cirrostratus, may be observed in the lower stratosphereabove the polar tropopause, but mainly below the levelof the jetstream core. The cirrus clouds of the equatorialzone also generally extend to the tropopause. There isa general tendency for the mean height of the bases toincrease from high to low latitudes more or lessparalleling the mean tropopause height, ranging from24,000 feet at 70°to 80°atitude to 35,000 to 4,000 feetor higher in the vicinity of the equator. The thickness ofindividual cirrus cloud layers are generally about 800feet in the midlatitudes. The mean thickness of cirrusclouds tends to increase from high to low latitudes. Inpolar continental regions in winter, cirrus clouds arevirtually based at the surface. In the midlatitudes and inthe tropics, there is little seasonal variation.Cirrus Clouds in Relation to the JetstreamA discussion of cloud types associated with thejetstream is contained in the AG2 TRAMAN, volume 1.In addition to this information, we will discuss a fewstudies pertaining to cloud types. All of these studiesagree that most of the more extensive and dense cloudsclouds are on the equatorward of the jet axis. Theobserved frequency of high clouds poleward of the jetaxis can be accounted for as the upper reaches of a coldfront, or cold lows, not directly related to the jetstream.In some parts of a trough, these high clouds may tend tobe dense, and in other areas thin.PREDICTION OF SNOWVS RAINLEARNING OBJECTIVES: Evaluate thesurface and upper-level synoptic situations indetermining the form of precipitation in yourforecast.Typically, an inch or so of precipitation in the formof rain will cause no serious inconvenience. On theother hand the same amount of precipitation in the formof snow, sleet, or freezing rain can seriously interferewith naval operations. In such cases, the snow versusrain problem may become a factor of operationalsignificance.Sleet and freezing rain, which often may occur inthe intermediate period between snow and rain, aregenerally grouped with snow in our discussion. Anydecision arrived at for the snow versus rain problemwould, naturally, have to be modified, dependant onyour geographical location. This should be easilyaccomplished through a local study of the optimumconditions. The various techniques and systemspresented here will often complement each other. Theapproach used here is a discussion of the generalsynoptic patterns and the thermal relationship; that is,the use of temperatures at the surface and aloft, and thepresentation of an objective technique to distinguish thetypes of precipitation.GEOGRAPHICAL AND SEASONALCONSIDERATIONSThe forecasting problem of snow versus rain arises,naturally, during the colder months of the year. Inmidwinter when the problem is most serious in thenorthern states, the southern states may not beconcerned.PHYSICAL NATURE OF THE PROBLEMThe type of precipitation that reaches the ground ina borderline situation is essentially dependent on twoconditions. There must be a stratum of above-freezingtemperatures between the ground and the level at whichprecipitation is forming, and this stratum must besufficiently deep to melt all of the falling snow prior tostriking the surface. Thus, a correct prediction of rainor snow at a given location depends largely on theaccuracy with which the vertical distribution of thetemperature, especially the height of the freezing level,can be predicted.On the average, it is generallysatisfactory to assume that the freezing level must be atleast 1,200 feet above the surface to ensure that most ofthe snow will melt before reaching the surface.Effects of AdvectionIn the lower troposphere, above the surface,horizontal advection is usually the dominant factoraffecting local temperature changes. In mostprecipitation situations, particularly in borderlinesituations, warm air advection and upward motion areoccurring simultaneously, giving rise to the fact thatwarming generally accompanies precipitation.However, this effect is frequently offset when there isweak warm advection, or even cold advection, in thecold air mass in the lower layers.In situations where precipitation is occurring inassociation with a cold upper low, upward motion isaccompanied by little, if any, warm advection. In suchborderline cases, precipitation may persist as snow, or4-21
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