Lightning is obviously the most spectacular of electrometers and is directly related to the thunderstorm even though classified independ-ently. It is the bright flash of light accom-panying a sudden electrical discharge. Most lightning has its beginning in clouds; however, it generates from high structures on the ground and mountains, although much less frequently. The thunderstorm changes the normal electric field, in which the ground is negatively charged with respect to the air above it. Because the upper portion of the thunderstorm cloud is positive and the lower part is negative, the negative charge induces a positive charge on the ground. The distribution of the electric charges in a typical thunderstorm is shown in figure 5-4-3. The lightning first occurs between the upper positive charge area and the negative charge area immediately below it. Lightning discharges are considered to occur most fre-quently in the area bracketed roughly by the 32F and the 15F temperature levels. However, this does not mean that all discharges are con-fined to this region; as the thunderstorm develops, lightning discharges may occur in other areas and from cloud to cloud, as well as from cloud to ground.

Figure 5-4-3.Location of electric charges inside a typical thunderstorm cell. There are four main types of lightning. All can do considerable damage to aircraft, especially to radio equipment.

1. CLOUD TO GROUND LIGHTNING (CG). Lightning occurring between cloud and ground

2. CLOUD DISCHARGES (IC). Lightning taking place within the cloud

3. CLOUD TO CLOUD DISCHARGES (CC). Streaks of lightning reaching from one cloud to another

4. AIR DISCHARGES (CA). Streaks of lightning passing from a cloud to the air that do not strike the ground

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