$_{Figure 4-31.}$  Radar summary chart. •  Type of precipitation—the type of precipitation is  marked on the chart using specific symbols. •  Echo configuration—echoes are shown as areas, cells,  or lines. •  Weather watches—severe weather watch areas for  tornadoes and severe thunderstorms are depicted by  boxes outlined with heavy dashed lines. Stability/Lifted Index Chart Composite Moisture Stability Chart A valuable tool for preflight planning, the SD chart has  several limitations. Since it depicts only areas of current  precipitation, it will not show areas of clouds and fog with no  appreciable precipitation, or the height of the tops and bases  of the clouds. SD charts should be used in conjunction with  current METAR and weather forecasts. The composite moisture stability chart is a chart composed  of four panels depicting stability, precipitable water,  freezing level, and average relative humidity conditions.  This computer-generated chart contains data obtained from  upper-air observations, is updated twice a day, and shows  the relative stability of the air mass and the potential for  thunderstorms or thermal activity.  The LI (numerator of the fraction) is a common measure of  atmospheric stability. It is calculated by hypothetically lifting  a parcel of air to the 500 mb level, approximately 18,000 feet  MSL, and analyzing its stability at that level. A positive LI  indicates that a particular parcel of air is stable and resists  further upward motion. Large positive values (+8 or greater)  would indicate very stable air. Conversely, a negative LI  means that a lifted surface parcel of air is unstable, and more  likely to rise. Large negative values (–6 or more) indicate  very unstable air.  A subdisplay of the composition moisture stability chart is  the stability or lifted index (LI) chart, a valuable tool for  determining the stability of the atmosphere. The stability  or LI chart is the upper left hand panel of the composite  moisture stability chart. Two indexes represent the moisture  and stability of the air: the K index (KI) and the LI, with  these numbers composing a fraction. KI (denominator of the  fraction) provides moisture and stability information. Values  range from high positive values to low negative values. A  high positive KI indicates moist, unstable air. KI values are  considered high when at or above +20, and low when less  than +20.  [Figure 4-32] 4-32