Since water is an important source of these odd hydrogen species, higher abundance of ozone is usually observed in the regions with lower water vapor content.[49] Measurements showed that the total column of ozone can reach 2–30 μm-atm around the poles in winter and spring, where the air is cold and has low water saturation ratio.[50] The actual reactions between ozone and odd hydrogen species may be further complicated by the heterogeneous reactions that take place in water-ice clouds.[51]