Ground flash density, frequently referred to as GFD or Ng, is defined as the number of lightning flashes striking ground per unit area and per year. Usually it is a long-term average value and ideally it should take into account the yearly variations that take place within a solar cycle—believed to the period within which all climatic variations that produce different GFD levels occur.

A 10-year average GFD map of the continental U.S. obtained by and reproduced here with permission from Vaisala, Inc. of Tucson, Arizona, is presented in Fig. 6.3. Note the considerable large GFD levels affecting remarkably the state of Florida, as well as all the southern states along the Gulf of Mexico (Alabama, Mississippi, Louisiana, and Texas).

High GFD levels are also observed in the southeastern states of Georgia and South Carolina. To the west side, Arizona is the only state with GFD levels as high as 8 flashes=km2=year. The lowest GFD levels (<0.5 flashes=km2=year) are observed in the western states, notably in California, Oregon, and Washington on the Pacific Ocean, in a spot area of Colorado and in the Northeastern state of Maine on the Atlantic Ocean.

It is interesting to mention that a previous (a 5-year average) version of this map showed levels of around 6 flashes=km2=year also in some areas of Illinois, Iowa, Missouri, and Indiana, not seen in the present version. This is often the result of short-term observations that do not reflect all climatic variations that take place in a longer time-frame.

The low incidence of lightning does not necessarily mean an absence of lightning-related problems. Power lines, for example, are prone to failures even if GFD levels are low when they pass through high resistiv it y soils like deserts or when lines span across hills or mountains, where ground w ire or lightning arrester ear thing becomes difficult.

The GFD level is an important parameter to consider for the design of electric power and telecommunication facilities. This is due to the fact that power line performance and damage to power and telecommunication equipment are considerably affected by lightning .

Worldwide, lightning accounts for most of the power supply interruptions in distribution lines and it is a leading cause of failures in transmission systems. In the U.S. alone, an estimated 30% of all power outages are lightning-related on annual average, w ith total costs approaching $1 billion (Kithil, 1998). In De la Rosa et. al. (1998), it is discussed how to determine GFD as a function of TD (thunder days or keraunic level) or TH (thunder-hours).

This is import ant where GFD data from lightning location systems are not available. Basically, any of these parameters can be used to get a rough approximation of ground flash density. Using the expressions described in Anderson et al. (1984) and MacGorman et al. (1984), respectively :
Ng ¼ 0:04TD 1:25 flashes= km2 = year (6:1)
Ng ¼ 0:054TH1: 1 flashes =km2 =year (6:2)

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