AbstractThe variations of the ozone content of surface air during depressions have been investigated. Three definite conditions were found to occur, apparently without exception.A drop in the ozone content is found up to 300 miles in advance of warm fronts, with high values of ozone after their passage.An occluded front of the warm front type behaved in exactly the same way as a front with a warm sector.A drop in the ozone content is found after the passage of backbent occlusions, persisting until the front has moved on up to 200 miles, after which the ozone returns to its normal concentration.This seems to indicate that the observed change under these conditions in the ozone content is due to the increased stability of air not at the surface but at the upper warm front inversion. This would prevent the ozone (constantly destroyed near the ground) from being restored by turbulent interchange with air from greater heights.