atmospheric ignition
Oppie says, in reference to the possibility of atmospheric ignition possibly destroying the Earth, "in one hour 58 minutes we'll know" a couple of hours before the test.
Actually when you think about it, no they wouldn't know.
If atmospheric ignition did not happen at the Trinity Test, they would still not know what would happen at Hiroshima where the lower elevation (even accounting for the altitude of the bomb) might create critical mass due to the thicker air at lower elevations where it might not at Trinity.
If atmospheric ignition did happen at the Trinity Test, they would still not know because they would be dead.