Miombo woodland occupies large areas in Africa south of the equator, including 1/2 the surface area of Tanzania. This vegetation type is found where there is a single-season rainfall regime producing between 50 and 120 cm annually. A number of studies of the environment and vegetation were made during 1962-64 at Lupa North Forest Reserve in the Southern Highlands Region of Tanzania, a site typical of much of the miombo in Western Tazania. This report describes the results of analysis of meteorological, hydrological and tree growth data by the method of Principal Component Analysis. The analysis greatly simplifies presentation and interpretation of the results. In the case of meteorological observations it is suggested that 2 basic independent weather "components'' can be found which account for most of the variation in the annual march of the 10 parameters measured. Plant growth variation in the miombo can be related to changes in these factors, and together with hydrological observations, they are used to provide an explanation of the pre-rain flush of miombo trees. Correlations between meteorological and hydrological components and girth increment of individual trees are also described.