Five thousand tons of high explosives detonated on Helgoland, April 18, 1947, created air pressure perturbations recorded on microbarographs between 66 and 1000 km SSE from the blast. Instruments responded to frequencies 0.05–5 c.p.s. Arrival times of abnormal signals at six stations more distant than 220 km, supplemented by high altitude meteorological data and the assumption of negligible winds above 30 km, permit upper atmosphere temperature calculations. Temperatures agree with N.A.C.A. values up to 42 km, but show a reduced gradient above that altitude, and a maximum value 294°K in the temperature hump between 30 and 70 km. This temperature maximum establishes critical ray which is refracted to infinity. A new explanation for observed outer boundaries of abnormal zones is therefore proposed, and substantiated by recorded evidence of dispersion near the temperature maximum. In the signal received near the abnormal zone outer boundary, high frequency content predominates.