The fireball, an extremely hot and highly luminous spherical mass of air and gaseous weapon residues, occurs within less than one millionth of one second of the weapon's detonation. Immediately after its formation, the fireball begins to grow in size, engulfing the surrounding air.
This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time the fireball rises, like a hot-air balloon. Within seven-tenths of one millisecond from the detonation, the fireball from a 1-megaton weapon is about 440 feet across, and this increases to a maximum value of about 5,700 feet in 10 seconds. It is then rising at a rate of 250 to 350 feet per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 4.5 miles from the point of burst.