It is important to recognize, however, that regional hazes are not necessarily caused by local emissions, nor do they depend on stagnant meteorological conditions. In the absence of precipitation, airborne particles (and their gaseous precursors) can exist in the atmosphere for many days and can be carried great distances by winds. Studies have shown that regional hazes are often associated with transport from distant sources. Haze episodes at eastern sites are well correlated with previous occurrences (more than 36 hours earlier) of low wind speeds (periods of stagnation) in upwind source regions (Samson, 1978). High SO₄^2- concentrations at rural areas of upstate New York (including the Adirondack Mountains) most often are associated with winds coming from the south and southwest (Galvin et al., 1978). High SO₄^2- concentrations at Shenandoah National Park, Virginia, usually are associated with moderate wind flows from the west to northwest (Wolff et al., 1982).

Figure 2-1 shows isopleths of median visual range at rural U.S. sites (Trijonis et al., 1990). The spatial patterns in this map are based on airport observations of visual range which differ from measurements of standard visual range.¹ Airport visual range measurements are based on the identification by human observers of targets at known distances from the observation point, whereas standard visual range is calculated from light extinction measurements. However, airport observations have been calibrated against the results of instrumental studies for standard visual range, and the airport data presented in these figures have been adjusted accordingly.

Figure 2-1 shows that the mountainous Southwest has the best visibility in the country. Median standard visual range exceeds 150 km in the region comprising Utah, Colorado, Nevada, northern Arizona, north