The discovery of high altitude cloud formations was made possible in 1885 after the eruption of Krakatoa volcano in Indonesia increased the amount of water vapour in the air.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/climate-change-causing-high-altitude-clouds-becoming-increasingly-visible
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/asian-age-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/volcanic-eruptions
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/atmosphere
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/green-house-gases