Humidity is a measure of the concentration of water vapor suspended in the air. Its presence makes the difference between a dry heat and the oppressive “muggy” feeling common in the South. Arkansas, classified as having a humid subtropical climate, experiences a particularly notable presence of this moisture, especially during the long summer months. The scientific reasons behind Arkansas’s high humidity involve a continuous, massive supply of moisture, a persistent atmospheric transport system, and local geographic features that trap the damp air.
The Primary Moisture Source
The single largest contributor to Arkansas’s humidity is the Gulf of Mexico, which acts as a vast, continuous reservoir of water vapor. Because of its latitude and the influence of warm ocean currents, the Gulf maintains high sea surface temperatures, particularly in the summer. These waters often reach into the 80s Fahrenheit, which facilitates high rates of evaporation. This sustained evaporation creates an enormous air mass that is consistently warm and heavily saturated with moisture. The resulting air mass, rich in water vapor, is constantly replenished and ready for transport inland toward the central United States.
Air Circulation and Prevailing Winds
The mechanism that transports this Gulf moisture nearly 500 miles north to Arkansas is a persistent, large-scale atmospheric circulation pattern. During the warmer seasons, the dominant wind flow across the central United States is from the south. This southerly flow is responsible for the atmospheric process known as advection, which is the horizontal movement of air masses. A significant component of this transport is the Great Plains low-level jet (GPLLJ), a fast-moving stream of air that operates near the Earth’s surface. The GPLLJ efficiently draws the warm, moist air mass from the Gulf of Mexico directly northward through the Mississippi River Valley.
Local Geography and Inland Hydrology
Once the moisture-rich air reaches Arkansas, local geography helps to trap and concentrate the humidity. Much of eastern Arkansas lies within the Mississippi Alluvial Plain, also known as the Delta, which is a region of remarkably low elevation and flat terrain. This topography allows the moist air carried by the southerly winds to pool and settle over the landscape. The Mississippi River and its major tributaries, including the Arkansas and White Rivers, also add to the local moisture load. These extensive river systems, along with numerous lakes and agricultural fields, provide surfaces for local evaporation and transpiration, supplementing the main flow from the Gulf.