signature, with a double intensity maximum, is characteristic of atmospheric nuclear explosions and is the result of the Earth's atmosphere becoming opaque to visible light and transparent again as the explosion's shock wave travels through it. The effect occurs because the surface of the early fireball is quickly overtaken by the expanding "case shock", the atmospheric shock wave composed of the ionised plasma of what was once the casing and other matter of the device. Although it emits a considerable amount of light itself, it is opaque and prevents the far brighter fireball from shining through. The net result recorded is a decrease of the light visible from outer space as the shock wave expands, producing the first peak recorded by the bhangmeter. As it expands, the shock wave cools off and becomes less opaque to the visible light produced by the inner fireball. The bhangmeter starts eventually to record an increase in visible light intensity. The expansion of the fireball leads to an increase of its surface area and consequently an increase of the amount of visible light radiated off to space. The fireball continues to cool down so the amount of light eventually starts to decrease, causing the second peak observed by the bhangmeter. The time between the first and second peaks can be used to determine its nuclear yield. The effect is unambiguous for explosions below about altitude, but above this height a more ambiguous single pulse is produced. Origin of the name The name of the detector is a pun which was bestowed upon it by Fred Reines, one of the scientists working on the project. The name is derived from the Urdu and Hindi word "bhang", a locally grown variety of cannabis which is smoked or drunk to induce intoxicating effects, the joke being that one would have to be on drugs to believe the bhangmeter detectors would work properly. This is in contrast to a "bangmeter" one might associate with detection of nuclear explosions. See also Vela incident WC-135 Constant Phoenix Nuclear MASINT Electro-optical MASINT References Further reading Nuclear weapons Nuclear warfare Electromagnetic radiation meters Chemical, biological, radiological and nuclear defense