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

[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.

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 concluded that the 1815 eruption of Mount Tambora was responsible for the subsequent cooling known as the Year Without a Summer.