Figure 1-1.—Orographic lift.height, but the saturated air within the cloud continuesto rise, forming the puffy, cumuliform buildups.Stratiform clouds may form where stable air isbrought to saturation by either the addition of moistureor by cooling the air. Most stratiform clouds, however,form when a layer of stable air is forced upward by alifting mechanism. The entire layer cools as it is lifted,reaches saturation, and forms a cloud layer.There are four processes that cool the air by liftingthe air mass: mechanical lift, convective lift,convergence, and vorticity.Mechanical lift is a process by which a physicalbarrier forces air aloft. The barrier may be a slopingplain, a rising coastline, or a mountain. Those landbarriers cause a type of mechanical lift calledorographic lift (fig. 1-1). The barrier may also be airmasses of different density; for instance, when fast-moving, warm air overrides the slower moving, coolerair in a warm front, or when fast-moving, cold air forcesslower moving warm air aloft in a cold front. Frontalbarriers cause a type of mechanical lift known as frontallift (figs. 1-2 and 1-3). Turbulent lift is mechanical liftcaused by friction between the earth’s surface and the airmoving above it or between adjacent layers of air inwhich wind speed (rig. 1-4) or direction is different.Turbulent lift appears to be the key factor in thedevelopment of cloud layers with both stratiform andcumuliform characteristics at all levels in theatmosphere.Convective lift is a process that occurs when cool airis heated from the surface and rises (fig. 1-5).Convective lift is the key factor in cumuliform clouddevelopment within an air mass.Convergence occurs when windflow at a particularlevel forces air to "pile up" in a general area, whichcreates a lifting action. For instance, where straight-linewinds of higher speed decrease, more air is transportedinto an area than is carried away, and a mass of air buildsup vertically. This is known as speed convergence.Alternatively, directional convergence occurs whenwinds of different directions come together and mergeat a certain location. Convergence plays a key role incumuliform cloud development in the tropical regions.The last lifting mechanism is vorticity. Vorticity isthe rotational motion of molecules in the atmosphere,Figure 1-2.—Frontal lift—conditionally unstable air causing cumuliform cloud development along a cold front.1-4
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