John Day was born in Salina, Kansas, and as a child loved to be outdoors. He played piano in a dance band to support himself while attending Colorado College, where he was studying physics and math. He graduated in 1936 and married in 1937. The couple settled near Oakland, California, and Day took a job as an airport guide. He joined the first class of the Boeing School of Aeronautics and learned meteorology. He continued to work for the company during World War II when the Army took it over. In 1946, at the end of the war, Day resigned from PanAm to teach physics and meteorology at Oregon State University while continuing his studies. He received a M.S. degree in 1953 and in 1956 he receive a Ph.D. degree in cloud physics. He taught at the University of Redlands in California, then returned to Oregon in the late 1950s to teach at Linfield College. He wrote several books, both textbooks and general interest books. In 1978, he retired from teaching, although he continued several projects, including two small businesses, and volunteer work. In his eighties, he established the website cloudman.com to share his love of clouds with people all over the world.