Method for determining the water content in the atmosphere by processing images acquired by an out-of-focus interferometric imaging device, said method is implemented by a processing unit comprising a calculation unit and a storage unit, said storage unit comprising at least a predetermined distribution (34, 84) of the number of particles (10, 12) according to the size of these particles outside said given range of sizes, said predetermined distribution (34, 84) comprising a number of particles having said threshold size (Dse, Dsm, Dsg), said device being solely able to image water and ice particles having a size (di) lying in a given range of sizes, said method comprising the following step For at least one image, a) determining the number of particles lying the given range of sizes from said image; b) measuring the mass of particles (10, 12) having a size (di) lying in the given range of sizes from the number of particles determined; c) estimating (60, 62, 70, 78, 80, 86) the mass of particles (10, 12) having a size outside the given range of sizes, said estimation step comprising the following steps: i) determining the number of particles having a size equal to a threshold size (Dse, Dsm, Dsg); ii) evaluating (62, 78, 80, 86) the number of particles outside the given range of sizes by multiplying the predetermined distribution (34, 84) by the ratio between the number of particles determined at step i) and the number of particles in said distribution having said threshold size (Dse, Dsm, Dsg); iii) estimating the mass of the particles outside the given range of sizes from the number of particles evaluated; d) determining the water content by adding the estimated mass and the measured mass. a) determining the number of particles lying the given range of sizes from said image; b) measuring the mass of particles (10, 12) having a size (di) lying in the given range of sizes from the number of particles determined; c) estimating (60, 62, 70, 78, 80, 86) the mass of particles (10, 12) having a size outside the given range of sizes, said estimation step comprising the following steps: i) determining the number of particles having a size equal to a threshold size (Dse, Dsm, Dsg); ii) evaluating (62, 78, 80, 86) the number of particles outside the given range of sizes by multiplying the predetermined distribution (34, 84) by the ratio between the number of particles determined at step i) and the number of particles in said distribution having said threshold size (Dse, Dsm, Dsg); iii) estimating the mass of the particles outside the given range of sizes from the number of particles evaluated; i) determining the number of particles having a size equal to a threshold size (Dse, Dsm, Dsg); ii) evaluating (62, 78, 80, 86) the number of particles outside the given range of sizes by multiplying the predetermined distribution (34, 84) by the ratio between the number of particles determined at step i) and the number of particles in said distribution having said threshold size (Dse, Dsm, Dsg); iii) estimating the mass of the particles outside the given range of sizes from the number of particles evaluated; d) determining the water content by adding the estimated mass and the measured mass.