While planning the Trinity test, Los Alamos scientists were concerned about weather. Clear skies were necessary in order to observe and document the test. There was also concerns that rain immediately before or during the test would damage the electrical machinery and instruments. Another reason for being concerned about the weather was fallout. This was first suggested by Joseph Hirschfelder a few months earlier. High winds threatened to send the radioactive cloud over inhabited areas. If it began to rain too soon after the test it would cause concentrated amounts of fallout onto a small area. A meteorologist, J. M. Hubbard, was hired to pick the final day. He felt that the ideal dates were between July 18 and the 21st. The window of July 12 to the 14 was his second choice. On June 30 all division leaders had to submit the earliest possible date that their work would be ready. After a meeting on this day July 16th was decided as the final date.
https://toxipedia.org/display/wanmec/Tri ... New+MexicoThe test was originally scheduled for 4 a.m., Monday July 16, but was postponed to 5:30 due to a severe thunderstorm that would have increased the amount of radioactive fallout, and have interfered with the test results. The rain finally stopped and at 5:29:45 a.m. Mountain War Time, the device exploded successfully and the Atomic Age was born. The nuclear blast created a flash of light brighter than a dozen suns. The light was seen over the entire state of New Mexico and in parts of Arizona, Texas, and Mexico. The resultant mushroom cloud rose to over 38,000 feet within minutes, and the heat of the explosion was 10,000 times hotter than the surface of the sun! At ten miles away, this heat was described as like standing directly in front of a roaring fireplace. Every living thing within a mile of the tower was obliterated. The power of the bomb was estimated to be equal to 20,000 tons of TNT, or equivalent to the bomb load of 2,000 B-29, Superfortresses!
After witnessing the awesome blast, Oppenheimer quoted a line from a sacred Hindu text, the Bhagavad-Gita: He said: "I am become death, the shatterer of worlds."[6] In Los Alamos 230 miles to the north, a group of scientists' wives who had stayed up all night for the not so secret test, saw the light and heard the distant sound. One wife, Jane Wilson, described it this way, "Then it came. The blinding light [no] one had ever seen. The trees, illuminated, leaping out. The mountains flashing into life. Later, the long slow rumble. Something had happened, all right, for good or ill."[7]
General Groves' deputy commander, Brigadier General T. F. Farrell, described the explosion in great detail: "The effects could well be called unprecedented, magnificent, beautiful, stupendous, and terrifying. No man-made phenomenon of such tremendous power had ever occurred before. The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined..."[8]
https://www.cddc.vt.edu/host/atomic/trin ... nity1.htmlso, clear weather and skies were important, but it looks overcast in the pictures. Don't know when the pictures were taken, however.
