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National Fire Danger Rating System
Development of a fire danger rating system began in 1922, and in 1972, the NFDRS was released for use among agencies across the United States. The rating system defines areas of homogenous fuel, climate and topography and assesses the likelihood of a fire starting in a particular area, as well as the intensity with which it is likely to spread. The NFDRS follows a color code: green signifies low fire danger, blue is moderate, yellow is high, orange is very high and red is extreme. The Lightning Activity Level rating is an important factor in the ignition component of the NFDRS.
Rating Lightning Activity
The LAL rating is a number from 1 to 6 that denotes the frequency and intensity of cloud-to-ground lightning within a 48-kilometer (30-mile) radius of the point of observation. When determining the fire danger rating, officials include two values for LAL. The first measures lightning activity from the time of the last observation until midnight, and the second measures activity from midnight until the present observation. Along with such other influences as relative humidity, moisture content of foliage, ambient temperature and wind conditions, the LAL contributes to the probability of ignition of a fire, which is the ignition component of the NFDRS.
Six Levels of Activity
LAL 1 indicates that there are no observable thunderstorms or cloud-to-ground lightning strikes. The next level -- 2 -- denotes one or two building cumulus clouds and the possibility of lightning, while level 3 indicates scattered thunderstorms and one or two ground strikes per minute. At level 4, two to three ground strikes occur every minute, and at level 5, lightning is frequent and intense, striking the ground more than three times a minute. Level 6 indicates dry lightning -- which is cloud-to-ground strikes without accompanying rainfall.
Strike Frequencies Similar at Levels 3 and 6
At LAL 3, towering cumulus clouds have formed, but they cover less than 20 percent of the sky, and while at least two thunderstorms occur during the observation period, they are infrequent. It is raining moderately. Lightning activity at LAL 6 is similar to that at LAL 3, but the key difference is the lack of rain at LAL 6. Lightning that develops in dry conditions is often positive lightning, which is the most powerful and dangerous type. It builds above the clouds, where there is a net positive charge, and can strike as far as 10 miles away from the clouds that produce it.