Ozone may be a life-saver in the upper atmosphere where it shields us from harmful ultraviolet rays, but closer to the ground it is a pollutant causing asthma in humans and stunted growth in crops. In large cities such as Los Angeles, ozone forms a key ingredient of smog, produced when sunlight reacts with car exhaust and other fumes. Ozone can also reach ground level, however, in smaller amounts through “intrusions” from the stratosphere. Now, Wayne Hocking at the University of Western Ontario and other physicists in Canada have shown that it should be possible to forecast when and where such ozone intrusions occur (Nature 450 281).