![]() The study analyses and discusses two recently discovered weather diaries from the agricultural region of Entre-Douro-e-Minho, in northwest Portugal (along the Atlantic coast of the Iberian Peninsula, southwest Europe). This paper aims to assess the impact of hydrometeorological phenomena on agricultural production in northwest Portugal, from the end of the eighteenth century through the first three decades of the nineteenth century. This date has important implications for the associated hemispheric climate impacts and temporal pat-tern of aerosol dispersal. ![]() ![]() Moreover, these observations confirm that the Unknown eruption, pre-viously identified and tentatively assigned to February 1809 (±4 months) from analysis of ice core sulfate records, oc-curred in late November or early December 1808 (4 Decem-ber 1808 ☗ days). These two accounts provide direct evidence of a persistent stratospheric aerosol veil that spanned at least 2600 km into both Northern and Southern Hemispheres and establish that the source was a tropical volcano. The second, made by physician José Hipólito Unanue in Lima, Peru, describes sunset after-glows (akin to well-documented examples known to be caused by stratospheric volcanic aerosols) from mid-December 1808 to February 1809. The first, Francisco José de Caldas, describes a stratospheric aerosol haze, a "transparent cloud that ob-structs the sun's brilliance", that was visible over the city of Bogotá, Colombia, from 11 December 1808 to at least mid-February 1809. The ob-servations were made by two highly respected Latin Ameri-can scientists. Here we report on two me-teorological observations dating from the end of 1808 that describe phenomena we attribute to volcanic-induced atmo-spheric effects caused by the Unknown eruption. However, no eyewitness accounts of the event, and therefore its location, or the atmospheric optical effects associated with its aerosols have been docu-mented from historical records. ![]() The Unknown eruption of 1808/1809 was the sec-ond most explosive SO 2 -rich volcanic eruption in the last two centuries, eclipsed only by the cataclysmic VEI 7 Tamb-ora eruption in April 1815. ![]()
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