Detailed numerical simulations have been performed to study the ignition behavior of hot spherical particles at atmospheric conditions. The particles move relative to a combustible gas with a velocity of 0-30 m s, which spans different flow regimes, from creeping flow to unsteady vortex shedding. The temperature of the particles' surface increases linearly over time and is recorded at ignition for methane/air and hydrogen/air mixtures. For low relative velocities m s or Reynolds numbers , increases proportionally to or and the flow field is axisymmetric. For higher relative velocities, an unsteady vortex street forms behind the particle so that three-dimensional simulations are required. A correlation employing the van't Hoff criterion yields linear correlations based on the Nusselt number and for both the low- and high-velocity ranges. For rich hydrogen flames at high velocities, the flame temporarily stabilizes near the hot particle in the recirculation zone downstream. As the surface temperature increases further, the flame suddenly starts to propagate downstream, leading to two distinct ignition events: local ignition at the particle's surface and start of the propagation into the surrounding gas. The latter yields a much steeper increase of ignition temperature with incoming flow velocity.