Index, devised by H. H. Lamb, of the amount of finely divided material suspended in the atmosphere after great volcanic eruptions, and of the duration of an effective veil intercepting the Sun’s radiation. It can be calculated from estimates of the amount of solid matter thrown up, or from the reduction of intensity of the solar beam, or from the reduction of temperatures prevailing at the surface of the Earth. The latitude of the volcano affects the index value also, because the maximum extent of the veil over the Earth varies, being greatest after eruptions in low latitudes. (The great eruption of Krakatoa in Indonesia in 1883, which ejected about 17 km³ of particulate matter into the atmosphere, where it remained for three years, gave an index value of 1000.)