original level moist adiabatically, By connecting the points   on   a   sounding,   a   wet-bulb   curve   can   be constructed. If  the  wet-bulb  curve  slopes  to  the  right  with increasing  altitude,  the  potential  wet-bulb  temperature increases with height, and the layer is potentially stable. If it slopes to the left with increasing height, more than the saturation adiabats, the layer is potentially unstable. If none of the potential curves intersect the sounding, thunderstorms are not likely to occur. AIR-MASS  THUNDERSTORMS This method is a reasonably simple method for forecasting  air-mass  thunderstorms  in  the  Eastern United  States.  It  does  require  prediction  of  short-range changes  in  the  vertical  distribution  of  temperature  and moisture. l The first consideration is to eliminate those areas whose  soundings  disclosed  one  or  more  of  the  following moisture  inadequacies: . The Dewpoint depression is 13°C or more at any level from 850 through 700 hPa. . The Dewpoint depression sum is 28°C or more at  700-  and  600-hPa  levels. . There is dry or cool advection at low levels. . The surface dewpoint is 60°F or less at 0730 local with  no  substantial  increase  expected  before  early afternoon. l There is a lapse rate of 21°C or less from 850 to 500 hPa. . There is a freezing level below 12,000 feet in an unstable cyclonic flow, producing only light showers. After eliminating all soundings that meet one or more of the previous six conditions, you should use the following  parameters  to  make  the  forecast. . The lapse rate between 850 and 500 hPa. . The sum of the dewpoint depressions at 700 and 600 hPa in degrees C. NOTE:  The  lapse  rate  is  the  difference  in temperature between these two pressure levels. For example, if the temperature at 850 hpa was 15°C and at 500hPa it was –10°C, the difference would equal 25°. These  two  computations  are  used  as  arguments  for the graph in figure 5-5. 25NP0046 Figure 5-5.-Local area thunderstorm graph. Area “A” iS isolated thunderstorms, with a 12 to 1 chance of at least one rain gauge in dense network receiving rain. Area “B” is scattered thunderstorms, with a 4 to 1 chance of reported rain. Area “C” is no rain. One  further  condition  for  the  development  of thunderstorms is the absence of large anticyclonic wind shear, which is measured at 850 hPa. No horizontal shear at this level may exceed 20 knots in 250 miles measured  toward  the  low-pressure  area  from  the sounding station. Figure  5-6  illustrates  how  this measurement is made. STABILITY INDEXES AS AN INDICATION OF INSTABILITY The overall stability or instability of a rawinsonde sounding  is  sometimes  conveniently  expressed  in  the form of a single numerical value called the stability index. Such indexes have been introduced mainly as aids   in   connection   with   particular   forecasting techniques. Most of the indexes take the form of a difference   in   temperature,   dewpoint,   wet-bulb temperature, or potential temperature in height or pressure  between  two  arbitrarily  chosen  surfaces. These  indexes  are  generally  useful  only  when combined, either objectively or subjectively, with other data and synoptic considerations. Used alone, they are less valuable than when plotted on stability index charts and analyzed for large areas. In this respect they have the value of alerting the forecaster to those soundings, routes, or areas that should be more closely examined by  other  procedures. 5-8