AEROGRAPHER'S MATE 3 & 2 



becomes saturated when it is lifted dry adia- 

 batically To obtain, find the intersection of 

 the saturation mixing-ratio line of the surface 

 dewpoint and the dry adiabat of the surface 

 temperature. (See figure 11-36.) 



2. CONVECTION CONDENSATION LEVEL 

 (CCL)-This is the height at which a parcel 

 of air, if heated sufficiently from below, will 

 rise adiabatically until it is just saturated. To 

 obtain, proceed upward along the saturation 

 mixing-ratio line of the surface dewpoint until 

 you intersect the temperature curve of the 

 sounding. This is the CCL. (See figure 11-35.) 



3. CONVECTION TEMPERATURE (CT)-This 

 is the surface temperature that must be at- 

 tained, if convective clouds are to form by 

 solar heating of the surface- air layer. To ob- 

 tain, proceed from the CCL dry adiabatically 

 to the surface level and read the temperature 

 at this point. (See figure 11-35.) 



4. LEVEL OF FREE CONVECTION (LFC)- 

 The level at which a parcel of saturated air 

 becomes warmer than the surrounding air and 

 begins to rise freely. To determine, start at the 

 LCL and draw a line upward, parallel to the near- 

 est saturation adiabat until you intersect the 

 temperature curve; this is the LFC. All sound- 

 ings, however, do not have a LFC. 



5. POSITIVE AND NEGATIVE AREAS-When 

 a parcel on a sounding lies in a stable layer, 

 energy has to be supplied to it, if it is to 

 move either up or down. This is called a 

 negative area. A positive area is when a parcel 

 can move freely because it is in a layer where 

 the adiabat it follows is warmer than the sur- 

 rounding environment. 



To determine the positive/negative areas 

 for convective lifting, construct a saturation 

 adiabat from the CCL to the top of the chart 

 and a dry adiabat downward to the surface from 

 the CCL. Any area bounded by the tempera- 

 ture curve on the left and the drawn satura- 

 tion adiabat on the right is positive and shaded 

 red. Any area bounded by the temperature curve 

 on the right and the drawn saturation adiabat 

 on the left is negative and shaded blue. (See 

 figure 11-37.) 



To determine the positive/negative areas for 

 mechanical lifting, continue the saturation adiabat 

 from the LFC (which was previously determined) 



to the top of the chart. Below the LFC the area 

 bounded on the right by the temperature curve 

 and on the left by the dry adiabat, to the LCL, 

 then bounded by the saturation adiabat to the 

 LFC again, is a negative area and shaded blue. 

 Above the LFC, any area bounded by the tem- 

 perature curve on the left and the saturation 

 adiabat on the right is positive and shaded 

 red, while areas bounded by the temperature 

 curve on the right and the saturation adiabat 

 to the left are negative and shaded blue. (See 

 figure 11-38.) 



6. STABILITY INDEX (SI)-This is a computed 

 value used to forecast the probability of thun- 

 derstorm and tornado occurrence. Values greater 

 than +3 indicate that shower activity is unlikely. 

 Values +3 to -2 indicate shower and thunder- 

 storm activity are likely, while values of -3 

 or less are associated with severe thunder- 

 storm activity, and a value of below -6 in- 

 dicates the possibility of tornadoes. 



Although several methods are available to 

 compute this value, only one method will be 

 discussed here. This is the SHOWALTER 

 METHOD. In this method, determine the LCL 

 for the 850-mb level; from the 850-mb LCL, 

 draw a line upward parallel to the nearest 

 saturation adiabat until it intersects the 500-mb 

 level. Read the temperature (T') at this point, 

 then read the actual temperature (T500) °* the 

 500-mb level. Now algebraically subtract (T') 

 from (T500); the value of the remainder, in- 

 cluding its algebraic sign, is the value of the 

 SI. (See figure 11-39.) 



There are many more parameters that can 

 be determined on the SKEW-T Diagram. 



To aid in determining parameters and for 

 a more detailed description of procedures used 

 to determine the parameters already discussed, 

 the Aerographer's Mate should refer to NA50- 

 1P-5, Use of the SKEW-T Diagram in Anal- 

 ysis and Forecasting; NA50-1P-6, Forecasting 

 of Aircraft Condensation Trails; NA50-1P-7, 

 Computation of Atmospheric Refractivity on the 

 USAF SKEW-T Diagram; and NA50-1P-8, Mem- 

 orandum of Density-Altitude. 



FILING AND DISPLAY 



An extensive amount of weather data is 

 constantly received in most weather offices 

 via teletypes, facsimile, satellite and other 

 means. This data must be sorted, identified, 

 and displayed properly, so that is available 



238 



