Photochemical processes in the atmosphere are driven by solar ultraviolet radiation. The photodissociation rate coefficients of atmospheric species are determined by the actinic flux, which is defined as 4&pgr; times the mean ultraviolet intensity. Because of the presence of clouds the actinic flux can change drastically throughout the atmosphere. Therefore clouds have large effects on photodissociation rate coefficients. At cloud top, photodissociation rate coefficients can be 300% higher than in clear sky conditions. We use Monte Carlo simulations to investigate the reflectance, the transmittance, and the actinic flux for cloud fields at various degrees of cloudiness. Scattering processes in the clouds are due to cloud particles only. We do not take absorption of radiation into account. The atmosphere outside the clouds is assumed to be completely transparent. The simulated reflectance and transmittance of plane-parallel cloud fields and in broken cloud field conditions reproduce the results of previous model studies within statistical uncertainties. The results of actinic flux calculations for plane-parallel cloud fields agree with the results obtained with a doubling-adding algorithm. Horizontal and vertical actinic flux profiles in broken cloud fields are studied for various solar zenith angles and for different cloud optical thicknesses. The aim of the present model study is to obtain insight into the effect of broken cloud fields on the actinic flux.¿ 1997 American Geophysical Union