to 16,000 feet, and there was dry air above at 500-hPa (approximately  18,000  feet),  you  wouldn’t  suspect, from  the  500hPa  analysis,  the  existence  of  clouds below the 500-hPa level. However, an analysis of the extension of the moist layers  in  three  dimensions  can  be  obtained  simply  by scrutinizing  individual  RAOBs.  Those  selected  should be in the general vicinity of, and the area 500 to 1,200 miles upstream of, the area of interest, depending on the forecast period. The heights of the bases and tops can be  indicated,  though  there  is  little  advantage  in indicating  a  dry  layer  2,000  to  3,000  feet  thick sandwiched  between  thicker  moist  layers.  Usually,  it  is sufficient to indicate the entire moist layer, without bothering about any finer stratums. A survey of the cloud field is made easier by writing the heights of the bases and tops in different colors. A moist layer for the sake of simplicity may be defined as a layer having a frost point depression of  3°C or less (i.e., a dewpoint depression of 4°C at –10°C;  5°C at -20°C; 6°C at –30°C). PRECIPITATION AND CLOUDS The type and intensity of precipitation observed at the surface is related to the thickness of the cloud aloft, and particularly to the temperatures in the upper portion of the cloud. The   results   of   a   study   relating   cloud-top temperatures to precipitation type and intensity are as follows: . From aircraft ascents through stratiform clouds, along   with   simultaneous   surface   observations   of precipitation, it was found that 87 percent of the cases where  drizzle  occurred,  it  fell  from  clouds  whose cloud-top  temperatures  were  warmer  than  –5°C.  The frequency of rain or snow increased markedly when the cloud-top  temperature  fell  below  –12°C. l   When   continuous   rain   or   snow   fell,   the temperature of the coldest part of the cloud was below –12°C in 95 percent of the cases. l Intermittent rain was mostly associated with cold cloud-top   temperatures. .  When  intermittent  rain  was  reported  at  the surface, the cloud-top temperature was colder than –12°C in 81 percent of the cases, and colder than –20°C in 63 percent of the cases. From this, it appears that when minor snow (continuous or intermittent) reaches the ground from stratiform clouds, the clouds (solid or layered) extend in most cases to heights where the temperature is well below –12°C, or even –20°C. This rule cannot be reversed. When rain or snow is not observed at the surface, middle clouds may well be present in regions where the temperature is below -12°C or –20°C. Whether or not precipitation reaches the ground will depend on the cloud thickness, height of the cloud base, and the dryness of the air below the base. INDICATIONS OF CIRRUS CLOUDS IN RAOB Cirrus clouds form at temperatures of –40°C or colder. At these temperatures, as soon as the air is brought  to  saturation,  the  condensate  immediately freezes.  The  ice  crystals  often  descend  in  altitude slowly,  to  levels  that  have  air  temperatures  of  –30°C, and persist if the humidity below the formation level is high  enough  to  support  saturation.  In  general,  cirrus clouds  are  found  in  layers  that  are  saturated,  or supersaturated, with respect to ice at temperatures colder than 0°C. OBSERVATION AND FORMATION OF CIRRUS Cirrus,  or  cirriform  clouds,  are  divided  into  three general  groups: cirrus  (proper),  cirrostratus,  and cirrocumulus.  Cirrus  clouds,  detached  or  patchy, usually do not create a serious operational problem. Cirrostratus and extensive cirrus haze, however, may be troublesome  in  high-level  jet  operations,  aerial photography, interception, rocket tracking, and guided missile  navigational  systems.  Therefore,  a  definite requirement for cirrus cloud forecasting exists. The  initial  formation  of  cirrus  clouds  normally requires that cooling take place to saturation, and to have temperatures  near  –40°C.  Under  these  conditions, water  droplets  are  first  formed,  but  most  of  them immediately freeze. The resulting ice crystals persist as long as the humidity remains near saturation with respect to ice. There is some evidence that the speed of the cooling, and the kind and abundance of freezing nuclei, may have an important effect on the form and occurrence  of  cirrus  clouds.  Slow  ascent  starts crystallization   at   humidities   substantially   below saturation; this is presumably the case in extensive cirrostratus  clouds  associated  with  warm  frontal altostratus clouds. If slow ascent occurs in air that has insufficient freezing nuclei, a widespread haze may result,  which  at  –30°  to  –40°C  is  predominantly composed  of  water  droplets.  In  the  case  of  more  rapid 4-19