The Chaco is a northward extension of the foothill plains of Argentina into western Paraguay and eastern Bolivia. The region is visited by traveling weather disturbances that bring cold fronts from the south into contact with the moist air masses from the northeast. Extensive cloudiness and a catalogue of cloud types comes with these disturbances.
Most large disturbances have thinner spots and holes where mobile observers can gather.
When the fronts are far to the south, unorganized afternoon cloudiness and thundershowers may build. These usually start well after the appointed time of the shadow passage, leaving the mornings with brighter prospects. Figure 9 suggests that no particular direction is favored in central and eastern Paraguay and in Argentina. The large scale weather patterns affect them all quite equally. These conflict with the climatology of the area and are probably due to a very short period of record or erroneous data collection.