Measurements of temperature-fluctuation statistics in a locally homogeneous, isotropic turbulent atmosphere above level land are shown to enable the quality of telescopic images of celestial objects to be calculated. Data obtained with captive-balloon-borne apparatus are used, together with other existing data, to calculate the modulation transfer function and Strehl Definition for both long- and short-exposure images formed by typical solar telescopes operating under a variety of meteorological conditions. The size of telescope may thus be matched to the expected atmospheric conditions; the improvement in performance and utilization of the instrument, obtainable by raising it on a tower, may be calculated. An 11-cm aperture telescope, for example, is shown to be atmosphere-limited for long exposures in average daytime conditions; raising it from 2 to 30 m above the ground makes an almost threefold improvement in definition. If observations are confined to intervals of good ‘seeing’ the full resolution of such a telescope may be realized. The existence of such intervals is attributed to convective thermal structure in the atmosphere and observational statistics of periods of below-average temperature fluctuation are given for a range of general meteorological conditions.