METEOROLOGISTS are indebted to Wing-Commander L. W. B. Rees for some very valuable observations during two aeroplane ascents at Cranwell (Lines.) on October 19, showing the change which took place in upper-air conditions during the passage of a “linesquall” or, in the phraseology of Bjerknes, a “cold front.” Observations of this kind are sufficiently rare, and the features exhibited by the present ones are so specially interesting that it seems very desirable to lay some emphasis on them.