Nuclear fallout maps: wind, distance and limits
Blast contours describe pressure thresholds near an explosion. Fallout maps describe the distribution of radioactive material after it is transported and deposited. This explorer offers an idealised textbook reference, not live weather or a radiation forecast.
What is nuclear fallout?
Fallout forms when radioactive residues become associated with particles that descend from the explosion cloud. A contact surface burst can draw soil and debris into the cloud, creating a local contamination hazard beyond the prompt blast and heat effects. Fine particles can be transported farther than heavier particles. Glasstone & Dolan, chapter IX, §§9.01–9.09 describes these processes.
How wind changes a fallout pattern
A wind described as from the west blows toward the east. The simulator uses that meteorological convention. Its effective wind is one assumed wind representing transport across the cloud’s height, not a measured wind at street level.
The controls allow 8–45 mph, the approximate range specified by the source’s simple wind-scaling rule. Changing effective speed adjusts downwind length. The approximation holds widths fixed; real winds can also widen, bend or split a pattern. Rain can create uneven deposition and hot spots. See the effective-wind discussion in §9.97.
How to read H+1 reference contours
The map labels idealised H+1 reference dose rates in rad/h. They are normalised to a one-hour reference. They are not a cloud’s location after one hour, a forecast of arrival, an accumulated dose, or a person’s exposure inside shelter.
Contour dimensions come from Table 9.93. The assumed fission fraction rescales the reference-rate labels under §9.94. A smooth outline is reconstructed from the table’s downwind length and widths. The outline’s detailed shape is a drawing convention, not a full deposition calculation.
Why the airburst option suppresses this plume
The fallout approximation is for contact surface bursts. It is not applied to the fixed-reference airburst, whose fireball remains above ground. Suppressing this particular local-fallout model does not mean an airburst produces no radiation or no globally dispersed radioactive material.
How far can fallout travel, and how long does it last?
There is no single distance or duration that applies to every event. Fallout can be significant well beyond the blast-affected area. Winds, precipitation, particle sizes and the event itself determine the distribution. Radioactive decay changes dose rate, while newly arriving particles or removal can change the contamination present.
The textbook’s approximate seven–ten rule describes a tenfold decrease in dose rate when time increases sevenfold for a fixed quantity of early fallout. It is an educational decay relationship, not a countdown to safety or an instruction to leave shelter. Read §9.15 and its limitations.
Nuclear fallout questions
What is nuclear fallout?
Nuclear fallout is material contaminated with radioactive residues that settles from an explosion’s cloud. In surface bursts, soil and debris can be incorporated into that material. The resulting contamination can extend far beyond the blast-affected area.
How far does nuclear fallout travel?
There is no universal fallout radius. The distribution depends on yield, burst conditions, particle settling, winds through the cloud layer, precipitation and surface conditions. This map illustrates an idealised pattern rather than predicting actual contamination.
How long does nuclear fallout last?
Radiation levels change as radioactive materials decay, but there is no universal time after which an area is safe. For a fixed quantity of early fallout after deposition, the approximate seven–ten rule describes a tenfold reduction in dose rate for a sevenfold increase in time. It does not account for newly arriving fallout, removal or a specific site’s conditions.
Does H+1 mean fallout arrives after one hour?
No. H+1 is a normalised one-hour reference dose rate, not an arrival time. Fallout at a particular location may arrive later. These contours do not calculate actual dose rate at a selected time or a person’s accumulated dose.
Preparedness and official guidance
This map cannot select a safe area, shelter or evacuation route. In a radiation emergency, follow local authorities. The CDC’s core guidance is to get inside, stay inside and stay tuned. The UK Government Prepare website provides general emergency-preparedness advice.
Model sources: Glasstone & Dolan, Table 9.93 and §§9.94, 9.97–9.100. Read the complete methods and limitations or open the simulator.