cirrostratus,  may  be  observed  in  the  lower  stratosphere above the polar tropopause, but mainly below the level of the jetstream core. The cirrus clouds of the equatorial zone also generally extend to the tropopause. There is a general tendency for the mean height of the bases to increase  from  high  to  low  latitudes  more  or  less paralleling  the  mean  tropopause  height,  ranging  from 24,000 feet at 70°to 80°atitude to 35,000 to 4,000 feet or higher in the vicinity of the equator. The thickness of individual  cirrus  cloud  layers  are  generally  about  800 feet in the midlatitudes. The mean thickness of cirrus clouds tends to increase from high to low latitudes. In polar continental regions in winter, cirrus clouds are virtually based at the surface. In the midlatitudes and in the tropics, there is little seasonal variation. Cirrus Clouds in Relation to the Jetstream A discussion of cloud types associated with the jetstream is contained in the AG2 TRAMAN, volume 1. In addition to this information, we will discuss a few studies  pertaining  to  cloud  types.  All  of  these  studies agree that most of the more extensive and dense clouds clouds  are  on  the  equatorward  of  the  jet  axis.  The observed  frequency  of  high  clouds  poleward  of  the  jet axis can be accounted for as the upper reaches of a cold front, or cold lows, not directly related to the jetstream. In some parts of a trough, these high clouds may tend to be dense, and in other areas thin. PREDICTION OF SNOW VS RAIN LEARNING  OBJECTIVES:  Evaluate   the surface  and  upper-level  synoptic  situations  in determining  the  form  of  precipitation  in  your forecast. Typically, an inch or so of precipitation in the form of rain will cause no serious inconvenience. On the other hand the same amount of precipitation in the form of  snow,  sleet,  or  freezing  rain  can  seriously  interfere with naval operations. In such cases, the snow versus rain  problem  may  become  a  factor  of  operational significance. Sleet  and  freezing  rain,  which  often  may  occur  in the intermediate period between snow and rain, are generally grouped with snow in our discussion. Any decision arrived at for the snow versus rain problem would, naturally, have to be modified, dependant on your  geographical  location.  This  should  be  easily accomplished through a local study of the optimum conditions.   The   various   techniques   and   systems presented  here  will  often  complement  each  other.  The approach  used  here  is  a  discussion  of  the  general synoptic  patterns  and  the  thermal  relationship;  that  is, the use of temperatures at the surface and aloft, and the presentation of an objective technique to distinguish the types  of  precipitation. GEOGRAPHICAL  AND  SEASONAL CONSIDERATIONS The forecasting problem of snow versus rain arises, naturally, during the colder months of the year. In midwinter when the problem is most serious in the northern  states,  the  southern  states  may  not  be concerned. PHYSICAL NATURE OF THE PROBLEM The type of precipitation that reaches the ground in a  borderline  situation  is  essentially  dependent  on  two conditions. There must be a stratum of above-freezing temperatures between the ground and the level at which precipitation  is  forming,  and  this  stratum  must  be sufficiently deep to melt all of the falling snow prior to striking the surface. Thus, a correct prediction of rain or snow at a given location depends largely on the accuracy with which the vertical distribution of the temperature,  especially  the  height  of  the  freezing  level, can  be  predicted. On  the  average,  it  is  generally satisfactory to assume that the freezing level must be at least 1,200 feet above the surface to ensure that most of the snow will melt before reaching the surface. Effects of Advection In  the  lower  troposphere,  above  the  surface, horizontal advection is usually the dominant factor affecting  local  temperature  changes.  In  most precipitation  situations,  particularly  in  borderline situations, warm air advection and upward motion are occurring simultaneously, giving rise to the fact that warming  generally  accompanies  precipitation. However,  this  effect  is  frequently  offset  when  there  is weak warm advection, or even cold advection, in the cold air mass in the lower layers. In situations where precipitation is occurring in association with a cold upper low, upward motion is accompanied by little, if any, warm advection. In such borderline cases, precipitation may persist as snow, or 4-21