Red Snow was a British thermonuclear weapon. Its physics package was based on United States' W28 nuclear warhead used in the B28 nuclear bomb and AGM-28 Hound Dog missile, with an explosive yield of approximately 1.8 megaton.
The Red Snow warhead was developed after a September 1958 decision to adopt the US warhead for British use, following the 1958 US-UK Mutual Defence Agreement. It entered service in 1961, remaining in use until 1972, when it was replaced by the WE.177 bomb. Perhaps 150 were produced.
Red Snow was used as both a free-fall bomb and as the warhead of the Blue Steel missile. In the gravity bomb role, it was fitted into the casing of the Yellow Sun weapon, even though the Red Snow warhead was considerably smaller than that of the original Yellow Sun bomb.
The Red Snow physics package was later reduced in size, weight and yield, and fitted with a smaller more modern primary, intended as a Red Beard replacement. Known as Una, this was later reduced in diameter and renamed Ulysses as the physics package intended for the UK warhead on the Skybolt project.
Red Snow is the British version of the U.S. Mk.28, which has been tested by the American Authorities. It follows that all current British nuclear weapons are based on tested designs.