 Drift-diffusion decay time

For constant diffusion coefficient $D$ and drift velocity $v\ne 0$, the balance $|v|t\sim \sqrt{Dt}$ gives the characteristic length $D/|v|$ and time $D/v^2$. The fundamental Gaussian distribution of the advection-diffusion equation has a fixed-position exponential factor $\exp [-v^{2}t/(4D)]$. Total probability is still conserved: this describes probability moving away from a fixed observation region, rather than disappearance of the particle. At zero drift, this finite exponential time scale is absent.