Fuel burns at a rate that itself depends on how much flame surface is exposed, so
a fresh log burns faster than a nearly-spent one — an exponential-ish decay rather
than a straight countdown. Flame height is modeled as proportional to the square
root of remaining fuel times the current combustion rate, matching how a real fire
shrinks fastest right after it catches and tapers more slowly as embers.
Wind shears the flame plume and every ember sideways in proportion to their
height above the fuel bed (taller means more time exposed to the draft), and raises
the effective burn rate slightly by feeding the flame more oxygen.