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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 

outflow can provide that boost, leading to new a Cb, which is nearby but not connected to the original Cb. 

## 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. 

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## K-Index 

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. 

