Rate of spread [Forward rate of spread, Spread rate]
n. The linear rate of advance of a fire front in the direction perpendicular to the fire front.
The above definition allows calculation of rate of spread or fireline intensity for any portion of the fire perimeter . When estimating rate of spread by observing the time interval between flaming front passage at two points, it is important that the two points be oriented perpendicular to the fire front.
The term "forward rate of spread" is used in place of rate of spread by some authors to indicate rate of spread in the heading direction. In that case, the preferred phrase is “head fire rate of spread.” Some authors may also use forward rate of spread to distinguish from other measures of fire growth rate, such as rate of area increase.