References:

Weapon Effects v2.1 (21 December 1984) by Horizons Technology under contract to the Defense Nuclear Agency (CLICK HERE to run it in your browser)
LA-8819 The yields of the Hiroshima and Nagasaki nuclear explosions. Malik, J. S. (1.6 MB PDF)

Hiroshima

Little Boy Unit L-11 initiated at a height of 580 meters above the ground with a yield of 15 kilotons. Visibility at the time was 9,855 meters. Figure used in calculations was 10 km as that was the lower bound of the validated model limits in Weapon Effects.

Peak Static Overpressure Ranges:

1.5 PSI: 3,374 meters
2.5 PSI: 2,432 meters
3 PSI: 2,169 meters
5 PSI: 1576 meters
10 PSI: 988 meters
15 PSI: 552 meters
20 PSI 359 meters
25 PSI: 204 meters
28 PSI: 80 meters <--Roughly the highest pressure obtained at Hiroshima.

Thermal Fluence Ranges:

3 cal/cm²: 3,100 meters
5 cal/cm²: 2,500 meters ← Flash burns on wooden telephone poles.
8 cal/cm²: 2,000 meters
8.5cal/cm²: 1,920 meters ← Flash burned shadow of valve onto adjacent painted surface. (PHOTO)
14 cal/cm²: 1,500 meters
25 cal/cm²: 1,100 meters
34 cal/cm²: 870 meters ← Light-on-dark shadows of railings on pavement at Yorozuyo Bridge, 880~m from hypocenter. (PHOTO)
50 cal/cm²: 650 meters
97 cal/cm²: 250 meters ← Human Shadow impressions on the steps of Sumito Bank, some 250~m from hypocenter. (PHOTO)
100 cal/cm²: 225 meters
115 cal/cm²: 36 meters ← Roughly the highest fluence obtained.

Prompt Radiation Dose Ranges

Measured as Total Dose (tissue)

100 rad (tis): 1,524 meters
250 rad (tis): 1,343 meters
500 rad (tis):: 1,212 meters
1000 rad(tis): 1,086 meters
5000 rad(tis): 805 meters
10,000 rad(tis): 684 meters
20,000 rad(tis): 561 meters
50,000 rad(tis): 381 meters
100,000 rad(tis): 197 meters

Nagasaki

Fat Man Unit F-31 initiated at a height of 469 meters above the ground with a yield of 21 kilotons.

Peak Static Overpressure Ranges:

1.5 PSI: 3,505 meters
2.5 PSI: 2,505 meters
3 PSI: 2,225 meters
5 PSI: 1,613 meters
10 PSI: 1,051 meters
15 PSI: 817 meters
20 PSI 682 meters
25 PSI: 592 meters
30 PSI: 516 meters
40 PSI: 293 meters
50 PSI: 200 meters
60 PSI: 101 meters

Thermal Fluence Ranges:

NOTE: Not calculated as I was unable to find a visibility for that day at Nagasaki.

Prompt Radiation Dose Ranges

Measured as Total Dose (tissue)

100 rad (tis): 1517 meters
250 rad (tis): 1327 meters
500 rad (tis):: 1190 meters
1000 rad(tis): 1059 meters
5000 rad(tis): 775 meters
10,000 rad(tis): 657 meters
20,000 rad(tis): 540 meters
50,000 rad(tis): 377 meters
100,000 rad(tis): 227 meters