Ideal pattern

The time sequence illustrated is for a 1-megaton fission yield weapon and a 15-mile-per-hour effective wind speed. The dose rates are external gamma dose rates. As shown, 20 miles downwind from the explosion at 1 hour after the detonation is a 3-rad-per-hour fallout field.

The ideal fallout pattern is based on a given weapon and yield and assumes a smooth, open terrain. Using the fission yield of the weapon, an average fallout pattern is determined for a given steady state wind condition. This results in ideal isodose rates with a cigar-like shape, having ideal width and ideal distance upwind and downwind.

In the example, although not illustrated, the dose rate would increase to more than 500 rads per hour between 1 and 2 hours after detonation. By 6 hours after the explosion, the dose rate would decrease to less than 100 rads per hour. The increase in dose rates after 1 hour would indicate that, at the specified location, the fallout is not yet complete. The subsequent drop after the maximum dose rate is achieved would be attributed to the natural decay of the fission products. At 18 hours after the burst, the dose rates would decrease even farther to near 30 rads per hour at 20 miles directly downwind.