摘要： Clouds exhibit significant horizontal inhomogeneities of their optical and microphysical properties, which complicate their realistic representation in weather and climate models. In order to investigate the directional, horizontal structure of cloud inhomogeneities, two-dimensional (2D) horizontal fields of optical thickness of subtropical cirrus and Arctic stratus with a spatial resolution of <10 m are investigated. The cloud optical thickness fields were derived from downward (transmitted) solar spectral radiance measurements from the ground beneath four cirrus clouds, and upward (reflected) radiances measured from aircraft above ten Arctic stratus clouds. The data were collected during the two major field campaigns Clouds, Aerosol, Radiation, and tuRbulence in the trade wInd regime over BArbados (CARRIBA) and VERtical Distribution of Ice in Arctic clouds (VERDI). Scalar one-dimensional (1D) and 2D autocorrelation functions, as well as power spectral densities are derived from the retrieved τ fields. Decorrelation lengths and scale breaks are identified and used to characterize the size range of the inhomogeneities and their influence on three-dimensional (3D) radiative effects. These studies reveal that there are considerable directional cloud inhomogeneities along and across the prevailing cloud structures. Therefore it is not sufficient to quantify horizontal cloud inhomogeneities by scalar 1D inhomogeneity parameters; 2D parameters are necessarily required.