High Quality Content by WIKIPEDIA articles! Yellow Sun was the first British operational high-yield strategic nuclear weapon. The name actually refers only to the outer casing; the warhead (or physics package) was known as "Green Grass" (in Yellow Sun Mk.1) and "Red Snow" (in Yellow Sun Mk.2). The ENI or electronic neutron initiator (generator) was Blue Stone. The casing was some 21 feet (6.4 metres) long, 48 inches (1.2 m) in diameter. The Mark 1 version with the Green Grass warhead weighed 7,250 lb (3.3 tonnes). The Mk.2 version with the lighter 1,700 lb Red Snow warhead had ballast added to maintain overall weight, ballistic and aerodynamic properties, and avoid further lengthy and expensive testing, and changes to the electrical power generating and airburst fusing equipment.