Imagine several million of the Beirut explosions happening simultaneously in densely populated cities, and look at the men who hold the launch codes for these weapons.
The Bulletin of the Atomic Scientists maintains a "Doomsday Clock" which is currently closer than ever to midnight — and they started in 1947.
There's apparently a small number of 5-7 kT "W76 mod 2" warheads as well.
Russian warheads tend to be bigger, around 500-1000 kT, presumably to compensate for poorer accuracy.
If it were a nuke it would have gotten bright very quickly, but it quickly would be obscured by clouds and dust.
For example, I believe the bombs dropped on Hiroshima and Nagasaki detonated at about 500m above the ground.
> It was also intended that the munition could be used against targets in coastal and near-coastal locations. One person carrying the weapon package would parachute from an aircraft and place the device in a harbor or other strategic location that was accessible from the sea. Another parachutist without a weapon package would follow the first to provide support as needed. The two-man team would place the weapon package in the target location, set the timer, and swim out into the ocean, where they would be retrieved by a submarine or a high-speed surface water craft.
> A security source said the explosive power of the stored ammonium nitrate was equivalent to at least 1,200 tonnes of TNT
So we are basically looking at a 1kt nuke blast (minus the heat and fallout). Hiroshima nuke, for scale, was at 13–18kt.