Chapter 5 Fog, Clouds, and Precipitation

Transcription

1 5.1 Prductin f Dew, Fg, and Cluds We have seen that air becmes saturated whenever the relative humidity is equal t 100%. Any further cling f the air causes water vapr t cndense ut f the air. As we saw in chapter 4, cndensatin is the change f state frm water vapr t liquid water. Cndensatin can ccur in tw ways: (1) by increasing the amunt f water vapr int the air, and (2) by cling the air dwn t the dew pint temperature. When the air is cled t the dew pint, the relative humidity becmes 100% and the air becmes saturated. Any further cling prduces cndensatin. If the cndensatin ccurs directly n the surface f the earth the result is called dew r frst. If the tiny water drplets frmed becme suspended in the air then either a fg r a clud is prduced depending n whether the cndensatin ccurs clse t the surface f the earth r alft. In mst f what fllws we assume that the temperature f the air is high enugh fr the cndensatin t ccur as water drplets. If the temperature f the air is belw freezing, the water vapr will cndense as ice crystals. Thus fg and/r cluds can cnsist f water drplets, ice crystals, r bth. 5.2 Fg Fg results when atmspheric water vapr cndenses (r sublimes) t the extent that the new frms, water drplets r ice crystals, becme visible and have their base in cntact with the grund. Saturatin f the air and sufficient cndensatin nuclei are rdinarily prerequisites fr the frmatin f fg. One f the great hazards assciated with fg is a lack f visibility. Visibility in fg is defined as the greatest distance in a given directin at which cmmn bjects like buildings r trees are visible t the unaided eye. Prevailing visibility is defined as the greatest visibility that prevails ver at least ne-half f the hrizn. The principle prcesses that cause saturatin are cling f the air and evapratin f water int it. Classificatin f Fg. Fg is classified accrding t the way it is prduced. Fg can be caused by adding water vapr t the air t cause saturatin r cling the air t prduce saturatin. (1) Fgs resulting frm evapratin. (a) Steam Fg. Steam fg is fg that is prduced by intense evapratin f water int relatively cld air. Saturatin ccurs, then cndensatin, then fg. Steam fg is bserved ver bdies f water in mid- and high latitudes. Smetimes it ccurs ver warm, wet land immediately after a rain. (b) Frntal Fg. Frntal fg, as the name implies, is fg fund alng the bundary f tw air masses. Evapratin frm warm rain falling thrugh the

2 drier air belw may be fllwed by saturatin and cndensatin in cler layers t frm frntal fg, figure 5.1. cl air warm air Fg Figure 5.1 Frntal Fg (2) Fgs resulting frm cling. (a) Radiatin Fg r Grund Fg. Radiatin fg is fg prduced when fairly calm mist air, which is in cntact with the grund, is cled t saturatin and then cndensatin by nighttime radiatin. If air is cmpletely calm nly dew r frst will frm. Slight turbulence increases the depth f the fg, but if it is vilent enugh it will prevent r dissipate the fg. Valleys are particularly susceptible t radiatin fg r frst. In plar regins fgs are cmpsed f ice crystals and are called ice fg. The prcess f frmatin f fg by radiatin is called a diabatic prcess, that is, it is a nn-adiabatic prcess. The cling ccurs by taking heat energy away frm the air. Anther frm f radiatin fg ccurs when direct radiatin frm the mist air itself causes a cling f the air dwn t saturatin; further cling causes cndensatin and the frmatin f the fg. (b) Advectin Fg. Advectin fg is fg that is frmed when mist air is transprted ver a cld surface. The cld surface causes the air in cntact with it t cl. If the air cls t saturatin, cndensatin ccurs and dew is frmed n the grund. Further cling causes the air clse t the surface t als becme saturated, and the mtin f the air causes a mixing f the air and causes the cndensatin t ccur at higher levels abve the surface until the fg cvers the entire area. Advectin fg is especially cmmn at sea. Winds blwing nshre tend t carry the fg inland. The fg als ccurs ver land when warm mist air is transprted ver snwy surfaces. (c) Upslpe Fg. Upslpe fg is fg that frms when there is a gradual rgraphic ascensin f mist air up a slping plain r hilly regin. The mist air will cl adiabatically t frm upslpe fg prviding the air is already clse t saturatin. (d) Mixing Fg. Mixing fg is fg that ccurs when warm mist air cmes in cntact with cl mist air. The mixture may have a temperature lw enugh t prduce saturatin and cndensatin, prducing the mixing fg. Mixing fg ccurs at frnts between air masses f maritime rigin. A special case f the physical prcess assciated with mixing fg ccurs when yu see yur breath n a cld day. The warm mist air expelled frm yur lungs mixes with the clder air. Fr a mment, the mixture becmes saturated and yu 5-2

3 can see the cndensatin in the air cming frm yur muth. Of curse, this prcess desn t last lng because the mixed air mixes with even mre drier air until the air is n lnger saturated and the water drplets frm yur breath quickly evaprate. 5.3 The Frmatin f Cluds A clud is physically an aersl, that is, a visible aggregate f minute water drplets, ice crystals r a mixture f bth suspended in the air. In rder t frm the clud, the water vapr in the air must cndense int these minute water drplets. As we saw previusly, cndensatin f water vapr int water drplets can ccur in tw ways: (1) by increasing the amunt f water vapr int the air, and (2) by cling the air dwn t the dew pint temperature. Since a clud frms at a significant height abve the surface f the earth, there is n surce f water available t evaprate int the air at that level. Whatever misture that is presently in the air was btained when water evaprated int the air when it was at the lwer level. Thus increasing the amunt f water vapr int the air is nt the main technique fr causing cndensatin fr the frmatin f cluds. Therefre the nly practical way that the relative humidity can be increased t 100% is t cl the air dwn t saturatin. Hence, the prcess that is respnsible fr the frmatin f cluds is the cling f the air dwn t saturatin. There are tw necessary cnditins fr the frmatin f cluds. They are: (a) A Cling Mechanism. (b) A Lifting Mechanism. (a) A Cling Mechanism. The mechanism fr the prductin f cluds is the adiabatic cling f rising air. An adiabatic prcess is a thermal prcess that ccurs in which there is n heat exchanged. Mst heating r cling prcesses that yu are familiar with are actually nn-adiabatic prcesses. Fr example, if yu wish t warm water, yu put it int a pt, place it n the stve and apply heat. The warming ccurs because yu have applied heat. Similarly, if yu wish t cl water, yu put it int a glass and place it in the refrigeratr. The refrigeratr uses electrical energy t remve the heat frm the water thereby cling it. Bth these prcesses are nn-adiabatic, because yu either added heat r remved it during the prcess. When a parcel f air rises int the atmsphere it finds it self at a new level. Hwever as we mentined in chapter 1 the pressure f the air decreases with height. Thus, the pressure f the air alft is less than the pressure at the surface f the earth. Cnsider the parcel f air as thugh it were a balln. At the surface, the air inside the balln has a certain pressure and this pressure is exerted utward against the wall f the balln. The pressure f the atmspheric air utside the balln is pushing inward n the balln, and an equilibrium cnditin exists between the frce pushing in n the wall f the balln and the frce pushing ut. When the balln rises int the atmsphere the utside pressure becmes less but 5-3

4 the inside pressure is the same. Therefre the balln will expand until the pressure inside the balln is the same as the pressure utside the balln and equilibrium is again established. The air inside the balln must d wrk in rder fr the balln t expand. It takes energy fr the air t d that wrk, and that energy cmes frm the internal energy f the air. But the internal energy f the air cmes frm the mtin f the air mlecules and is directly prprtinal t the temperature f the air. S if the internal energy f the air decreases, s des the temperature f the air. Therefre, when the air expands it is cled. Ntice, hwever, that the cling has taken place withut sme external agent remving the energy, as in the case f the glass f water in the refrigeratr. Thus this cling is an adiabatic cling. Therefre the rising air expands and cls adiabatically. When the air is cled t the dew pint temperature Td the air becmes saturated, the relative humidity becmes 100%, and any further cling will cause cndensatin. The tiny water drplets frmed in the air becmes the clud. (b) A Lifting Mechanism. As we have just seen, rising air can cl t the pint where cndensatin can began and clud drplets can frm. But hw d we get rising air? The air must be lifted by sme mechanism s that it rises int the atmsphere. The fur lifting mechanisms fr clud frmatin are: (1) Cnvectin Nrmal heating f the grund by shrt-wave radiatin frm the Sun causes the grund t warm up. The grund radiates lng-wave radiatin that is absrbed by water vapr and carbn dixide in the air. The air at the surface f the earth is thus warmed. The warm air expands and becmes lighter. The lighter air nw rises by cnvectin, and expands and cls adiabatically. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm. The usual type f clud that is frmed by cnvectin is the cumulus clud. (2) Cnvergence The cnvergence f wind currents r air masses causes a lifting f the air. As pinted ut in the quick survey f meterlgy in chapter 1, air spirals int a lw-pressure surface at the surface f the earth. Where can all this air g? The nly place fr it t g is upward. Hence there is vertical mtin upward in a lw-pressure area. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm. Cnvergence als ccurs ver a regin like Flrida. Air blws inward frm the Gulf f Mexic and the Atlantic Ocean. The air cnverges ver Flrida. The nly place fr it t g is upward. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm (3) Frntal Lifting A frnt is a bundary between tw different air masses. When these tw air masses cllide the warmer air mass, being lighter, will mve up ver the clder air mass. Hence, the frnt will cause lifting f the air. (4) Orgraphic Lifting Orgraphic lifting ccurs when air pushes up against a muntain barrier. There is n place fr the air t g but upward. Hence the air is frced t rise. 5-4

5 5.4 Classificatin f Cluds Of all the many varied cluds that are bservable in the sky, they fall int nly tw basic types f cluds. Thse cluds assciated with strng rising air currents have vertical develpment and a puffy appearance and are called cumulus cluds. Thse resulting frm gentler lifting tend t spread ut int layers and are called stratus cluds. Cluds are primarily classified n the basis f their height int the fllwing scheme: (1) High Cluds. High cluds are fund at levels abve 6000 m abve the grund and can extend up t the trppause. High cluds belng t the family f Cirrus cluds. These cluds are made up f ice crystals. The different types f high cluds are: (a) Cirrus. Cirrus cluds are nearly transparent, white, fibrus r silky. Figure 5.2 Cirrus cluds (b) Cirrcumulus. A cirrifrm layer, r patch f small white flakes arranged in grups r lines. Smetimes they have the appearance f ripples, similar t sand n a beach. (This is nt a very cmmn clud.) 5-5

6 Figure 5.3 Cirrcumulus cluds. (c) Cirrstratus. A thin white veil f cirrus, nearly transparent (the sun, mn and stars can be seen thrugh them). Cirrus cluds create hals. A hal is refractin f light by ice crystals. Figure 5.4 Cirrstratus cluds. (2) Middle Cluds. Middle cluds are fund at levels between 2000 m and 6000 m abve the grund. Middle cluds belng t the family f Alt cluds. These cluds 5-6

7 are made up f water drplets, ice crystals, r bth. The different types f middle cluds are: (a) Altcumulus. An altcumulus clud is in the frm f layers r patches f glbular cluds. The cluds may build upward. Frm the grund, the cluds ften lk very much like cirrcumulus cluds, but they are lwer. Figure 5.5 Altcumulus cluds. (b) Altstratus. An altstratus clud is defined as a fibrus veil f cluds that is gray r blue gray. When the clud becmes thick and rain starts t fall it is called a nimbstratus clud. Figure 5.6 Altstratus cluds. 5-7

8 (3) Lw Cluds. Lw cluds are fund at levels just abve the grund t 2000 m abve the grund. Lw cluds all have the prefix stratus. These cluds are made up f water drplets. The different types f lw cluds are: (a) Stratus. A stratus clud is a lw unifrm layer f clud resembling fg but nt resting n the grund. Because the thickness f the clud is small, precipitatin, if it ccurs, is light. Figure 5.7 Stratus cluds. (b) Stratcumulus. Stratcumulus cluds are a lw, gray layer f cluds cmpsed f glbular masses r rlls. They have the same appearance as altcumulus clud nly they are lwer. Figure 5.8 Stratcumulus cluds 5-8

9 (4) Cluds f Vertical Develpment. Cluds f vertical develpment are fund at levels frm just abve the grund and can extend all the way up t the trppause. These cluds are made up f water drplets and ice crystals. The different types f cluds are: (a) Cumulus. Cumulus cluds are dense, dme-shaped cluds that have flat bases. Cumulus with little vertical develpment and a slightly flattened appearance are usually assciated with fair weather. Figure 5.9 Cumulus cluds. (b) Cumulnimbus. A clud f great vertical develpment, twering t 18 km r mre where they spread ut t leeward and frm an anvil f cirrus. The cumulnimbus is the thunderstrm clud that has heavy shwers f rain, snw, r hail, lightning and thunder. (a) Side view. Clud still building up. (Prbably better called a Cumulus cngestus clud at this stage f its develpment.) 5-9

10 (b) When viewed frm beneath the clud, yu cannt make ut the detail f the twering clud.) Figure 5.10 Cumulnimbus cluds. 5.5 Clud Observatins Because f the bvius relatin between cluds and weather, clud bservatins are an integral part f every weather bservatin. The items listed in every clud bservatin include: (1) Clud Types. A statement as t what types f cluds are present in the atmsphere. That is, cirrus, cumulnimbus etc. (2) Sky Cver. This is a statement f the fractin f the sky that is cvered by cluds. The categries are (a) Clear. The sky is cnsidered clear if the sky is cmpletely clear r cntains less than 1/10 f cluds. (b) Scattered r smetimes called partly cludy. The sky is cnsidered scattered if the sky is cvered by mre than 1/10 f cluds but less than 6/10 f cluds. (c) Brken r smetimes called cludy. The sky is cnsidered brken if the sky is cvered by mre than 6/10 f cluds but less than 9/10 f cluds. (d) Overcast. The sky is cnsidered vercast if the sky is cvered by mre than 9/10 f cluds. (3) Clud Height. The clud height is the distance frm the grund t the base f the clud. The ceiling is the distance frm the grund t the lwest brken r vercast clud cver. T determine the distance frm the grund t the base f the clud, the fllwing techniques are used (a) Ceiling Balln. A weather balln is released frm the surface f the earth and is bserved as it rises int the atmsphere. The balln rises at a cnstant rate v in the atmsphere. Hence, by measuring the time t fr the 5-10

11 balln t rise t the pint where it enters a clud, figure 5.11, the base f the clud is determined. That is, if the vertical velcity is v, then the height h that the balln will rise t in the time t is h = v t h = v t Figure 5.11 Ceiling balln. The ceiling balln is mstly used in the day time, but it can als be used at night if it carries a light surce. This is nt a very gd technique fr very high cluds. The balln will mve t far in the hrizntal while it is rising and can mve ut f the field f bservatin. (b) Ceiling Light. A vertical light is prjected nt the base f a clud, figure The angle θ is measured and the distance d is knwn. The height f the base f the clud is determined by trignmetry. That is, Slving fr the height h we get tan = h d h = d tanθ θ d h Figure 5.12 The ceiling light. (c) Ceilmeter. The ceilmeter is an autmatic ceiling light. The same principle f the ceiling light is used but a phtelectric element which reacts selectively t the light spt gives an autmatic indicatin f the height. (d) Pilt Reprts. Pilts regularly radi infrmatin n the types and bases f cluds back t the airprt. 5-11

12 (4) Clud Directin. The mtin f the cluds is usually determined by radar and satellite bservatins. 5.6 Stability and Cluds In the sectin 5.4 we saw the many different types f cluds that can frm in the atmsphere. Why des ne type f clud frm rather than anther? One f the things that determines the type f clud that will frm is the stability f the atmsphere. Let us, therefre, first try t understand the cncept f stability. The simplest way t understand the cncept f stability is t visualize a ball placed in the bttm f a bwl as seen in figure 5.13a. The bttm f the bwl is called the (a) stable (b) unstable (c) neutral Figure 5.13 The cncept f stability. equilibrium psitin. If the ball is given a slight push it will mmentarily mve away frm the bttm f the bwl but then the ball will rll back tward the bttm f the bwl, the equilibrium psitin. The ball may scillate a few times abut this equilibrium psitin but eventually it will cme t rest at the bttm f the bwl. The ball in the bttm f the bwl is said t be stable because when displaced frm the equilibrium psitin the ball always returns t its equilibrium psitin. Nw cnsider the ball placed at rest at the tp f the inverted bwl, figure 5.13b. If the ball is given a slight push, the ball will nw mve away frm the equilibrium psitin and will keep n mving, never returning t the equilibrium psitin. The ball at the tp f the inverted bwl is said t be unstable because when displaced frm the equilibrium psitin the ball cntinues t mve away frm the equilibrium psitin. Nw cnsider the ball placed n the level surface in figure 5.13c. If the ball is given a slight push, the ball will mve away frm the equilibrium psitin. It will nt, hwever, return t the equilibrium psitin, as in stable mtin, nr will it cntinue in mtin as in the case f unstable mtin. The ball n the level surface is said t be in neutral stability because when displaced frm the equilibrium psitin the ball neither cntinues t mve away frm the equilibrium psitin, nr returns t the equilibrium, but stays at the place where it was displaced. Let us nw apply this cncept f stability t the atmsphere. If atmspheric air is stable, when displaced frm its equilibrium psitin, the air will return t its riginal psitin. If atmspheric air is unstable, when displaced frm its equilibrium psitin, the air will cntinue t mve away frm its riginal psitin. If 5-12

13 atmspheric air is neutral, when displaced frm its equilibrium psitin, the air will remain at the new psitin. Hw d we determine if the atmspheric air is stable? The stability f the atmsphere is determined by the atmspheric lapse rate. As yu recall, the lapse rate is defined as the change in temperature T with height h, that is, L = T h Knwing that the atmsphere is heated by the absrptin f lng wave infrared radiatin frm the surface f the earth, and the fact that we see permanent snw caps n high muntains, even in the trpics indicates that the temperature f the air decreases with altitude. The nrmal r average atmspheric lapse rate is C/km. This means that fr each kilmeter that we mve upward int the atmsphere the temperature will decrease by C. If the temperature at the surface f the earth is 20 0 C then the temperature at 1 km will have cled t 20 0 C C = C. At a height f 2 km the air temperature will nw be C C = C. Using the temperature lapse rate in this way we can find the temperature at any height h. We shuld nte that the nrmal lapse rate is an average ver time and space and the actual lapse rate at a particular time and place will prbably be different. A typical graph f the variatin f temperature with altitude is shwn in figure h 4 km 3 km 2 km 1 km T 0 C Figure 5.14 The variatin f temperature T with altitude h fr the nrmal lapse rate. Remember that when air rises, it cls adiabatically. The rate at which unsaturated air cls as it rises is called the dry adiabatic lapse rate and is L DA = 10 0 C/km. The dry adiabatic lapse rate is shwn in figure Air will cl at this rate as it rises and will warm at this rate if it subsides. Ntice that the dry adiabatic lapse rate (10 0 C/km) is greater numerically than the nrmal lapse rate (6.5 0 C/km). But because the rate is negative, the slpe f the line fr the dry adiabatic rate is nt as steep as the line fr the nrmal lapse rate. 5-13

14 h 4 km 3 km 2 km 1 km L DA Dry Adiabatic Rate Nrmal rate T 0 C Figure 5.15 The dry adiabatic lapse rate. If the air cls t saturatin and cndensatin begins, then the heat f cndensatin (L v = 600 kcal/kg) is released int the air reducing the rate f cling. The new rate f cling is called the wet r mist adiabatic lapse rate. It is the dry adiabatic lapse rate mdified by the heat f cndensatin. Hence, the saturated r wet adiabatic rate f cling is less than the dry adiabatic rate f cling. The wet adiabatic lapse rate varies between 5 0 C/km and 9 0 C/km depending upn the amunt f water vapr in the air. We will take the wet adiabatic lapse rate t be L WA = 6 0 C/km in all ur examples. The wet adiabatic lapse rate is smetime called the pseudadiabatic lapse rate because sme latent heat is added t the air as the water vapr cndenses. Figure 5.16 shws the wet and dry adiabatic lapse late n the same graph. h 4 km 3 km 2 km 1 km Wet Adiabatic rate L WA L DA Dry Adiabatic Rate T 0 C Figure 5.16 The wet and dry adiabatic lapse rates. An example f adiabatic cling as air rises int the atmsphere is shwn in figure Surface air is at a value f 30 0 C. If it is lifted int the atmsphere it cls at the dry adiabatic rate L DA = 10 0 C/km. Hence, at a height f 1 km the temperature f the air will be 30 0 C 10 0 C = 20 0 C. That is the air will have cled by 10 0 C t a value f 20 0 C at the height f 1 km. At 2 km it will have cled, 20 0 C 10 0 C = 10 0 C, and at 3 km we will have 10 0 C 10 0 C = 0 0 C. Let us assume that the actual dew pint temperature f the air is 0 0 C at 3 km. When the air cls t this temperature at 3 km, the air is cmpletely saturated and cndensatin begins. Nw, if there is any further cling, the air will cl at the wet adiabatic lapse rate 5-14

15 f LWA = 6 0 C/km. As the air rises anther kilmeter it nw cls as 0 0 C 6 0 C = 6 0 C, and then fr the next kilmeter, 6 0 C 6 0 C = 12 0 C. Figure 5.17 shws the temperature f the rising air at each f these heights. 5 km 4 km - 12 C - 6 C Wet adiabatic rate L = 6 C/km 3 km 2 km 0 C Cndensatin Level 10 C 1 km Surface 20 C 30 C Dry adiabatic rate L = 10 C/km Figure 5.17 Adiabatic cling in the atmsphere. An example f an abslutely stable atmsphere is shwn in figure Let us assume that the actual lapse rate fund in the atmsphere at this time and place is 5 0 C/km. This means that the actual temperature f the air at each kilmeter is 5 km 4 km Actual Lapse rate L = 5 C / km 5 C 10 C - 12 C - 6 C Wet adiabatic rate L = 6 C/km 3 km 15 C 0 C Cndensatin Level 2 km 20 C 10 C 1 km 25 C 20 C Dry adiabatic rate L = 10 C/km Surface 30 C 30 C at 1 km 30 0 C 5 0 C = 25 0 C at 2 km 25 0 C 5 0 C = 20 0 C Figure 5.18 An Abslutely stable atmsphere. 5-15

16 at 3 km 20 0 C 5 0 C = 15 0 C at 4 km 15 0 C 5 0 C = 10 0 C at 5 km 10 0 C 5 0 C = 5 0 C As the air starts t rise it cls at the dry adiabatic rate f L DA = 10 0 C/km, until the air becmes saturated at 3 km and thereafter it cls at the wet adiabatic rate f L WA = 6 0 C/km, as shwn in figure 5.17 and nw again in figure Ntice frm figure 5.18, that as the air rises and cls in the atmsphere, it is always cler than the air f the riginal atmsphere. Being cler it is mre dense than the riginal air, and hence heavier. Therefre, if it can, it will fall back t the riginal height it had in the atmsphere. This air is thus abslutely stable. The type f cluds that are frmed in stable air are the layered r stratus type f clud, because there is n tendency fr the air t rise n its wn. Instead it spreads ut int layers. When flying in an airplane in stable air, the ride will usually be very smth due t a lack f turbulence in the stable air. The criteria fr air t be abslutely stable is that the actual lapse rate be less than the wet adiabatic lapse rate. That is, L < L WA Cnditin fr abslute stability That is, if the actual lapse rate is 5 r 4 r 3 0 C/km etc., this lapse rate is less than the wet adiabatic lapse rate f 6 0 C/km, and hence the air is abslutely stable. Thus if the air is lifted t any level in the atmsphere it will be cler than the surrunding air at that level and will fall back t its riginal level if pssible. An example f an abslutely unstable atmsphere is shwn in figure Let us assume that the actual lapse rate fund in the atmsphere at this time and L = 12 C/ km 5 km -30 C 4 km Actual Lapse rate -18 C - 12 C - 6 C Wet adiabatic rate L = 6 C/km 3 km -6 C 0 C Cndensatin Level 2 km 6 C 10 C 1 km 18 C 20 C Dry adiabatic rate L = 10 C/km Surface 30 C 30 C Figure 5.19 An abslutely unstable atmsphere. 5-16

17 place is 12 0 C/km. This means that the actual temperature f the air at each kilmeter is at 1 km 30 0 C 12 0 C = 18 0 C at 2 km 18 0 C 12 0 C = 6 0 C at 3 km 6 0 C 12 0 C = 6 0 C at 4 km 6 0 C 12 0 C = 18 0 C at 5 km 18 0 C 12 0 C = 30 0 C As the air starts t rise it cls at the dry adiabatic rate f L DA = 10 0 C/km, until the air becmes saturated at 3 km and thereafter it cls at the wet adiabatic rate f L WA = 6 0 C/km, as shwn in figure 5.17 and again in figure Ntice frm figure 5.19, that as the air rises and cls in the atmsphere, it is always warmer than the air f the riginal atmsphere. Being warmer it is less dense than the riginal air, and hence lighter. Therefre, it will cntinue t mve upward in the atmsphere. This air is thus abslutely unstable. The type f cluds that are frmed in unstable air are the cumulus r puffy building up type f clud, because nce displaced frm its riginal psitin, the air will cntinue t rise n its wn. When flying in an airplane in unstable air, the ride will be very bumpy due t a great deal f turbulence in the unstable air. The criteria fr air t be abslutely unstable is that the actual lapse rate be greater than the dry adiabatic lapse rate. That is, L > L DA Cnditin fr abslute instability As an example, if the actual lapse rate is 11 r 12 r 13 0 C/km etc., this is greater than the dry adiabatic lapse rate f 10 0 C/km, and the air is abslutely unstable. An example f a cnditinally unstable atmsphere is shwn in figure km 4 km Actual Lapse rate L = 9 C/ km -15 C -6 C - 12 C - 6 C Unstable Air Wet adiabatic rate L = 6 C/km 3 km 3 C 0 C Cndensatin Level 2 km 12 C 10 C Stable Air 1 km 21 C 20 C Dry adiabatic rate L = 10 C/km Surface 30 C 30 C Figure 5.20 A cnditinally unstable atmsphere. 5-17

18 As can be seen, as the air rises and cls in the atmsphere at the dry adiabatic rate it is always cler than the air f the riginal atmsphere. Being cler it is mre dense than the riginal air, and hence heavier. Therefre, if it can it will fall back t the riginal height it had in the atmsphere. This air is thus stable. If the air is frced t cntinue t rise, cndensatin begins and the air will nw cl at the wet adiabatic rate, L WA = 6. Nw the rate at which the rising air cls is less, and when the air reaches the 4 km level it will have cled t 6 0 C which is the same temperature f the riginal air at the 4 km level. Hence at this level the air is said t be neutral because if the cause f the lifting is remved the air will remain at this 4 km level. As the air cntinues t rise it becmes warmer than the riginal air and nw becmes unstable. Thus this air is cnditinally unstable. The cnditin fr the air t be cnditinally unstable is that the actual lapse rate fall between the wet adiabatic lapse rate and the dry adiabatic lapse rate. That is, LWA < L < LDA Requirement fr cnditinal instability Hence, if the actual lapse rate is 7 r 8 r 9 0 C/km, this is greater than the wet adiabatic lapse rate f 6 0 C/km, and yet is less than the dry adiabatic lapse rate f 10 0 C/km, and the air is cnditinally unstable. Fr this example, if the air is nly lifted t levels belw the 4 km level the air will be stable. But if it is lifted abve the 4 km level the air will be unstable and will cntinue t rise up int the atmsphere. 5.7 Precipitatin Precipitatin is defined as water in liquid r slid frms falling t the earth. It is always preceded by cndensatin r sublimatin r a cmbinatin f the tw, and is primarily assciated with rising air. Fg, dew, r frst are nt cnsidered t be precipitatin because they d nt fall frm the sky. A clud is an aersl - a suspensin f minute water drplets r ice crystals. These water drplets r ice crystals are held up by the rising air. In rder fr the water drplet r ice crystal t fall frm the clud, the drplets r crystals must grw t sizes which can n lnger be held up by the rising air. The simplest prcess fr the cnversin f a clud drplet t a rain drplet is by calescing with ther clud drplets until the size f the drplet is s large that it can nt be suspended by the rising air. Precipitatin is classified in tw ways: (1) accrding t the frm taken by the falling water, and (2) n the basis f the prcess which leads t its frmatin. I. Precipitatin based upn the frm taken by the falling water. Rain is the mst cmmn frm f precipitatin. It falls frm cluds that are frmed in rising air when the temperature, at least at the lwer levels, is abve the freezing level f 0 0 C. Raindrps may begin as snw but melt as they descend int the warmer air belw. 5-18

19 Snw is frmed when the temperature is belw 0 0 C when the saturatin and cndensatin prcess ccurs. Ice crystals are then frmed. If these ice crystals reach the grund we have snw. Sleet (Ice Pellets) Sleet, r as it is nw called, ice pellets, is rain which freezes as it falls frm a warmer layer f air alft thrugh a cld layer f air near the surface, figure cld air warm air rain sleet Figure 5.21 The frmatin f sleet. Freezing Rain Freezing rain is similar t sleet in that it is rain that falls frm warm air alft thrugh cld air belw and freezes upn striking the cld surfaces at the grund. The air itself is nt cld enugh t freeze the rain as in the frmatin f sleet, but the surfaces are all belw freezing and when the rain hits the surface it freezes n cntact. Freezing rain is very dangerus. Autmbiles slide all ver the rad when it ccurs and many accidents ccur. Many tree limbs and shrubs are brken frm the weight f the ice, figure Freezing rain usually presages a warmer mass f air that mves in behind a warm frnt. Figure 5.22 The effects f freezing rain. Drizzle Drizzle are minute drplets f water that fall s slwly that they seen t flat in the air fllwing the slightest mvement f the air. Drizzle falls 5-19

20 cntinuusly frm lw stratus cluds. It is ften accmpanied by fg and pr visibility. Freezing Drizzle Freezing drizzle is drizzle that freezes when it strikes the cld surfaces at the grund. The frmatin f freezing drizzle is similar t the frmatin f freezing rain except that the size f the drizzle particle is much smaller than the rain particle. Hail Hail is a frm f precipitatin that falls frm vilent summer thunderstrms. It starts as a rain drp in the cnvective clud but is carried t higher levels by the cnvective currents. As the rain drp passes the freezing level, it freezes. As the size f the ice crystal increases, it falls t lwer altitudes belw the freezing level where water drps nw frm all ver the ice crystal. It is again caught in a cnvective current and is again carried t higher levels abve the freezing level where the water n the ice crystal freezes again. Repeated rising and falling f the hailstne ccurs until the hailstne becmes s large that it falls t the earth. The largest hailstne ever recrded had a mass f 1 kilgram. Snw Pellets Snw pellets are small paque balls f cmpacted snw crystals that are white in appearance but they d nt have the ice cat fund n a hailstne. Snw pellets are usually fund in cnvective strms f winter and spring. II. Precipitatin based upn the prcesses which lead t its frmatin. (a) Cnvectinal Precipitatin Cnvectinal precipitatin is precipitatin that results frm cnvective verturning f mist air and is fund mstly in thunderstrm type cluds. The precipitatin is usually heavy and shwery and cnsists usually f either rain, snw shwers, hail, r snw pellets. (b) Oragraphic Precipitatin Oragraphic precipitatin is precipitatin that frms when air rises and cls as it tries t rise ver a tpgraphic barrier such as a muntain. An example f a lcatin that receives large amunts f ragraphic precipitatin is Cherrapunji, India. During the mnsn seasn mist air frm the Indian Ocean mves ver India heading nrth. As the air tries t rise up and ver the Himalayas muntains it cls t saturatin, then cndensatin, and then eventually rain ccurs. The mean annual amunt f precipitatin thrughut the wrld is abut 86 cm (34 inches). The average rainfall in Cherrapunji is 1144 cm, and the recrd is 2299 cm in (c) Frntal precipitatin Frntal precipitatin is precipitatin that frms when air currents cnverge and rise ver a frntal surface. In warm frnts, figure 5.23, the frntal cnvergence is characterized by the mre gradual slping ascent f warm air ver cler air. Usually the air is mre stable and hence layered type cluds usually frm and the type f precipitatin is usually steady rather than shwery. 5-20

21 Warm frnt cl air warm air rain Figure 5.23 Warm frnt precipitatin. In cld frnts, figure 5.24, the frnt is steep and the warm mist air is pushed up with greater vertical velcity and the cluds tend t be mre f the cumulus type. Usually the air is a little mre unstable and hence the frm f the precipitatin is usually f the shwery cnvectinal type. If the cld frnt is fast mving a squall line may develp ut in advance f the frnt. cld frnt cl air rain warm air Figure 5.24 Cld frnt precipitatin. The Language f Meterlgy 1. Fg - results when atmspheric water vapr cndenses (r sublimes) t the extent that the new frms, water drplets r ice crystals, becme visible and have their base in cntact with the grund. Fgs resulting frm evapratin. (a) Steam Fg - fg that is prduced by intense evapratin f water int relatively cld air. Saturatin ccurs, then cndensatin, then fg. (b) Frntal Fg - fg fund alng the bundary f tw air masses. Evapratin frm warm rain falling thrugh the drier air belw may be fllwed by saturatin and cndensatin in cler layers t frm frntal fg, Fgs resulting frm cling. (a) Radiatin Fg r Grund Fg - fg prduced when fairly calm mist air, which is in cntact with the grund, is cled t saturatin and then cndensatin by nighttime radiatin. (b) Advectin Fg - fg that is frmed when mist air is transprted ver a cld surface. The cld surface causes the air in cntact with it t cl and the air clse t the surface becmes saturated, and the fg frms. 5-21

22 (c) Upslpe Fg - fg that frms when there is a gradual rgraphic ascensin f mist air up a slping plain r hilly regin. The mist air will cl adiabatically t frm upslpe fg prviding the air is already clse t saturatin. (d) Mixing Fg. Mixing fg is fg that ccurs when warm mist air cmes in cntact with cl mist air. A clud is physically an aersl, that is, a visible aggregate f minute water drplets, ice crystals r a mixture f bth suspended in the air. The prcess that is respnsible fr the frmatin f cluds is the cling f the air dwn t saturatin. The tw necessary cnditins fr the frmatin f cluds are: (a) A Cling Mechanism. The mechanism fr the prductin f cluds is the adiabatic cling f rising air. An adiabatic prcess is a thermal prcess that ccurs in which there is n heat exchanged. Therefre the rising air expands and cls adiabatically. When the air is cled t the dew pint temperature the air becmes saturated, the relative humidity becmes 100%, and any further cling will cause cndensatin. The tiny water drplets frmed in the air becmes the clud. (b) A Lifting Mechanism. Rising air can cl t the pint where cndensatin can began and clud drplets can frm. The fur lifting mechanisms fr clud frmatin are: (1) Cnvectin Air at the surface f the earth is warmed. The warm air expands and rises by cnvectin, and cls adiabatically. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm. The usual type f clud that is frmed by cnvectin is the cumulus clud. (2) Cnvergence The cnvergence f wind currents r air masses causes a lifting f the air. Air spirals int a lw-pressure surface at the surface f the earth. The nly place fr this air t g is upward. Hence there is vertical mtin upward in a lw-pressure area. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm. Cnvergence als ccurs ver a regin like Flrida. Air blws inward frm the Gulf f Mexic and the Atlantic Ocean. The air cnverges ver Flrida. The nly place fr it t g is upward. If the rising air cls t saturatin, cndensatin ccurs and clud drplets frm (3) Frntal Lifting A frnt is a bundary between tw different air masses. When these tw air masses cllide the warmer air mass, being lighter, will mve up ver the clder air mass. Hence, the frnt will cause lifting f the air. (4) Orgraphic Lifting Orgraphic lifting ccurs when air pushes up against a muntain barrier. There is n place fr the air t g but upward. Hence the air is frced t rise. Types f Cluds - cluds assciated with strng rising air currents have vertical develpment and a puffy appearance and are called cumulus cluds. 5-22

23 Thse resulting frm gentler lifting tend t spread ut int layers and are called stratus cluds. Cluds are primarily classified n the basis f their height int the fllwing catagries: (1) High Cluds. High cluds are fund at levels abve 6000 m abve the grund and can extend up t the trppause. High cluds belng t the family f Cirrus cluds. These cluds are made up f ice crystals. The different types f high cluds are: (a) Cirrus. Cirrus cluds are nearly transparent, white, fibrus r silky (b) Cirrcumulus. A cirrifrm layer, r patch f small white flakes arranged in grups r lines. Smetimes they have the appearance f ripples, similar t sand n a beach. (c) Cirrstratus. A thin white veil f cirrus, nearly transparent (the sun, mn and stars can be seen thrugh them). (2) Middle Cluds. Middle cluds are fund at levels between 2000 m and 6000 m abve the grund. Middle cluds belng t the family f Alt cluds. These cluds are made up f water drplets, ice crystals, r bth. The different types f middle cluds are: (a) Altcumulus. An altcumulus clud is in the frm f layers r patches f glbular cluds. The cluds may build upward. Frm the grund, the cluds ften lk very much like cirrcumulus cluds, but they are lwer (b) Altstratus. An altstratus clud is defined as a fibrus veil f cluds that is gray r blue gray. When the clud becmes thick and rain starts t fall it is called a nimbstratus clud. (3) Lw Cluds. Lw cluds are fund at levels just abve the grund t 2000 m abve the grund. Lw cluds all have the prefix stratus. These cluds are made up f water drplets. The different types f lw cluds are: (a) Stratus. A stratus clud is a lw unifrm layer f clud resembling fg but nt resting n the grund. Because the thickness f the clud is small, precipitatin, if it ccurs, is light (b) Stratcumulus. Stratcumulus cluds are a lw, gray layer f cluds cmpsed f glbular masses r rlls. They have the same appearance as altcumulus clud nly they are lwer. (4) Cluds f Vertical Develpment. Cluds f vertical develpment are fund at levels frm just abve the grund and can extend all the way up t the trppause. These cluds are made up f water drplets and ice crystals. The different types f cluds are: (a) Cumulus. Cumulus cluds are dense, dme-shaped cluds that have flat bases. Cumulus with little vertical develpment and a slightly flattened appearance are usually assciated with fair weather. (b) Cumulnimbus. A clud f great vertical develpment, twering t 18 km r mre where they spread ut t leeward and frm an anvil f cirrus. The cumulnimbus is the thunderstrm clud that has heavy shwers f rain, snw, r hail, lightning and thunder. 5-23

24 Chapter 5 Cluds and Fg Sky Cver. This is a statement f the fractin f the sky that is cvered by cluds. The categries are (a) Clear. The sky is cnsidered clear if the sky is cmpletely clear r cntains less than 1/10 f cluds. (b) Scattered r smetimes called partly cludy. The sky is cnsidered scattered if the sky is cvered by mre than 1/10 f cluds but less than 6/10 f cluds. (c) Brken r smetimes called cludy. The sky is cnsidered brken if the sky is cvered by mre than 6/10 f cluds but less than 9/10 f cluds. (d) Overcast. The sky is cnsidered vercast if the sky is cvered by mre than 9/10 f cluds. Clud Height. The clud height is the distance frm the grund t the base f the clud. The ceiling is the distance frm the grund t the lwest brken r vercast clud cver. Atmspheric stability. If atmspheric air is stable, then when it is displaced frm its equilibrium psitin, the air will return t its riginal psitin. If atmspheric air is unstable, when displaced frm its equilibrium psitin, the air will cntinue t mve away frm its riginal psitin. If atmspheric air is neutral, when displaced frm its equilibrium psitin, the air will remain at the new psitin. Precipitatin is defined as water in liquid r slid frms falling t the earth. Rain is the mst cmmn frm f precipitatin. It falls frm cluds that are frmed in rising air when the temperature, at least at the lwer levels, is abve the freezing level f 0 0 C. Raindrps may begin as snw but melt as they descend int the warmer air belw. Snw is frmed when the temperature is belw 0 0 C when the saturatin and cndensatin prcess ccurs. Ice crystals are then frmed. If these ice crystals reach the grund we have snw. Sleet (Ice Pellets) Sleet, r as it is nw called, ice pellets, is rain which freezes as it falls frm a warmer layer f air alft thrugh a cld layer f air near the surface. Freezing Rain - Rain that falls frm warm air alft thrugh cld air belw and freezes upn striking the cld surfaces at the grund. Drizzle Drizzle are minute drplets f water that fall s slwly that they seen t flat in the air fllwing the slightest mvement f the air. Drizzle falls cntinuusly frm lw stratus cluds. It is ften accmpanied by fg and pr visibility. Freezing Drizzle Freezing drizzle is drizzle that freezes when it strikes the cld surfaces at the grund. Hail Hail is a frm f precipitatin that falls frm vilent summer thunderstrms. It starts as a rain drp in the cnvective clud but is carried t higher levels by the cnvective currents where it freezes. Repeated rising and falling f the hailstne ccurs until the hailstne becmes s large that it falls t the earth. 5-24

25 Chapter 5 Cluds and Fg Questins fr Chapter 5 1. On a very clear night, radiatin fg can develp if there is sufficient misture in the air. Explain. 2. Discuss the cncept f stability and hw it applies t the weather in the atmsphere. 3. If yu are n the grund withut any weather instruments and yu see sme cluds that might be stratus cluds r alt stratus cluds. Is there any way that yu can really tell the difference. 4. If yu are n the grund and yu see sme cluds that are building up very rapidly. Can yu guess what type f cluds they are and what type f weather yu are prbably ging t get. T g t anther chapter, return t the table f cntents by clicking n this sentence. 5-25