William Jackson Humphreys (February 3, 1862 – November 10, 1949) was an American physicist and atmospheric researcher.

Humphreys was born on February 3, 1862, in Gap Mills, Virginia, to Jackson and Eliza Ann (née Eads) Humphreys.^[1] He studied physics at Washington & Lee University in Virginia and later at Johns Hopkins University in Baltimore, where he earned his Ph.D. in 1897, studying under Henry Augustus Rowland.^[2]

He worked in the fields of spectroscopy, atmospheric physics and meteorology. In the field of spectroscopy he found the shift of spectral lines under pressure. In atmospheric physics he found a very good model for the stratosphere in 1909. He wrote numerous books, including a textbook titled Physics of the Air, first published in 1920 and considered a standard work of the time,^[2] though it was last published in 1940.^{[citation needed]} He also held some teaching positions at universities. He concluded that the 1815 eruption of Mount Tambora was responsible for the subsequent cooling known as the Year Without a Summer.^[3]

From 1905 to 1935 he worked as a physicist for the U.S. Weather Bureau, predecessor of the National Weather Service.^[2] In 1919, he served as president of the Philosophical Society of Washington.^[4] He was elected to the American Academy of Arts and Sciences in 1921.^[5] In 1924 he was an Invited Speaker of the ICM in Toronto.^[6] He was elected to the American Philosophical Society in 1929.^[7]

He died on November 10, 1949, in Washington, D.C.

• Physics of the Air (1920)
• Weather proverbs and paradoxes (1923)
• Fogs and clouds, The Williams & Wilins Co. (1926)
• Rain making and Other weather vagaries (1926)
• Snow crystals (1931)