suppression acts as a lid, holding the imported and locally generated moisture close to the surface, which prevents it from dispersing and results in the lingering, high summer humidity throughout much of the state.