BOUNDARY LAYER (ATMOSPHERIC) AND AIR POLLUTION | Diurnal Cycle

The diurnal cycle over land is driven by solar heating in the daytime, and longwave cooling at night. In summer, the maximum temperature decreases with increasing cloud cover, because clouds reflect sunlight. In winter, the minimum temperature falls steeply under clear skies, because clouds reduce the longwave cooling to space. Over moist soils, increased evaporation reduces the diurnal temperature and humidity ranges. A few hours after sunrise, there is a transition when the nighttime stable layer is eroded by surface heating. Carbon dioxide shows a dawn maximum as nighttime respiration is trapped near the surface, and an afternoon minimum.