“The first test began at dawn on Jan. 27, 1951, as a United States B-50 bomber dropped a nuclear warhead from nearly 20,000 feet onto Frenchman Flats in the Nevada desert. The device, codenamed Able, detonated 1060 feet above the desert floor, shaking the earth and echoing through the nearby mountains. The test would be the first of more than 900 documented nuclear detonations that would take place at the Nevada Test Site between 1951 and 1992.
“Mushroom clouds could be seen from Vegas,” some forty-five miles away, says Karen Green, curator at the Atomic Testing Museum. “Viewing parties were held on casino rooftops and people drove out of town to watch.”” – http://www.neh.gov/news/nuclear-nevada
A source says the test in the photo was part of the Operation Plumbbob but I cant verify this. http://en.wikipedia.org/wiki/Operation_Plumbbob