TEMPERATURE.—  The 0°C (-3°C east coast) isotherm at the 850-hPa level is used as a basis for snow-rain areas, This isotherm should be carefully analyzed by using all data at 850 hpa. It should then be checked against the surface chart. Keep in mind the following  two  points: 1. In areas of precipitation, locations reporting snow should lie on the cold side of the 0°C (–3°C east coast)  isotherm;  for  locations  reporting  mixed  types  of precipitation (e.g., rain and snow, sleet and snow), the 0°C (–3°C) isotherm will lie very close to or through the location. 2. In areas of no precipitation, the 0°C (–3°C east coast) isotherm will roughly parallel the 32°F isotherm at the surface. In cloudy areas, the separation will be small, and in clear areas, the separation will be larger. At the 850-hPa level, the 0°C wet-bulb temperature should  be  sketched  in,  particularly  in  the  area  where precipitation may be anticipated within the next 12 to 24 hours. This line will serve as the first approximation of the future position of the 0°C isotherm. MOISTURE.—  At the 850-hPa level, the –5°C dewpoint line is used as the basic defining line; at the 700-hPa level, the –10°C dewpoint line is used as the basic defining line. The area at 850 hpa that lies within the overlap of the 0°C isotherm and the –5°C dewpoint line is the first approximation of the maximum snowfall area. All locations within this area have temperatures less than 0°C and spreads of 5°C or less. This area is further  refined  by  superimposing  the  sketched  –10°C dewpoint line at 700 hPa upon the area. Now the final area is defined by the 0°C isotherm and the overlapped minimum dewpoint lines from both levels. This final area becomes the area where moderate or heavy snow will be reported, depending upon the particular synoptic situation. See figure 4-25, MOVEMENT.— The first basic rule for moving the area of maximum snowfall is that it maintains the same relative position to the other synoptic features of the 850-hPa level and surface charts. However, in order to forecast the expansion or contraction of the area, it is necessary  to  forecast  the  lines  that  define  it.  The  0°C isotherm  should  be  forecast  according  to  the  roles  set forth in the section treating this particular phase. The moisture lines may be advected with the winds. The 0°C isotherm  should  also  be  moved  with  rules  stated previously in this chapter. The area of maximum snowfall can be forecast for 12 hours with considerable accuracy, and for 24 hours with fair accuracy, provided a reasonable amount of care is exercised according to rules and subjective ideas mentioned  previously. TEMPERATURE LEARNING  OBJECTIVES:  Analyze  synoptic features in determining temperature forecasts. Temperature  ranks  among  the  most  important forecast elements. Temperatures are not only important in the planning and execution of operational exercises, but also are of keen interest to all of us in everyday life. FACTORS AFFECTING TEMPERATURES Many factors are involved in the forecasting of temperatures.   These   factors   include   air   mass characteristics, frontal positions and movement, amount and  type  of  cloudiness,  season,  nature  and  position  of pressure systems, and local conditions. Temperature, which is subject to marked changes from day to night, is not considered a conservative property of an air mass. Too, it does not always have a uniform lapse rate from the surface up through the atmosphere.  This  means  that  the  surface  air  temperature will  not  be  representative  because  of  the  existence  of inversions,  which  may  be  a  condition  particularly prevalent  at  night. Usually,  the  noonday  surface  air temperature is fairly representative, Let’s  look  at  factors  that  cause  temperature variations. These  factors  include  insolation  and terrestrial radiation, lapse rate, advection, vertical heat transport,  and  evaporation  and  condensation. Insolation and Radiation In  forecasting  temperatures,  insolation  and terrestrial radiation are two very important factors. Low latitudes, for instance, receive more heat during the day than stations at high latitudes. More daytime heat can be expected in the summer months than in the winter months, since during the summer months the sun’s rays are more direct and reach the earth for a longer period of time. Normally, there is a net gain of heat during the day and a net loss at night. Consequently, the maximum temperature is usually reached during the day, and the minimum at night. Cloudiness will affect insolation and terrestrial  radiation.  Temperature  forecasts  must  be made only after the amount of cloudiness is determined. Clouds  reduce  insolation  and  terrestrial  radiation, 4-30