Two recent volcanic eruptions (Volcan del Fuego in Guatemala, October 1974, and Ngauruhoe in New Zealand, 19 February 1975) may have populated the stratosphere with sufficient ash to create large-scale stratospheric dust veils. Optical effects on twilight sky radiance, apparently created by the eruption of Fuego, were observed at College, Alaska, three months after the eruption. In lower latitudes the Fuego dust veil effects were observed to peak and then decline one or two months following the eruption. Based on these observations, it is surmised that the Fuego dust veil underwent northerly meridional transport in winter and spring.
On 20 March 1975, however, a strong volcanic twilight was observed at Mauna Loa, Hawaii. Lidar echoes showed the presence of a two-layer dust cloud. It is suggested that this episode may have been caused by dust from Ngauruhoe that was mixed across the Hadley cell circulation system from the Southern Hemisphere.