where \(\lvert W_{r} \rvert\) is the relative velocity between the firebrand terminal velocity and the ambient vertical velocity,  \(d_p\) is the particle diameter,  \(\rho_{ap}\) is the air density surrounding the particle,   \(\rho_{a}\) is the air density, \(\nu_{air} \) and \(\nu_{p}\) are the ambient kinematic viscosity and the kinematic viscosity immediately surrounding the firebrand, respectively. The kinematic viscosities are calculated as