Humboldt, viewing qobar in Peru, says […] . When travelling in Spain, Willkomm remarked qobar at a distance of 3 or 4 miles, yet, on reaching the actual spot, he saw nothing. [...] Bravais saw qobar on the Faulhorn, when his hygrometer was at 51, air saturated with moisture marking 100. In Ethiopia, where I have observed it so low as 20, the hygrometer's mean reading was 41 when qobar was conspicuous. Above 72 it disappeared.
1889 January 3, Antoine d'Abbadie, in a letter to the editor of Nature, volume 39, pages 247-248