Thunderstorms Summer (June–August) 20 20 10 10 5 5 1 1 20 20 20 20 30 30 40 40 50 50 50 40 40 40 30 Figure 9-15.  Thunderstorm frequency in the summertime. outflow can provide that boost, leading to new a Cb, which  is nearby but not connected to the original Cb. is in a headwind situation. With the likely outcome of that  combination of factors, and the hazards of ground handling,  operations in such conditions must be severely discouraged. Pilots need to watch apparently  dissipating thunderstorms  closely for new dark, firm bases that indicate a new cell  forming. In addition, outflow from one Cb may flow several  miles before encountering an area where the air is primed  for lifting given an extra boost. The relatively cool air in the  The KI is used to determine the probability of thunderstorm  occurrence and uses information about temperature and  moisture at three levels. It is given by the equation KI =  (T850 – T500) + Td850 – (T700 – Td700). Here, T stands  for temperature, Td is the dewpoint, and 500, 700, or 850  indicates the level in mb. All values are obtained from  a morning sounding. Using the following values from a  morning sounding KI = (16 – [–9]) + 12 – (6 – 0) = 31. This  indicates about a 60 percent probability that thunderstorms  will occur.  [Figure 9-18]  As discussed below, charts showing  both the LI and KI for all the sounding sites in the continental  United States are produced daily. K-Index Lifted Index The LI is determined by subtracting the temperature of a  parcel that has been lifted to 500 mb from the temperature  of the ambient air. This index does not give the likelihood  of thunderstorm occurrence; rather, it gives an indication of  thunderstorm severity if one does occur. In  Figure 9-17 , an LI of  –5 indicates moderately severe thunderstorms if they develop.  Figure 9-16.  A cumulus growing to a towering cumulus. 9-12 Lifted Index 0 to –2  –3 to –5 ≤ –6 Chance of Severe Thunderstorm Weak Moderate Strong Figure 9-17.  Lifted index (LI) versus thunderstorm severity.