The effective wavefront outer scale L0 was monitored continuously during 16 nights in August-September 1997 using the new Grating Scale Monitor (GSM) instrument. Simultaneous comparison with the data form open-loop adaptive optics system on the ESO 3.6 m telescope revealed a good agreement of the L0 derived by both methods. The GSM principle is similar to the Shack-Hartmann sensor: angle of arrival fluctuations are measured with four 10-cm telescopes and the outer scale is computed from their normalized covariances. The seeing and effective wavefront velocity are measured as well. GSM does not suffer form dome and mirror seeing effects and can be used anywhere for site evaluation. Outer scale has log-normal distribution, its median is found to be 24 m, with a rms scatter of log L0 of +/- 0.22. Short isolated bursts of high L0 values were sometimes observed. A weak correlation of L0 and seeing angle exists for some nights. Decametric values of L0 found here imply that at baselines of few meters the influence of finite L0 on the energy balance between tip-tilt and higher-order modes must be properly taken into account. This can sensibly modify the predicted performance of adaptive optics systems.