Did the Americans even understand the effect nukes would have on humans pre-Hiroshima? Also, did the US think about dropping a bomb in the Japanese countryside/water/unpopulated area as a show of force?
Hi there, this is not do discourage further responses, but these questions appear fairly regularly on /r/AskHistorians -- so much so that there's a section in our FAQ about the atomic bombs:
http://www.reddit.com/r/AskHistorians/wiki/wwii#wiki_the_atomic_bombs
You may be interested in particular in these threads:
http://www.reddit.com/r/AskHistorians/comments/ydn10/could_america_have_used_the_atomic_bomb_on_a/
Hope this helps! Again, this is not to discourage further responses.
The simple answer is "yes".
The minds involved with the "Manhattan Project" knew full well of the power they worked to unleash, at least on the theoretical level, with plenty of fatal, unintended consequences along the way.
One such incident known as the "The First Demon Core Incident" resulted in the death of Physicist Harry Daghlian when he was exposed to a lethal burst of neutron radiation during a neutron refection experiment.
Radiation was a known hazard long before his untimely death, the works of Wilhelm Rontgen, Marine Curie, and others paved the way in nuclear physics.
Hundreds of thousands of hours was put into the Manhattan Project, with decades of research in theoretical mathematics, theoretical physics, nuclear physics, ect... prior to the establishment of the project.
The Trinity Test (aka The Gadget) was a proof of concept test done prior to the bombing of Nagasaki and Hiroshima. A Plutonium Implosion Fission device, with a 20KT yield was detonated at Alamogordo Bombing and Gunnery Range in New Mexico.
The device created a 30ft. wide, 5ft. deep crated, turned the desert sand into glass which is know as Trinitite. Various testing instruments had been employed recording everything from blast overpressure to seismic effect.
The effects were well known and documented at the time of the bombings.
An sort of warning was decided against, in the event Japan tried to prevent or interfere in some capacity, the uncertainly of detonation and to maximize psychological shock. Prior to the bombing, regular air raid warning leaflets had been dropped up until are week before. You also have to understand Japanese culture at the point in time. It was illegal to be in possession of warning leaflets, which were seen as enemy propaganda and Japan still had a sitting Emperor that was believed to be a god.
That had all been discussed, but ultimately decided against for many of the above reasons, such as psychological effect.