This paper describes cloud‐to‐ground lightning that lowers positive charge to earth over mountainous terrain in the northern Rocky Mountain region. A sample of 14 visually confirmed ground flashes provided criteria for identifying positive ground flashes by using only measured electric fields. According to these criteria, 75 positive ground flashes occurred within the 2830 km2 sampling area during three consecutive summer seasons. About 3% of all ground flashes lowered positive charge. The area density of positive ground flashes is about 0.01/km2 per summer season compared to an area density of negative ground flashes of 0.3/km2 per summer season. Positive ground flashes occurred within the sampling area on 16 of 48 thunderstorm days. Daily occurrences ranged from 1 to 11. When lightning could be associated with individual thunderstorms, the positive flashes usually occurred in sequence during the final 30 min of the storm. The 75 positive flashes are very similar. Each has a single R stroke followed by a continuing current interval. The continuing current duration distribution is approximately log normal, with range 5 to 160 ms and mean of about 60 ms. The prestroke intervals, the total period of field change preceding the first R stroke, are similarly distributed over the range 65 to 210 ms with mean of about 130 ms. Leader field change durations, the latter portion of the prestroke interval, range from 40 to 70 ms with mean of about 50 ms. The positive ground flashes occurred over mountainous and forested terrain free of man‐made structures. Available data indicate that the positive ground flashes for this region may, in most instances, be initiated by positive downward leaders following a horizontally elongated out‐of‐cloud path to ground.

The hypothesis that lightning fires are caused by a special type of lightning discharge has been presented several times in literature over the past 20 years. Working with laboratory sparks, McEachron and Hagenguth [1942] suggested that ignition by natural lightning is usually caused by a discharge having a long‐continuing current phase. This view is widely accepted [Berger, 1947; Malan, 1963; Loeb, 1966], even though field corroboration has been almost nil. To the authors' knowledge, the two discharges and resultant fires documented by Norinder et al. [1958] are the only natural events reported to date wherein both the discharge and its ignition effects were documented.

Of 16 documented lightning discharges 11 caused forest fires in western Montana forests. All 11 discharges exhibited a long‐continuing current (LCC) phase of at least 40 msec duration. Of the 5 non‐fire discharges, 2 had LCC phases and 3 did not. Results to date strongly support the hypothesis that forest fires are caused by discharges with long‐continuing currents. However, available data suggest that discrete discharges (cloud‐to‐ground discharges without LCC portions) cannot be entirely ruled out as a source of forest fuel ignition.

No abstract

No abstract