Measurements of r_0 and θ_0: two decades and 18 sites

We present a statistical analysis of r (0) and theta(0) measurements collected over the last 2 decades at 18 different sites. Although the site altitudes varied from sea level to 3 km, the major distinguishing feature between average r(0) values was the presence or absence of a turbulent, atmospheric boundary layer above the surface. For locations without a strong boundary-layer inversion, the mean coherence length, r(0), was 93 +/- 6 mm and the mean isoplanatic angle, theta(0), was 9.4 +/- 0.5 murad. Where a boundary-layer inversion was present, the corresponding values for r(0) and theta(0) were 53 +/- 2 mm and 6.5 +/- 0.5 murad, respectively.