tend to turn to snow, due to cooling, as a result of upward motion or advection. Nonadiabatic  Effects The most important of the nonadiabatic effects taking place during the precipitation process is the cooling, which takes place due to evaporation as the precipitation  falls  through  unsaturated  air  between  the clouds  and  the  surface.  This  effect  is  especially pronounced  when  very  dry  air  is  present  in  the  lower levels, with wet-bulb temperatures at or below freezing. Then, even if the dry-bulb temperature is above freezing in a layer deeper than 1,200 feet in the lower levels, the precipitation may still fall as snow, since the evaporation of the snow will lower the temperatures in the layer between  the  cloud  and  the  surface  until  the below-freezing  wet-bulb  temperatures  are  approached. The actual cooling that occurs during the period when evaporation is taking place may often be on the order of 5° to 10°F within an hour. After the low-level stratum becomes saturated, evaporation practically ceases, and advection brings a rise in temperature in the low  levels.  However,  reheating  often  comes  too  late  to bring a quick change to rain since the temperatures may have  dropped  several  degrees  below  freezing,  and  much snow may have already fallen. The lower levels may be kept  cool  through  the  transfer  of  any  horizontally transported  heat  to  the  colder,  snow-covered  surface. Melting of Snow Melting  snow  descending  through  layers  that  are above  freezing  is  another  process  which  cools  a  layer. To  obtain  substantial  temperature  changes  due  to melting,  it  is  necessary  to  have  heavy  amounts  of precipitation falling, and very little warm air advection. As  cooling  proceeds,  the  temperature  of  the  entire stratum will reach freezing, so that a heavy rainstorm could transform into a heavy snowstorm, Incidents of substantial lowering of the freezing level   due   to   melting   are   relatively   rare.   The combination of heavy rain, and little, if any, warm advection  is  an  infrequent  occurrence. Combined  Effects The combined effects of horizontal temperature advection,   vertical   motion,   and   cooling   due   to evaporation are well summarized by observations of the behavior of the bright band on radar (approximately 3,000 ft). Observers have found that within the first 1 1/2 hours after the onset of precipitation, the bright band  lowers  by  about  500  to  1,000  feet.  This  is attributable primarily to evaporational cooling, and probably secondary to melting. Since evaporational cooling  ceases  as  saturation  is  reached,  warm  air advection, partially offset by upward motion, again becomes dominant, and the bright band ascends to near its original level. The bright band will ascend to its original level approximately 3 hours after the onset of precipitation, and may ascend a few thousand additional feet. Other  nonadiabatic  effects,  such  as  radiation  and heat  exchange  with  the  surface,  probably  play  a relatively  smaller  role  in  the  snow-rain  problem. However, it is likely that the state of the underlying surface (snow-covered land versus open water) may determine whether the lower layers would be above or below  freezing. Occasionally,  along  a  seacoast  in winter,  heat  from  the  open  water  keeps  temperatures offshore  above  freezing  in  the  lower  levels.  Along  the east coast of the United States, for example, coastal areas may have rain, while a few miles inland snow predominates. This  situation  is  associated  with low-level  onshore  flow,  which  is  typical  of  the  flow associated with many east coast cyclones. Actually, this situation cannot be classified as a purely nonadiabatic effect since the warmer ocean air is being advected on shore. GENERAL SYNOPTIC CONSIDERATIONS The snow versus rain problem usually depends upon relatively small-scale synoptic considerations, such as the exact track of the surface disturbance, whether the wind at a coastal station has an onshore component, the position of the warm front, and the orientation of a ridge east of the low. In the larger sense, the snow versus rain zone is directly  related  to  the  position  of  the  polar  front.  The location of the polar front is, in turn, closely related to the position of the belt of strong winds in the middle and upper  troposphere.  When  the  westerlies  extend  farther to the south, storm tracks are similarly affected, and the snow-rain zone may be farther to the south. As the westerlies  shift  northward  of  their  normal  position,  the storm  tracks  develop  across  Canada.  Concurrent  with this northward shift, the United States has above normal temperatures,  and  the  snow-rain  problem  exists  farther to the north. With  a  high  zonal  index  situation  aloft,  the snow-rain zone will extend in a narrower belt, often well 4-22