A Monte Carlo method has been used to simulate the transport of visible (0.45 μm) and near-infrared (0.87 μm) photons which are produced by transient light sources, such as lightning, within cubic, spherical and cylindrical clouds. Computations of the total absorption, the fractions of photons which escape various cloud surfaces, the flux density of photons at the cloud surface, and the angular distributions of the photons are given for various source locations. The results show that the total absorption with point and extended sources is negligible in the visible and less than 20% in the near-infrared with total optical depths up to 400. The fractions of photons which escape various cloud surfaces are high when the sources are close to the surface; and if a source is at the optical center of the cloud, these fractions are the same as if there were no cloud present at all, except for the absorption loss. Most cloud discharges and the in-cloud portion of cloud-to-ground flashes occur at or above t... Abstract A Monte Carlo method has been used to simulate the transport of visible (0.45 μm) and near-infrared (0.87 μm) photons which are produced by transient light sources, such as lightning, within cubic, spherical and cylindrical clouds. Computations of the total absorption, the fractions of photons which escape various cloud surfaces, the flux density of photons at the cloud surface, and the angular distributions of the photons are given for various source locations. The results show that the total absorption with point and extended sources is negligible in the visible and less than 20% in the near-infrared with total optical depths up to 400. The fractions of photons which escape various cloud surfaces are high when the sources are close to the surface; and if a source is at the optical center of the cloud, these fractions are the same as if there were no cloud present at all, except for the absorption loss. Most cloud discharges and the in-cloud portion of cloud-to-ground flashes occur at or above t...