The use of cell-size parameters from oak trees as a powerful proxy for past climate reconstruction needs to be more intensively studied. The freeware tool CATS will help pushing these high-volume cell-size measurements. In this study, we analyzed the response of vessel size chronologies to daily meteorological records. For this purpose, we developed a computer-aided tree-ring analysis software (CATS) for high-volume cell-size measurements. To stress test CATS, quantitative vessel parameters were measured in 22 living ring-porous oak trees from two riparian-like forests (Mainfranken, southern Germany). Climate response analysis was performed using daily meteorological records. To make the measurement of millions of such vessels within wooden samples feasible, semi-automated computer-aided software for high-volume analysis is crucial. To expedite the measurement of the cells within distinct tree rings, CATS was developed to enable the automated measurement of a high number of samples and tree rings. CATS uses common image segmentation techniques as well as novel software algorithms to detect earlywood vessel superclusters that are used to identify distinct tree rings. The software structure of CATS is described in detail. The climate–vessel response analysis shows a clear temperature signal from mid-March to mid-April (March 14–April 18). This effect is particularly pronounced for a 50-year span in the twentieth century (1915–1964).