What does the red mean?

[v0031]

The United States first detonated a hydrogen bomb in 1952 in the Marshall Islands, also in the Pacific. However, that bomb–and the others used in tests that followed–were large and unwieldy affairs that were exploded from the ground. The practical application of dropping the weapon over an enemy had been a mere theoretical possibility until the successful test in May 1956. The hydrogen bomb dropped over Bikini Atoll was carried by a B-52 bomber and released at an altitude of more than 50,000 feet. The device exploded at about 15,000 feet. This bomb was far more powerful than those previously tested and was estimated to be 15 megatons or larger (one megaton is roughly equivalent to 1 million tons of TNT). Observers said that the fireball caused by the explosion measured at least four miles in diameter and was brighter than the light from 500 suns.