Nuclear Fallout Maps: What They Can and Cannot Tell You

Many people search for a map of where fallout would land. No such map can exist in advance, and relying on an old one could put you in danger. Here is what is known, and what to do instead.

Why fallout cannot be mapped in advance

Fallout forms when a ground-level explosion lofts material into the air, and it is carried by the wind. According to FEMA's planning guidance, its pattern depends on the weather, and wind direction and speed change with altitude, which can deposit fallout in more than one direction. Any map drawn before the event is a sketch of an assumed wind on an assumed day.

What planners expect

• FEMA says the most dangerous concentrations often occur within tens of miles downwind of ground zero and typically fall within the first few hours.
• Federal planners define a dangerous radiation zone as 10 R/hr and above. It reaches its largest extent after the first few hours and then shrinks, perhaps from tens of miles to a mile or two in one day. These figures are for planning and depend on the event.
• The earlier edition of the guidance adds that fallout travels well beyond that zone, and that areas up to hundreds of miles away may warrant protective actions, such as sheltering or evacuation, food restrictions and water advisories. FEMA says sheltering may be warranted even beyond the dangerous zone.
• FEMA warns that lethal radiation levels may be present in the first few hours even where fallout is not apparent. You cannot rely on seeing it, and you cannot tell which zone you are in.
• Until the size and direction of fallout are established, FEMA's guidance is that people within 50 miles of a detonation who are not part of the response should seek adequate shelter.

How officials would map a real event

FEMA's 72-hour guidance describes a federal center, the Interagency Modeling and Atmospheric Assessment Center (IMAAC), that coordinates dispersion modeling and hazard predictions for responders. Measurements then refine those predictions. You would receive the result as instructions from officials, so keep a battery-powered or hand-crank radio.

Do not drive by a map

• CDC says not to stay in your car, because cars do not provide good protection, and to stay inside until an evacuation order is given. A short move from very poor shelter to adequate shelter nearby is different: see finding shelter.
• The planning guidance warns that early evacuation without adequate knowledge of the highest fallout hazard areas, even from poor shelters, can be extremely hazardous.
• Ready.gov says you will have 10 minutes or more after a detonation to find adequate shelter before fallout arrives. Use them to reach a building.

Scenario tools

Educational simulators can show a fallout plume for a chosen yield and wind. They are useful for understanding scale, but they show one assumed case. They do not predict your risk, and none of them replaces official instructions.

See how fast fallout fades, how to find adequate shelter and how to read radiation maps.