How nuclear fallout travels
Fallout does not spread in a circle, and it does not follow the wind you feel outside. Where it lands is decided by the winds several miles up.
What fallout is
When a nuclear weapon explodes on or near the ground, the fireball pulls up enormous amounts of soil and debris. That material mixes with radioactive products of the explosion, rises in the mushroom cloud, and then falls back to earth as dust and grit. This is fallout.
The larger particles come down first, within minutes to hours, and land closest to the explosion. This early fallout is the main radiation danger to people who survived the blast itself. The finest particles stay aloft for days or weeks and are spread so thinly by then that they are a much smaller hazard.
Ground bursts and air bursts
How much fallout an explosion produces depends heavily on whether the fireball touches the ground. A ground burst produces heavy local fallout. An air burst, set off high enough that the fireball stays clear of the ground, causes wider blast damage but little local fallout, because there is no soil for the radioactive material to attach to.
Hardened military targets such as missile silos would be attacked with ground bursts. Cities might be attacked either way. The map lets you switch cities between the two assumptions.
Why the upper winds matter
A nuclear cloud rises far above ordinary weather. For a small weapon it levels off around 3 to 5 miles up; for a large one, 7 to 12 miles (approximate figures from Glasstone and Dolan, The Effects of Nuclear Weapons (1977)). Fallout particles then drift down through every layer of air between the cloud and the ground, and each layer pushes them in its own direction.
Winds at those heights are usually much stronger than surface winds and frequently blow from a different direction. It is common for the ground-level wind to come from the southeast while the winds that would steer fallout come from the west. That is why this site reads the whole column of air rather than the surface wind.
How far and how fast
Fallout moves at roughly the speed of the winds carrying it. With steering winds of 30 mph, a place 60 miles downwind would see fallout begin to arrive in about two hours. Close to the explosion it can begin within minutes; Ready.gov advises that there are typically 10 minutes or more to reach shelter.
From a single ground burst, significant fallout typically reaches tens of miles to a few hundred miles downwind, in a long, uneven band that widens with distance. When winds change direction with height, the band is broader and less predictable.
What rain does
Rain or snow falling through a fallout cloud scrubs particles out of the air and deposits them on the ground below. This creates intense local hotspots, sometimes far from where dry fallout would have landed. Precipitation is the biggest single reason real fallout patterns differ from tidy predictions.
How quickly it fades
Fallout radiation is strongest at the start and decays fast. A widely used rule of thumb is the 7-10 rule: for every sevenfold increase in time after the explosion, the dose rate drops about tenfold (from Glasstone and Dolan, The Effects of Nuclear Weapons (1977)).
| Time after explosion | Dose rate compared with 1 hour |
|---|---|
| 1 hour | 100% |
| 7 hours | about 10% |
| 2 days (49 hours) | about 1% |
| 2 weeks | about 0.1% |
This is why sheltering through the first day or two matters so much. See what to do in the first 24 hours.
What this site models
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). The other factors described on this page, such as rain, particle size and the details of the weapon, are not modeled. See how it works for the full list.
Last reviewed November 13, 2026.