Cloudy and clear stratospheres before A.D. 1000 inferred from written sources

How can observational information about stratospheric transparency in the preinstrumental era be acquired today? It may be reasonably assumed that a high turbidity in the stratosphere is almost always caused by light‐scattering sulfate aerosols derived from large volcanic eruptions. Historical reports of a dimming of the Sun, red twilight glows, reddish solar haloes, and dark total eclipses of the Moon indicate a high turbidity; contrariwise, a ruddy disk shown by the eclipsed Moon betrays a clear stratosphere. On the basis of an extensive search of primary European and Middle Eastern written sources pertaining to the ancient and early medieval periods, seven stratospheric dry fogs, in addition to the four major ones already known, have been identified by using solar observations, and five smaller ones have been detected from dark total lunar eclipses. Seven of the eight most important dry fogs between 300 B.C. and A.D. 1000 can be either definitely or plausibly correlated with high levels of sulfate acidity observed in Greenland ice cores. An important conclusion is that this sample is probably very nearly complete for major dry fogs during this period. A second conclusion is that the ratio of dark to normal total lunar eclipses during early medieval times (A.D. 400–1000) appears to be equal, approximately, to the ratio that has prevailed for the past 40 years. These conclusions suggest that the frequency of volcanic eruptions, both large and moderate, throughout the world may have remained statistically constant (on a long timescale) since at least 300 B.C.