Warm, gusty afternoons and cool, calm mornings during fair weather over many midlatitude land surfaces create a daily cycle that is comfortably familiar. But we forget that such meteorological variations are not observed above about 2 kilometers from the Earth's surface. Below that altitude, the air feels the direct influence of the Earth's surface; and daily changes of temperature, wind, and other variables are driven by solar heating, infrared cooling, evaporation, carbon dioxide exchange, and frictional drag. This layer of air influenced by the ground is called the atmospheric boundary layer.