![]() These upward and downward currents or vertical eddies within the PBL transfer mass and energy upward from the surface. Convection and the prospect of breaking through the top of the PBL vary markedly across such heterogeneous landscapes. Cities, suburbs, fields, forests, lakes, and shopping centres both heat and evaporate water into the air of the PBL according to the nature of the surfaces involved. The landscapes of most human-dominated ecosystems are decidedly “patchy” in their geography. The result is a dynamic synergy between the atmosphere and biosphere. As the ratio of diffuse sunlight to direct beam sunlight increases, the rate of photosynthesis rises, and greater levels of biological productivity are favoured in the biosphere below. The small clouds just above the PBL are known as planetary boundary layer clouds. The cooling of the lifting air initiates the condensation of water vapour and the development of miniscule particles of liquid water called cloud droplets. ![]() If convective mixing of the air in the PBL is vigorous, convection currents may penetrate through the temperature inversion at the top of the PBL. By contrast, during the months before the spring leafing, the PBL thickens from solar heating as the sun rises higher in the sky and day length increases. In New England forests during the days following the spring leafing, it has been shown that the top of the PBL is lowered to between 200 and 400 metres (650 and 1,300 feet). For every 1 ☌ (1.8 ☏) increase in daily maximum surface temperature for a well-mixed PBL, the top of the PBL is elevated 100 metres (about 325 feet). The wetter the air advected into the region and the greater the additional water added by evaporation and transpiration, the lower the height of the top of the PBL. This sublayer, known as the planetary boundary. The lower levels of the troposphere are usually strongly influenced by Earth’s surface. In contrast, the PBL is less than 1,000 metres (3,300 feet) thick over ocean areas, since little surface heating takes place there because of the vertical mixing of water. Over deserts, the PBL may extend up to 4,000 or 5,000 metres (13,100 or 16,400 feet) in altitude. In general, the greater the heating of the surface, the deeper the PBL. The thickness of the PBL depends on the intensity of this surface heating and the amount of water evaporated into the air from the biosphere. During the day, the air within the PBL is thoroughly mixed by convection induced by the heating of Earth’s surface, and the very top of the PBL is an entrainment zone characterized by sporadic and weakening turbulence. The top of the PBL can also be denoted by a thin layer of haze often seen by passengers aboard airplanes during takeoff from airports. The boundary between the cooler PBL below and the warmer layer above can be visually marked by the base of the clouds in the area. The PBL is covered by a layer of warmer air, creating what is known as a temperature inversion. For this reason, the planetary boundary layer has also been called an Ekman layer, for Swedish oceanographer Vagn Walfrid Ekman, a pioneer in the study of the behaviour of wind-driven ocean currents. Planetary boundary layer (PBL), also called atmospheric boundary layer, the region of the lower troposphere where Earth’s surface strongly influences temperature, moisture, and wind through the turbulent transfer of air mass.Īs a result of surface friction, winds in the PBL are usually weaker than above and tend to blow toward areas of low pressure. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! ![]() ![]()
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