Live fuels can also be expected to burn actively at these levels. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels. The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior.
Ignition depends on the duration of the current and the kind of fuel the lightning hits.
Ignition in fuels with long and medium length needle cast, such as Ponderosa pine and Lodgepole pine, depend on the fuel moisture. Ignitions in short- needled species, such as Douglas fir depend far more on the depth of the duff layer than on the moisture. Spread of the fire after ignition usually depends on fuel moisture in all cases. The ignition efficiency on a 1 km pixel is given on a per discharge basis. The ratio of positive and negative discharges is built into the calculation.