Class Note for NATS 101 at UA
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Date Created: 02/06/15
Lesson 4 Vertical Motion and Atmospheric Stability This lesson describes the vertical structure of the atmosphere atmospheric stability and the corresponding vertical motion Adiabatic diagrams are introduced to help explain atmospheric conditions affecting pollutant dispersion Goal To familiarize you with the vertical temperature structure of the atmosphere and to introduce its relationship to plume dispersion Objectives Upon completing this lesson you will be able to do the following 1 Explain the concept of buoyancy 2 De ne lapse rate and distinguish between dry adiabatic wet adiabatic and environmental lapse rates 3 Describe stable unstable and neutral conditions 4 Given an adiabatic diagram identify the atmospheric stability category represented 5 Describe how atmospheric stability and inversions affect air pollutant dispersion 6 Describe how four different types of inversions form 7 List ve types of plume behavior and relate each to atmospheric conditions Introduction The previous lesson discusses horizontal motion of the atmosphere Vertical motion is equally important in air pollution meteorology for the degree of vertical motion helps to determine how much air is available for pollutant dispersal Vertical motions can be attributed to high and low pressure systems air lifting over terrain or fronts and convection There are a number of basic principles related to vertical motion that you must be familiar with before you can understand the mechanics and conditions of vertical motion These principles are presented rst and are followed by discussions of instability stability and plume behavior Inversion where the temperature of the air increases with height is also discussed Principles Related to Vertical Motion Parcel Throughout this lesson we will be discussing a parcel of air This theoretically in nitesimal parcel is a relatively wellde ned body of air a constant number of molecules that acts as a whole Selfcontained it does not readily miX with the surrounding air The exchange of heat between the parcel and its surroundings is minimal and the temperature within the parcel is generally uniform The air inside a balloon is an analogy for an air parcel Buoyancy Factors Atmospheric temperature and pressure in uence the buoyancy of air parcels Holding other conditions constant the temperature of air a uid increases as atmospheric pressure increases and conversely decreases as pressure decreases With respect to the atmosphere where air pressure decreases with rising altitude the normal temperature pro le of the troposphere is one where temperature decreases with height An air parcel that becomes warmer than the surrounding air for example by heat radiating from the earth s surface begins to eXpand and cool As long as the parcel s temperature is greater that the surrounding air the parcel is less dense than the cooler surrounding air Therefore it rises or is buoyant As the parcel rises it eXpands thereby decreasing its pressure and therefore its temperature decreases as well The initial cooling of an air parcel has the opposite effect In short warm air rises and cools while cool air descends and warms The eXtent to which an air parcel rises or falls depends on the relationship of its temperature to that of the surrounding air As long as the parcel s temperature is greater it will rise as long as the parcel s temperature is cooler it will descend When the temperatures of the parcel and the surrounding air are the same the parcel will neither rise nor descend unless in uenced by wind ow Lapse Rates The lapse rate is de ned as the rate at which air temperature changes with height The actual lapse rate in the atmosphere is approximately 76 to 77 C per km in the troposphere but it varies widely depending on location and time of day We de ne a temperature decrease with height as a negative lapse rate and a temperature increase with height as a positive lapse rate How the atmosphere behaves when air is displaced vertically is a function of atmospheric stability A stable atmosphere resists vertical motion air that is displaced vertically in a stable atmosphere tends to return to its original position This atmospheric characteristic determines the ability of the atmosphere to disperse pollutants emitted into it To understand atmospheric stability and the role it plays in pollution dispersion it is important to understand the mechanics of the atmosphere as they relate to vertical atmospheric motion Dry Adiabatic For the most part a parcel of air does not exchange heat across its boundaries Therefore an air parcel that is warmer than the surrounding air does not transfer heat to the atmosphere Any temperature changes that occur within the parcel are caused by increases or decreases of molecular activity within the parcel Such changes occur adiabatically and are due only to the change in atmospheric pressure as a parcel moves vertically An adiabatic process is one in which there is no transfer of heat or mass across the boundaries of the air parcel In an adiabatic process compression results in heating and expansion results in cooling A dry air parcel rising in the atmosphere cools at the dry adiabatic rate of 98 C1000m and has a lapse rate of 798 C 1000m Likewise a dry air parcel sinking in the atmosphere heats up at the dry adiabatic rate of 98 C1000m and has a lapse rate of 98 C1000m Air is considered dry in this context as long as any water in it remains in a gaseous state The dry adiabatic lapse rate is a xed rate entirely independent of ambient air temperature A parcel of dry air moving upward in the atmosphere then will always cool at the rate of 98 C 1000 m regardless of its initial temperature or the temperature of the surrounding air You will see later that the dry adiabatic lapse rate is central to the de nition of atmospheric stability A simple adiabatic diagram demonstrates the relationship between elevation and temperature The dry adiabatic lapse rate is indicated by a broken line as shown in Figure 41 beginning at various temperatures along the horizontal axis Remember that the slope of the line remains constant regardless of its initial temperature on the diagram 2 I I I I I I I I I E 3 g I Ta 1 gt 2 L I l I l I J I l I 10 20 30 40 50 0 Temperature C FIgure 41 Dry adiabatic lapse rate Wet Adiabatic A rising parcel of dry air containing water vapor will continue to cool at the dry adiabatic lapse rate until it reaches its condensation temperature or dew point At this point the pressure of the water vapor equals the saturation vapor pressure of the air and some of the water vapor begins to condense Condensation releases latent heat in the parcel and thus the cooling rate of the parcel slows This new rate called the wet adiabatic lapse rate is shown in Figure 42 Unlike the dry adiabatic lapse rate the wet adiabatic lapse rate is not constant but depends on temperature and pressure In the middle troposphere however it is assumed to be approximately 76 to 77 C 1000 m Elevation km Temperature C Figure 42 Wet adiabatic lapse rate Environmental As mentioned previously the actual temperature pro le of the ambient air shows the environmental lapse rate Sometimes called the prevailing or atmospheric lapse rate it is the result of complex interactions of meteorological factors and is usually considered to be a decrease in temperature with height It is particularly important to vertical motion since surrounding air temperature determines the eXtent to which a parcel of air rises or falls As Figure 43 shows the temperature pro le can vary considerably with altitude sometimes changing at a rate greater than the dry adiabatic lapse rate and some times changing less The condition when temperature actually increases with altitude is referred to as a temperature inversion In Figure 44 the temperature inversion occurs at elevations of from 200 to 350 m This situation is particularly important in air pollution because it limits vertical air motion Elevation km Elevation km Figure 44 Temperature C Temperature inversion 39 39 I l l l EnVIronmental lapse rate l l l l l l l l l l l l l l Dry adiabatic lapse rate l l l l l I 7 I 10 20 30 40 50 Temperature C Figure 43 ErIVIronmental lapse rate l l l l l l l l l l l l l Temperature InverSIon l l l l l l l l l l l l l l l l l l l I I I 2 4 6 8 10 Mixing Height Remember the analogy of the air parcel as a balloon Figure 45 shows three ways in which the adiabatic lapse rate affects buoyancy In each situation assume that the balloon is lled at ground level with air at 20 C then lifted manually to a height of 1 km for example lifted by the wind over a mountain ridge The air in the balloon will expand and cool to about 10 C Whether the balloon rises or falls upon release depends on the surrounding air temperature and density In situation quotAquot the balloon will rise because it remains warmer and less dense than the surrounding air In situation quotBquot it will sink because it is cooler and more dense In situation quotCquot however it will not move at all because the surrounding air is the same temperature and density Elevation km Temperature C Figure 45 Relationship of adiabatic lapse rate to airtemperature The same principles apply in real atmospheric conditions when an air parcel is heated near the surface and rises and a cool parcel descends to take its place The relationship of the adiabatic lapse rate and the environmental lapse rate should now be apparent The latter controls the extent to which a parcel of air can rise or descend In an adiabatic diagram as shown in Figure 46 the point at which the air parcel cooling at the dry adiabatic lapse rate intersects the ambient temperature pro le line is known as the mixing height This is the air parcel s maximum level of ascendance In cases where no intersection occurs when the environmental lapse rate is consistently greater than the adiabatic lapse rate the mixing height may extend to great heights in the atmosphere The air below the mixing height is the mixing layer The deeper the mixing layer the greater the volume of air into which pollutants can be dispersed 2 u Environmental lapse rate Dry adiabatic E mega Mixing height E 3 1 m gt 4 Airparcel with surface temperature of 30 10 20 30 40 50 Temperature C Figure 46 Mixing height Atmospheric Stability The degree of stability of the atmosphere is determined by the temperature difference between an air parcel and the air surrounding it This difference can cause the parcel to move vertically ie it may rise or fall This movement is characterized by four basic conditions that describe the general stability of the atmosphere In stable conditions this vertical movement is discouraged whereas in unstable conditions the air parcel tends to move upward or downward and to continue that movement When conditions neither encourage nor discourage air movement beyond the rate of adiabatic heating or cooling they are considered neutral When conditions are extremely stable cooler air near the surface is trapped by a layer of warmer air above it This condition called an inversion allows virtually no vertical air motion These conditions are directly related to pollutant concentrations in the ambient air Unstable Conditions Remember that an air parcel that begins to rise will cool at the dry adiabatic lapse rate until it reaches the dew point at which point it will cool at the wet adiabatic lapse rate This assumes that the surrounding atmosphere has a lapse rate greater than the adiabatic lapse rate cooling at more than 98 C 1000 In so that the rising parcel will continue to be warmer than the surrounding air This is a superadiabatic lapse rate As Figure 47 shows the temperature difference between the actual environmental lapse rate and the dry adiabatic lapse rate actually increases with height and buoyancy is enhanced Temperature difference Elevation krn Dry adiabatic lapse rate Environmental lapse rate 10 20 30 40 50 Tem perature C Figure 47 Enhanced buoyancy associated with instability superadiabatic lapse rate As the air rises cooler air moves underneath It in turn may be heated by the earth s surface and begin to rise Under such conditions vertical motion in both directions is enhanced and considerable vertical mixing occurs The degree of instability depends on the degree of difference between the environmental and dry adiabatic lapse rates Figure 48 shows both slightly unstable and very unstable conditions Dry adiabatic lapse rate Elevation km s t u t t t t t 10 20 30 40 50 Temperature C Figure 48 Unstable conditions Unstable conditions most commonly develop on sunny days with low Wind speeds where strong insolation is present The earth rapidly absorbs heat and transfers some of it to the surface air layer There may be one buoyant air mass if the thermal properties of the surface are uniform or there may be numerous parcels if the thermal properties vary The air warms becomes less dense than the surrounding air and rises Another condition that may lead to instability is the cyclone low pressure system which is characterized by rising air clouds and precipitation Neutral Conditions When the environmental lapse rate is the same as the dry adiabatic lapse rate the atmosphere is in a state of neutral stability Figure 49 Vertical air movement is neither encouraged nor hindered The neutral condition is important as the dividing line between stable and unstable conditions Neutral stability occurs on Windy days or when there is cloud cover such that strong heating or cooling of the earth s surface is not occurring Elevation km 40 50 Temperature C Figure 49 Neutral conditions Stable Conditions When the environmental lapse rate is less than the adiabatic lapse rate cools at less than 98 C 1000 m the air is stable and resists vertical motion This is a subadiabatic lapse rate Air that is lifted vertically will remain cooler and therefore more dense than the surrounding air Once the lifting force is removed the air that has been lifted will return to its original position Figure 410 Stable conditions occur at night when there is little or no wind 2 I Very stable Elevation krn adiabatic lapse rate 10 20 30 40 50 Temperature C Figure 410 Stable conditions Conditional Stability and Instability In the previous discussion of stability and instability we have assumed that a rising air parcel cools at the dry adiabatic lapse rate Very often however the air parcel becomes saturated reaches its dew point and begins to cool more slowly at the wet adiabatic lapse rate This change in the rate of cooling may change the conditions of stability Conditional instability occurs when the environmental lapse rate is greater than the wet adiabatic lapse rate but less than the dry rate This is illustrated in Figure 411 Stable conditions occur up to the condensation level and unstable conditions occur above it 2 l l I I 39 I 39 39 Wet adiabatic lapse rate 6 Ckm Unstable atmosphere C O a 2 x Dry adiabatic LIJ lapse rate Environmental 3998 COM stable lapse rate atmosphere 7 Ckm s I I I I I I 10 20 30 40 50 Temperature C Figure 411 Conditional stability Illustration of Atmospheric Stability Conditions Figure 412 illustrates the various stability categories These analogies are intended to illustrate the different atmospheric stability conditions Figure 412 a depicts stable atmospheric conditions Notice that when the lifting force is removed the car eventually returns to its original position Since the car resists displacement from its original position it is in a stable environment Figure 412 b depicts neutral conditions When a force is applied to the car it moves as long as the force is maintained When the force is removed the car stops and remains in its new position This condition represents neutral stability Figure 412 c depicts unstable conditions Once a force is applied to the car it will continue to move even after the force is removed 1 Roller coaster track 2 Car at rest An outside force moves car up track Force removed car oscillates Car eventually returns to original position 1 Roller coaster track 2 Car at rest on at section of track Force moves car up track Force removed car stops Car remains in new position a Stable conditions b Neutral conditions Roller coaster track Car at rest Slightest push and car rolls down track Car continues to roll Car will continue to roll c Unstable conditions Figure 412 Atmospheric stability conditions In versions An inversion occurs when air temperature increases with altitude This situation occurs frequently but is generally con ned to a relatively shallow layer Plumes emitted into air layers that are experiencing an inversion inverted layer do not disperse very much as they are transported with the wind Plumes that are emitted above or below an inverted layer do not penetrate that layer rather these plumes are trapped either above or below that inverted layer An example of the lapse rate for an inversion is depicted in Figure 413 High concentrations of air pollutants are often associated with inversions since they inhibit plume dispersion The four major types of inversions are caused by different atmospheric interactions and can persist for different amounts of time 2 39 39 39 I Dry adiabatic lapse rate 5 squot E 1 2 uJ Environmental lapse rate i i i i 10 20 30 40 50 Temperature C Figure 413 Temperature inversion Radiation The radiation inversion is the most common form of surface inversion and occurs when the earth s surface cools rapidly As the earth cools so does the layer of air close to the surface If this air cools to a temperature below that of the air above it becomes very stable and the layer of warmer air impedes any vertical motion Radiation inversions usually occur in the late evening through the early morning under clear skies with calm winds when the cooling effect is greatest The same conditions that are conducive to nocturnal radiation inversions are also conducive to instability during the day Diurnal cycles of daytime instability and nighttime inversions are relatively common Therefore the effects of radiation inversions are often shortlived Pollutants trapped by the inversions are dispersed by vigorous vertical mixing after the inversion breaks down shortly after sunrise Figure 414 illustrates this diurnal cycle Dry adiabatic lapse rate Elevation km Nocturnal inversion 10 20 30 40 50 Temperature C Figure 414 Diurnal cycle In some cases however the daily warming that follows a nocturnal radiation inversion may not be strong enough to erode the inversion layer For example thick fog may accompany the inversion and reduce the effect of sunlight the neXt day Under the right conditions several days of radiation inversion with increasing pollutant concentrations may result This situation is most likely to occur in an enclosed valley where nocturnal cool downslope air movement can reinforce a radiation inversion and encourage fog formation In locations where radiation inversions are common and tend to be relatively close to the surface tall stacks that emit pollutants above the inversion layer can help reduce surfacelevel pollutant concentrations Subsidence The subsidence inversion Figure 415 is almost always associated with anticyclones high pressure systems Recall that air in an anticyclone descends and ows outward in a clockwise rotation As the air descends the higher pressure at lower altitudes compresses and warms it at the dry adiabatic lapse rate Often this warming occurs at a rate faster than the environmental lapse rate The inversion layer thus formed is often elevated several hundred meters above the surface during the day At night because of the surface air cooling the base of a subsidence inversion often descends perhaps to the ground In fact the clear cloudless days characteristic of anticyclones encourage radiation inversions so that there may be a surface inversion at night and an elevated inversion during the day Although the mixing layer below the inversion may vary diumally it will never become very deep Subsiding air Top 24 gt Mixinglayer V lt 4quot Figure 415 Subsidence inversion Inversion lay Subsidence inversions unlike radiation inversions last a relatively long time This is because they are associated with both the semipermanent anticyclones centered on each ocean and the slowmoving migratory anticyclones moving generally west to east in the United States When an anticyclone stagnates pollutants emitted into a mixing layer cannot be diluted As a result over a period of days pollutant concentrations may rise The most severe air pollution episodes in the United States have occurred either under a stagnant migratory anticyclone for example New York in November 1966 and Pennsylvania in October 1948 or under the eastern edge of the Paci c semipermanent anticyclone Los Angeles Frontal Lesson 3 mentions frontal trapping the inversion that is usually associated with both cold and warm fronts At the leading edge of either front the warm air overrides the cold so that little vertical motion occurs in the cold air layer closest to the surface Figure 416 The strength of the inversion depends on the temperature difference between the two air masses Because fronts are moving horizontally the effects of the inversion are usually shortlived and the lack of vertical motion is often compensated by the winds associated with the frontal passage However when fronts become stationary inversion conditions may be prolonged Inversion layer Cold air gt f Warm air Figure 416 Frontal inversion cold front Advection Advection inversions are associated with the horizontal ow of warm air When warm air moves over a cold surface conduction and convection cools the air closest to the surface causing a surfacebased inversion Figure 417 This inversion is most likely to occur in Winter when warm air passes over snow cover or extremely cold land Inversion layer m Cold ground Cooler air Figure 417 Surfacebased advection inversion Another type of advection inversion develops when warm air is forced over the top of a cooler air layer This kind of inversion is common on the eastern slopes of mountain ranges Figure 418 where warm air from the west overrides cooler air on the eastern side of the mountains Denver often eXperiences such inversions Both kinds of advection inversions are vertically stable but may have strong winds under the inversion layer Warm air 5 gt Inversion layer gt Figure 418 Terrainbased advection inversion Stability and Plume Behavior The degree of atmospheric stability and the resulting miXing height have a large effect on pollutant concentrations in the ambient air Although the discussion of vertical miXing did not include a discussion of horizontal air movement or wind you should be aware that horizontal motion does occur under inversion conditions Pollutants that cannot be dispersed upward may be dispersed horizontally by surface winds The combination of vertical air movement and horizontal air ow in uences the behavior of plumes from point sources stacks Lesson 6 will discuss plume dispersion in greater detail However this lesson will describe several kinds of plumes that are characteristic of different stability conditions The looping plume of Figure 419 occurs in highly unstable conditions and results from turbulence caused by the rapid overturning of air While unstable conditions are generally favorable for pollutant dispersion momentarily high groundlevel concentrations can occur if the plume loops downward to the surface Eleva on km 40 50 Tem perature C Figure 419 Looping plume The fanning plume Figure 420 occurs in stable conditions The inversion lapse rate discourages vertical motion Without prohibiting horizontal motion and the plume may eXtend downwind from the source for a long distance Fanning plumes often occur in the early morning during a radiation inversion 2 I I I I S E u 393 1 u 39 g M E Iu Stable I I I I 10 20 30 40 50 Temperature C Figure 420 Fanning plume The coning plume Figure 421 is characteristic of neutral conditions or slightly stable conditions It is likely to occur on cloudy days or on sunny days between the breakup of a radiation inversion and the development of unstable daytime conditions Elevation km Figure 421 Coming plume Obviously a major problem for pollutant dispersion is an inversion layer which acts as a barrier to vertical mixing The height of a stack in relation to the height of the inversion layer may often in uence groundlevel pollutant concentrations during an inversion When conditions are unstable above an inversion Figure 422 the release of a plume above the inversion results in effective dispersion Without noticeable effects on ground level concentrations around the source This condition is known as lofting Elevation km 50 Term perature C Figure 422 Lofting plume If the plume is released just under an inversion layer a serious air pollution situation could develop As the ground warms in the morning air below an inversion layer becomes unstable When the instability reaches the level of the plume that is still trapped below the inversion layer the pollutants can be rapidly transported down toward the ground Figure 423 This is known as fumigation Groundlevel pollutant concentrations can be very high when fumigation occurs Su iciently tall stacks can prevent fumigation in most cases 2 I h I I l Inversion 5 C 2 1 5 Lu I I I 10 20 30 40 50 Temperature C Figure 423 Fumigation Thus far you have learned the basic meteorological conditions and events that in uence the movement and dispersal of air pollutants in the atmosphere Lesson 6 eXplores more fully the behavior of pollutants around point sources while the neXt lesson discusses the instrumentation used for meteorological measurement Review Exercise 1 An in nitesimally small wellde ned body of air that does not readily miX with the surrounding air is an a Air column b Air mass c Air parcel d Hot air balloon e b and c 2 The temperature of air iiiiii as atmospheric pressure increases a Increases b Decreases 3 What two atmospheric factors in uence the buoyancy of an air parcel 4 If the temperature of an air parcel is cooler than the surrounding air it will usually a Ascend b Descend c Stay in the same place 5 The environmental or prevailing lapse rate can be determined from the Rate of pressure change in the atmosphere Rate of wet air vs pressure change Temperature pro le of the atmosphere Rate of frontal system passage 9957 6 Changes in the temperature of an air parcel that are due to changes in atmospheric pressure are called a Advective b Adiabatic c Slope d Prevailing 7 The dry adiabatic lapse rate is 76 C1000 m lt 1 C1000 m 798 C1000 m 775 C1000 m 9957 10 11 12 13 14 15 16 True or False The dry adiabatic lapse rate is xed and entirely independent of ambient air temperature a True b False A displaced air parcel cools at the wet adiabatic lapse rate once it becomes At the wet adiabatic lapse rate the cooling rate of the air parcel is usually a The same as at the dry rate b Slower than at the dry rate c Faster than at the dry rate The actual temperature pro le of the ambient air can be used to determine the i if lapse rate True or False The environmental lapse rate in uences the extent to which a parcel of air can rise or descend a True b False The maximum level to which a parcel of air will ascend under a given set of conditions is known as the a Ascenddescend level b Mixing trough c Mixing height d Mixing layer The adiabatic lapse rate for a given air parcel will intersect the environmental lapse rate at the a Mixing trough b Moisture rate c Mixing height d None of the above A large mixing layer implies that air pollutants have a volume of air for dilution a Greater b Lesser True or False A stable atmosphere resists vertical motion a True b False 17 Vertical mixing due to buoyancy is increased when atmospheric conditions are a Unstable b Neutral c Stable d Extremely stable 18 Unstable atmospheric conditions most commonly develop On cloudy days On sunny days On cloudy nights On clear nights 9957 19 On cloudy days with no strong surface heating atmospheric conditions are likely to be a Unstable b Neutral c Stable d Extremely stable 20 In this diagram a displaced air parcel becomes saturated at an elevation of 2 km Which of the following stability conditions does the diagram depict 4 39l 39 39 39 l l Wet adiabatic lapse rate 6 C lkm Unstable atmosphere E x E g 2 x gt 2 Dry adiabatic quotu lapse rate Environmental 3998 OCkm Stable lapse rate atmos here 7 Ckm p quotl I 10 20 30 40 50 Temperature C Stable below 1 km Conditional stability above 1 km Neutral from 0 to 2 km Conditional instability above 2 km 9957 21 An 7 7 acts as a lid on vertical air movement 22 23 24 25 26 27 28 29 When the earth s surface cools rapidly such as between late night and early morning under clear skies an i 7 inversion is likely to occur When vigorous vertical mixing follows a radiation inversion pollutant plumes will a Be trapped near the surface b Be dispersed away from their source True or False A high pressure system can cause an elevated temperature inversion to form a True b False The subsidence inversion is associated with because it usually forms high above the surface during the day A subsidence inversion generally tends to last for a relatively period of time compared to a radiation inversion a Short b Long Surfacebased inversions associated with horizontal air ow such as when warm air moves over a cold surface are called i inversions a Subsidence b Frontal c Advection d Adiabatic The iiiiii plume is characteristic of neutral or slightly stable atmospheric conditions Fanning Looping Coning Lofting 9957 What is the name of the plume depicted in this illustration i i 30 31 32 33 Elevation km Temperature C Which plume is represented by this lapse rate and stack height777777 A fanning plume will occur when atmospheric conditions are generally a Highly unstable b Stable c Neutral The looping plume can cause 7 of air pollutants Ifa plume is releasedjust pollution situation could develop a Under b Over 7 groundlevel concentrations an inversion layer a serious air 34 The plume in this drawing is an example of 7 7 21 Coming b Looping c Fumigation d Lofting Review Exercise Answers 1 10 11 c Air parcel An in nitesimally small wellde ned body of air that does not readily miX with the surrounding air is called an air parcel a Increases The temperature of air increases as atmospheric pressure increases Temperature and pressure Temperature and pressure are two atmospheric factors that in uence the buoyancy of an air parcel b Descend If the temperature of an air parcel is cooler than the surrounding air it will usually descend c Temperature pro le ofthe atmosphere The environmental or prevailing lapse rate can be determined from the temperature pro le of the atmosphere b Adia batic Changes in the temperature of an air parcel that are due to changes in atmospheric pressure are called adiabatic c 98 C1000 m The dry adiabatic lapse rate is 798 C1000 m a True The dry adiabatic lapse rate is xed and entirely independent of ambient air temperature Saturated A displaced air parcel cools at the wet adiabatic lapse rate once it becomes saturated b Slower than at the dry rate At the wet adiabatic lapse rate the cooling rate of the air parcel is usually slower than at the dry rate Environmental The actual temperature pro le of the ambient air can be used to determine the environmental lapse rate 12 13 14 15 16 17 18 19 20 21 22 23 a True The environmental lapse rate in uences the extent to which a parcel of air can rise or descend c Mixing height The maximum level to which a parcel of air will ascend under a given set of conditions is known as the mixing height c Mixing height The adiabatic lapse rate for a given air parcel will intersect the environmental lapse rate at the mixing height a Greater A large mixing layer implies that air pollutants have a greater volume of air for dilution a True A stable atmosphere resists vertical motion a Unstable Vertical mixing due to buoyancy is increased when atmospheric conditions are unstable b On sunny days Unstable atmospheric conditions most commonly develop on sunny days b Neutral On cloudy days with no strong surface heating atmospheric conditions are likely to be neutral d Conditional instability above 2 km The diagram depicts conditional instability above 2 km Inversion An inversion acts as a lid on vertical air movement Radiation When the earth s surface cools rapidly such as between late night and early morning under clear skies a radiation inversion is likely to occur b Be dispersed away from their source When vigorous vertical mixing follows a radiation inversion pollutant plumes will be dispersed away from their source 24 25 26 27 28 29 30 31 32 33 34 a True A high pressure system can cause an elevated temperature inversion to form subsidence inversion Anticyclones The subsidence inversion is associated with anticyclones because it usually forms high above the surface during the day b Long A subsidence inversion generally tends to last for a relatively long period of time compared to a radiation inversion c Advection Surfacebased inversions associated with horizontal air ow such as when warm air moves over a cold surface are called advection inversions c Coning The coning plume is characteristic of neutral or slightly stable atmospheric conditions Looping A looping plume is depicted in this illustration Lofting A lofting plume is represented by this lapse rate and stack height h Stable A fanning plume will occur when atmospheric conditions are generally stable High The looping plume can cause high groundlevel concentrations of air pollutants 21 Under If a plume is released just under an inversion layer a serious air pollution situation could develop c Fumigation The plume in this drawing is an example of fumigation
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