How it works

Fallout Weather turns a forecast of the winds at many altitudes into an estimate of where fallout would be carried. This page describes the method and its limits.

The data

For your location, for each nearby target and for four points around you, the site fetches an hourly forecast of wind speed and direction at 14 altitudes from the ground to about 12 miles up, together with precipitation, for the next 48 hours. The forecasts come from the NOAA GFS weather model by way of Open-Meteo.

The fallout wind

A particle falling from a given height is pushed by every layer of air it passes through. Its overall drift is therefore the average of all the winds between that height and the ground. The site calculates this average for several heights within the stem and cloud of the assumed explosion. The average for the center of the cloud, where most of the radioactive material is, gives the main direction and speed. This is the long-established wind-vector method from civil defense planning.

How much those averages differ from one another shows how strongly the wind turns with height, and sets how quickly a path widens.

Following the path

Each path starts at the target and is stepped forward ten minutes at a time for up to 12 hours, using the fallout wind interpolated between the forecast points and moving through the hourly forecast as it goes. This is why paths bend. A path ends at a distance limit that depends on the assumed weapon: about 60, 150 or 300 miles. These limits, like the 12-hour cut-off and the 500-mile limit for missile fields, are settings chosen for this model.

Missile fields are treated differently, because they would be struck by many ground bursts across a wide area. Their paths start as wide as the field and extend to about 500 miles.

Reaching you

Your location is compared with each path. If it lies inside, the result is "reaches you"; if it lies just outside, "edge of path". The arrival time uses the fastest of the winds involved, so it tends to be early.

What it takes into account, and what it does not

Many factors affect where fallout travels and how dangerous it is. This educational tool takes only three into account: the forecast winds from the ground up to the top of the mushroom cloud, an assumed weapon size, and an assumed burst type (on the ground or in the air).

Taken into account:

• Forecast winds at every altitude: speed and direction from the ground to the top of the mushroom cloud, at each target and at points around you, hour by hour for up to 12 hours of travel.
• Assumed weapon size: the size you select, which sets how high the cloud rises and how far this model draws the path.
• Assumed burst type: ground bursts for military sites, and your choice of ground or air for cities. A ground burst produces heavy local fallout; an air burst produces little.

Not taken into account:

• The weapons actually used: their real size, design, number and height of burst, none of which can be known in advance.
• Radiation dose: how strong the radiation would be at any place or time.
• Rain and snow: precipitation is flagged, but the hotspots it creates are not mapped.
• Terrain and local weather: hills, valleys, storms and small-scale winds between forecast points.
• Particle sizes: heavier particles fall sooner and nearer than light ones, which makes real patterns uneven.
• Forecast error: the result is only as good as the weather forecast behind it.
• What would actually be targeted: the target list is illustrative, not a prediction.

The site also has no knowledge of real events: it describes the weather and does not detect or report explosions. For all these reasons the paths are a rough guide to what is likely, not a prediction.

Sources

• Glasstone and Dolan, The Effects of Nuclear Weapons, US Department of Defense and Department of Energy, 1977.
• Ready.gov: Radiation Emergencies.
• CDC: Radiation Emergencies.
• Open-Meteo GFS API, which serves NOAA GFS forecast data.

Last reviewed November 13, 2026.