Kaboomly / Fallout

Nuclear Fallout Map

Fallout is the part of a nuclear explosion that the wind carries away. It can land far from the blast, and where it lands depends on the weather at that moment. This nuclear fallout map page explains how fallout spreads and how our map draws an idealized version of it for a surface burst.

Nuclear bomb fallout radius

Fallout has no fixed radius: it is not a ring around ground zero. It is carried downwind as a long, narrow plume, and how far that plume reaches depends on the yield, the burst type (surface burst versus airburst), and the wind at the time. The table below gives the idealized downwind distance for each weapon preset in the simulator above, for a surface burst, using the same model the simulator itself uses.

Figures assume Glasstone and Dolan's idealized Table 9.93 pattern: a contact surface burst, a fixed effective wind of 15 mph, and no wind change with height beyond the table's own 15° shear. This is not a weather forecast for any real place or day, and "100 rad/h" is the unit-time (one hour after the explosion) reference dose rate of the contour, matching the "Fallout (idealized)" figure in the simulator's results panel.

Real fallout depends on changing winds and weather; very large distances in this idealised model should be read as an upper-bound illustration, not a prediction.

Airbursts are not in the table because they produce little local fallout: when the burst is high enough that the fireball does not touch the ground, there is little soil for the radioactive material to condense onto, so it forms fine particles that rise high and spread thinly instead of falling nearby (see why the burst type matters below).

Try it on the simulator to see the same idealized plume drawn on a map for a city of your choice.

What fallout is

A nuclear explosion creates radioactive fission products. In the fireball they are vaporized along with the weapon materials and anything else drawn in. As the cloud rises and cools, these materials condense onto particles of many sizes. When the particles fall back to Earth, the radioactive material they carry is called fallout. Glasstone and Dolan cover the subject in chapter IX of The Effects of Nuclear Weapons (1977). For a longer plain-language explainer, see what is nuclear fallout.

The book separates fallout by timing. Early fallout reaches the ground within about a day and is made of larger, heavier particles. It can deliver high radiation doses downwind of the explosion. Delayed fallout is made of very fine particles that stay in the upper atmosphere for months or years and settle over large parts of the world at much lower levels.

Why the burst type matters

Surface burst fallout is heaviest when the fireball touches the ground. In a surface burst, large amounts of soil and debris are pulled up into the fireball and the rising cloud. Radioactive material condenses onto these particles, and the heavier ones start to fall within minutes to hours. This produces heavy local fallout.

In an airburst where the fireball stays above the ground, there is little soil to carry the radioactive material down. It forms extremely small particles that rise high and spread thinly, so early fallout near the explosion is light. In the nuke simulator, the airburst option uses a fixed height at which the fireball stays clear of the ground for every preset weapon. This difference in how fallout spreads is why the simulator draws a downwind pattern only for surface bursts.

Why wind and weather decide the nuclear fallout map

How fallout spreads is decided mostly by the wind. Particles fall from different heights of the cloud, and the wind at each height can blow in a different direction and at a different speed. Heavy particles land close to the explosion; lighter ones travel much farther before they reach the ground.

With a steady wind in one direction, the result is a long area stretched downwind. Real weather is rarely that tidy. Wind that turns with height can bend or widen the pattern, and rain can wash particles out of the air in some places and not others, leaving patches of higher contamination. Glasstone and Dolan stress that the idealized patterns in chapter IX are simplified and that actual patterns can be very irregular.

A well-known example is the Castle Bravo test at Bikini Atoll in 1954, which Glasstone and Dolan discuss in chapter IX (§9.104 to §9.105). The yield was about 15 megatons, and significant contamination was found more than 300 miles downwind. The episode is one reason fallout became a major subject of study.

How fallout changes over time

Fallout radioactivity is highest soon after it lands and then drops, quickly at first and more slowly later. Glasstone and Dolan give a rule of thumb in chapter IX: for every sevenfold increase in time after the explosion, the fallout dose rate falls by a factor of about ten. The rule is an approximation. Real decay depends on the mix of materials in the fallout.

How this nuclear fallout map is drawn, and why it is idealized

Other simulators draw surface burst fallout with a simplified model usually credited to work by Carl F. Miller. Our standard is that every number on this site must trace back to a source we can cite in full: title, organization, year, and report number. We could not locate and verify the original Miller report, so instead of using it we use Glasstone and Dolan's own idealized fallout contours, Table 9.93 in chapter IX, which are fully cited (sources section of How It Works, and the fallout section above it explains the method in full).

That table describes an "idealized" pattern: a smooth, cigar-shaped area for one fixed effective wind of 15 miles per hour, not real weather for any place or day. For a surface burst of the chosen yield, the map shades four of the table's eight fallout dose rate contours (1,000, 100, 10, and 1 rad per hour, one hour after the explosion) downwind of ground zero, always toward the east. It never changes with the city, season, or real wind, and it is not a prediction for any real event. An airburst shows no fallout pattern at all, because Glasstone and Dolan state that its residues rise too high within about a minute to reach the ground in significant amounts (§9.04).

What the nuclear fallout map shows

• Blast: the 20 psi, 5 psi, and 1 psi overpressure rings.
• Heat: the distance for a 50% chance of third-degree burns on bare skin.
• Initial radiation: the 500 rem dose from neutrons and gamma rays in the first minute. This is not fallout; it comes from the explosion itself.
• Fireball: its maximum size.
• Surface burst fallout: the idealized downwind pattern described above, shown for surface bursts only, using one fixed 15 mph wind.

The nuclear blast radius page explains each ring with a table for every preset, and the nuclear bomb map guide covers how to explore the map.

Real-world information

This page is educational and is not safety guidance. For real-world preparedness information, see official sources such as Ready.gov in the United States, or your national or local emergency management agency.