Clouds – Part 2 (Mechanisms Behind Clouds) In the last segment, we discussed cloud formation in terms of condensation. But we also referred to air rising, many times while it was cooler than surrounding air, giving it no reason to rise. So what causes air to rise? The first, and most obvious reason air rises is because of simple convection. This occurs when air is less dense than the air around it (and usually also warmer), and thus lighter, causing it to rise. Convection was the main mechanism behind rising air discussed in the last segment. The second mechanism is frontal lifting. Frontal lifting occurs along frontal systems, boundaries between two air masses of different densities. The two major kinds of fronts are cold and warm fronts. There are other fronts, such as stationary and quasi-stationary fronts, and in extratropical cyclones, occluded fronts, but for our purposes, cold and warm fronts are the only important fronts. In cold fronts, the dense cool air forms a steep slope. In a warm front, the less dense warm air drifts over the denser cool air. Thus, air diverted by a cold front is pushed upward very steeply, and air in a warm front rises at a shallow angle. For this reason, air lifted by a cold front is more likely to be pushed up to its LCL and then its LFC, where it can rise on its own without help from the front. Cold fronts are associated with thunderstorms, supporting this statement. Air lifted by warm fronts is more likely to reach only its LCL, and warm fronts are associated with light rain from shallow clouds. The third cause of lifting with clouds is orographic lifting. This is a fancy term for lifting due to some form of obstruction, usually a mountain or mountain range. Air cannot simply move through a mountain. Instead, it must move over it. This lifting causes air to cool, sometimes to its LCL, more rarely to its LFC. And to support this claim, I ask you to consider the fact that mountains often have clouds on one side of them. Orographic lifting also gives rise to the Rainshadow Effect. The Rainshadow Effect occurs when air rising over a mountain deposits all its moisture on the side of the mountain. By the time it reaches the peak and crosses onto the far side of the mountain, it is bone dry. This effect has created many deserts around the world. Orographic lifting also gives rise to many unusual cloud formations, one of which is called a lenticular cloud. When air rises over the peak of a mountain, it ripples like water being hit by a stone. The air continues over the peak and sinks, before rising again, repeatedly rising and sinking slightly. This means that the air may repeatedly rise above the LCL and then sink back below it, depositing some of its moisture in the “crest” of each wave. Another result of orographic lifting, usually occurring when the slope of the far side of the mountain is very steep, is the hydraulic jump. When air that has risen over the peak of a mountain begins to sink rapidly, it is forced to become very hot, which compels it to rise rapidly again. This creates not only a downslope wind storm, which holds some of the most powerful winds on earth, but rotor clouds, which are formed when the air rises again, quickly reaching its LCL before sinking again.
Posted on: Wed, 12 Jun 2013 21:39:15 +0000
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