Figure 1-1.—Orographic lift.

Figure 1-1.—Orographic lift. height, but the saturated air within the cloud continues to rise, forming the puffy, cumuliform buildups. Stratiform clouds may form where stable air is brought to saturation by either the addition of moisture or by cooling the air. Most stratiform clouds, however, form when a layer of stable air is forced upward by a lifting 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 lifting the air mass: mechanical lift, convective lift, convergence, and vorticity. Mechanical lift is a process by which a physical barrier forces air aloft. The barrier may be a sloping plain, a rising coastline, or a mountain. Those land barriers cause a type of mechanical lift called orographic lift (fig. 1-1). The barrier may also be air masses of different density; for instance, when fast- moving, warm air overrides the slower moving, cooler air in a warm front, or when fast-moving, cold air forces slower moving warm air aloft in a cold front. Frontal barriers cause a type of mechanical lift known as frontal lift (figs. 1-2 and 1-3). Turbulent lift is mechanical lift caused by friction between the earth’s surface and the air moving above it or between adjacent layers of air in which wind speed (rig. 1-4) or direction is different. Turbulent lift appears to be the key factor in the development of cloud layers with both stratiform and cumuliform characteristics at all levels in the atmosphere. Convective lift is a process that occurs when cool air is heated from the surface and rises (fig. 1-5). Convective lift is the key factor in cumuliform cloud development within an air mass. Convergence occurs when windflow at a particular level forces air to "pile up" in a general area, which creates a lifting action. For instance, where straight-line winds of higher speed decrease, more air is transported into an area than is carried away, and a mass of air builds up vertically. This is known as speed convergence. Alternatively, directional convergence occurs when winds of different directions come together and merge at a certain location. Convergence plays a key role in cumuliform cloud development in the tropical regions. The last lifting mechanism is vorticity. Vorticity is the 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