Correct spelling for the English word "hyther" is [hˈɪðə], [hˈɪðə], [h_ˈɪ_ð_ə] (IPA phonetic alphabet).
A word coined by W.F. Tyler, F.R. Met. Soc., to indicate the combined effect upon the organism of the humidity and the temperature of the atmosphere.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.