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Chapter 6

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McMaster University
Environmental Science
Sean Mc Master

Chapter 6: Cloud development and forms Mechanisms that lift air  4 mechanisms lift air so that condensation and cloud formation can occur 1. Ist 2. Orographic lifting, focusing air above a mountain barrier 3. Frontal lifting, displacement of one air mass over another 4. Convergence, horizontal movement of air into an area at low levels 5. Localized convective lifting due to buoyancy Orographic Uplift  Air flowing toward a hill or mountain will be deflected around and over the barrier  The upward displacement of air that leads to adiabatic cooling and possible cloud formation is called Orographic uplift ( Orographic effect)  The height to which these Orographic clouds can rise is not limited to the height of the hill or mountain, their tops can extend many hundreds of meters higher and even into the lower stratosphere  The height of the cloud tops is strongly related to charactertics of their that vary from day to day  Downwind of a mountain ridge, on its leeward side, air descends the slope and warms by compression to create a rain shadow effect, an area of lower precipitation  The sierra Nevada mountain range provides a dramatic illustration of this effect  The ridge crest of the sierra runs north to south and is essentially perpendicular to the predominantly west-to-east airflow  With much of the range being higher than 3500 m precipitation on the eastern windward side is greatly enhanced because of Orographic lifting  The eastern slope, of the range is extremely steep in the valley floor is low, sometimes below sea level, therefore the descending air on the leeward slope creates one of the strongest rain shadow effects on Earth Frontal Lifting  Sometimes, however transition zones exist in which great temperature differences occur across relatively shrot distances  These transition zones called “fronts” are not like vertical walls separating arm and cool air but rather slope gently  Air flow along frontal boundaries results in the widespread development of clouds in either of two ways  When cold air advances toward warmer air ( a situation called a cold front) the denser cold air displaces the lighter warm air ahead of it  When warm air flows toward a wedge of cold air (a warm front) the warm air is forced upward in much the same way that the Orographic effect causes air to rise above the mountain barrier Convergence  Because the mass of the atmosphere is not uniformly distributed across Earth’s surface, large areas of high and low surface pressure exist.  These pressure differences set the motion in the familiar effect we call the wind  Not surprisingly, the pattern of wind that results is very much related to the pattern of pressure  In particular, when a low-pressure cell is near the surface winds in the lower atmosphere tend to converge on the center of the low from all directions  Horizontal movement toward a common location is called horizontal convergence, or just convergence for short  No instead vertical motions near the center of the low carry away about as much mass as is carried in  Air will rise  This will be explained in more detail later, but for now we can just make the connection between low-level convergence between low-level convergence and rising air with adiabatic cooling Localized Convection  That free convection is lifting that results from heating the air near the surface  It is often accompanied by updrafts strong enough to form clouds and precipitation  During the warm season, heating of Earth’s surface can produce free convection over a fairly limited area and create the brief afternoon thunderstorms that disrupt summer picnics  In Canada and the United States east of the rocky Mountains, high moisture content of air sometimes allows for tall clouds with bases at relatively low altitudes  Such conditions favor vigorous precipation over small regions free convection by its nature does not create updrafts more than several tens of meters in deiamter  Even in the deserts of the Southwest, which are usually low in water vapor, intense heating can lead to localized convection intense enough to cause thundershowers  Free convection arises from buoyancy, the tendency for a lighter fluid to float upward through a denser one  By itself, buoyancy can initate uplift  But buoyancy can also speed or slow the uplift begun by the orogrpahic effect, frontal lifting and convergence Static Stability and the Environment Lapse Rate  Sometimes the atmosphere is easily displaced and an air parcel given an initial boost upward continues to rise even after the original lifting process ceses  At other times, the atmosphere resists such lifting  The air’s susceptibility to uplift is called its static stability  Statically unstable air becomes buoyant when lifted and continues to rise if given an initial upward push; statically neutral air neither rises on its own following an initial life not sinks back to its original level; it simply comes to rest at the height to which it was displaced  Static stability is closely related to buoyancy  When a parcel of air is less dense than the air around it, it has a positive buoyancy and floats upward,  Air that is denser than its surroundings sinks if not subjected to continued lifting forces  Density differences between a parcel and its surroundings are determined by their temperatures  If the parcel is warmer than the surrounding air, it will be less dense and experience a lifting force  If it is colder, it will be more dense and have “negative “ buoyancy  If a rising parcel cools at a rate that makes it colder than the surrounding air, it will become warm relative to the surrounding air  Temperatures in the parcel are governed by either the dry or satured adiabatic lapse rate, whereas the surrounding are governed by the environemental lapse rate  Consier a parcel of air near the surface that is lifted through the surrounding air  The lifted air cools at one of the adiabatic lapse rates, and the surrounding air maintains its original temperature profile  The relative density of the rising parcel thus depends on two conditions 1. Whether or not the parcel is saturated 2. The ELR  These factors combine to produce different types of air with regard to their static stability  These are absolutely unstable, absolutely stable and conditionally unstable Absolutely Unstable Air  What happens when a parcel of unsaturated air is lifted and the ELR is greater than the dry adiabatic lapse rate (DALR)  The rising air is cooling more slowly than its surroundings  The lifted parcel is becoming progressively warmer and more buoyant than the surrounding air  The air in this instance is said to be absolutely unstable because once a parcel within is lifted, it continues to move upward  Not only does the parcel within is lifted it continues to move upward  Not only does the parcel rise, but it does so at an ever-increasing speed  This occurs because the temperature difference between it and the surrounding air continually increases, leading to greater buoyancy and also because it gathers momentum as it rises  LEF is till 1.5*C/100 m but he air is now saturated  The lifted parcel of air therefore cools more slowly at the saturated adiabatic lase rate (SALR)and will be warmer than an unsaturated parcel  The temperature difference between the warm parcel and colder surrounding air is greater, giving rise to a stronger buoyant force  Whenever the environmental lapse rate exceeds the dry adiabatic lapse rate, the air is absolutely unstable and a parcel contained within it will continue to rise once lifted, regardless of whether or not it is saturated Absolutely Stable Air  What happens when the ELR is less than the saturated adiabatic lapse rate  When a parcel of unsaturated air rises, its temperature drops more rapidly than the temperature of the air around it, making the parcel relatively heavier and less buoyant  Because of its negative buoyancy, the lifted air will sink back to its initial level if the lifting mechanism stops  Such air is absolutely stable  the saturated parcel becomes colder than the air around it  like the unsaturated parcel , it has a tendency to sink back to its original position  whenever the environmental lapse rate is less than the saturated adiabatic lapse rate, the air will be absolutely stable and will resist lifting ,regardless of whether or not it is saturated  note that it is possible for the ELR to be such that the temperature does not change at all with height or even for the temperature to increase with height Conditionally Unstable Air  the preceding examples describe what happens when the ELR is less than the SALR is greater than the DALR  but what happens when the ELR is between the dry and saturated adiabetic lapse rate  in this environment, the air is said to be conditionally unstable and the tendency for a lifted parcel to sink or continue rising depends on whether or not it becomes saturated and how far it is lifted  how the same environment lapse rate can lead to a lifted parcel of air becoming buoyant relative to its surroundings if it is lifted to a great enough height  it cools at the DALR until it reaches saturation at the 100 m level, where T=Ta=90*C  there the lifted parcel is colder and denser than the surrounding air; it is not buoyant and will not rise further unless something forces it to  if the parcel is lifted farther it will cool at the saturated adiabatic rate which is less than the ELR  at the 200 m level, the lifted parcel is still colder than the surrounding air is less than it was at the 100 m level  if the, it then becomes parcel is lifted to the 300 m level it then becomes warmer than the ambient air  the lifted parcel thus becomes buoyant and will now rise on its own, even in the absence of external lifting  thus if the atmosphere is conditionally unstable an air parcel becomes buoyant if lifted above some critical altitude , that altitude called the level of free convection is the height to which a parcel of air must be lifted for it to become buoyant and to rise on its own  the condition in the term conditionally unstable refers to a parcel’s ability to become buoyant only if it is lifted to some particular level  air not lifted to that level doesn’t become buoyant and will rise only if its is lifted to some particular level  air not lifted to that level doesn’t become buoyant and will rise only if subjected to some other lifting mechanism  when a parcel of conditionally unstable air rises above that level, it is common for clouds to rapidly increase in depth and yield precipitation Static and Potential Instability  the types of static stability in this section pertain to a parcel of air’s ability to rise when subjected to uplifting mechanisms  potential instability: sometimes called convective instability  while the static stability describes what would happen to a small parcel of air that is lifted or lowered while the surrounding air is kept in place, potential stability describes what happens when entire layers of air are displaced upward  when such layers are displaced upward, their environmental lapse rates may be altered so that statically stable conditions within the layer can change to statically unstable conditions, or vise versa  potential instability is a feature that is particularly important with the forecasting of sever thunderstorms Factors influencing the environmental lapse rate  ELRs are highly variable in space and time  Just as the surface air temperature at a location is subject to change so is the vertical temperature profile  The following 3 factors can
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