The southeasterly wind pattern measured at Chalco was the most consistent flow feature measured during the experimental period. These winds developed in the mid-afternoon in a layer up to 1 km deep and continued for several hours. The local upslope flows evident in the early afternoon in the southwestern part of the basin were weaker and shallower than anticipated. These flows have been associated with the prevalence of high ozone concentrations in this area of the valley. On about half of the days studied, late afternoon drainage flows from the northeast, beginning at about 1800 CST and persisting for several hours, ventilated the valley. Mixed layers grew.
