Reconstruction of fallout deposition from U.S. atmospheric nuclear tests conducted in New Mexico and Nevada

For decades, the sky above the United States carried a silent, invisible burden. Between 1945 and 1962, the nation detonated 101 nuclear weapons in the air over New Mexico and Nevada. These were not just explosions of power, but releases of radioactive dust that drifted on the wind, settling across the country in a pattern no one could fully see at the time. Scientists have long tried to map where this dust went, but their efforts were often limited by the sparse data available from the mid-20th century. They knew the tests happened, and they knew radiation was released, but the precise path of that radioactive cloud, how far it traveled, and exactly how much settled on the ground in every corner of the country remained a patchwork of estimates. Understanding this history is not merely an exercise in looking back; it is essential for evaluating the long-term health consequences for the population and for understanding how radioactive material moves through our atmosphere, a knowledge base that remains critical if such events were ever to happen again.

Now, a team of researchers has used modern technology to redraw that invisible map with remarkable clarity. By combining powerful computer models with the best available historical weather data, they have reconstructed the journey of radioactive fallout from 94 of those nuclear tests over the first ten days after each explosion. The result is a detailed, high-resolution picture of how fission products—tiny particles created when the atomic nuclei split—spread across the entire contiguous United States. This work does more than just fill in gaps; it corrects previous assumptions about how these particles behave, particularly when the bombs were detonated in the air rather than on the ground.

The researchers started by gathering every known detail about the tests: where they happened, when they happened, how powerful they were, and how high the radioactive clouds rose. They then fed this information into a sophisticated atmospheric transport model, a tool that simulates how particles move through the air. To make this simulation as accurate as possible, they drove it with ERA5, a modern, high-resolution reconstruction of global weather patterns that goes back to 1940. This dataset is unique because it incorporates digitized wind observations from the 1940s and 50s that were previously unavailable to scientists, allowing for a much sharper view of the atmosphere during the test years than ever before.

When the team first ran their simulations, they compared the results against historical records of iodine-131, a radioactive isotope that was widely measured at the time, to see if their model matched reality. For tests where the device was lowered on a tower or placed on the ground, the model worked perfectly, aligning closely with the historical data. However, for the tests where the bomb was dropped from a plane and exploded in the air, the model initially predicted far less fallout than what was actually recorded. This discrepancy led the researchers to a crucial discovery: the standard assumptions about the size of the radioactive particles created by airbursts were too narrow. By adjusting the model to allow for a much wider variety of particle sizes—ranging from extremely fine dust to larger grains—they found that the simulation finally matched the historical records. This adjustment was vital, as it revealed that airbursts create a more diverse mix of debris that settles differently than previously thought.

With the model calibrated and the particle behavior corrected, the team turned their attention to the very first nuclear test, Trinity, which took place in New Mexico in 1945. This event had received less modeling attention than the later Nevada tests, largely because the weather data from that era was so sparse. Yet, the new reconstruction showed that the fallout from Trinity was far-reaching. Within just ten days, the radioactive plume had traveled across the continent, depositing material in 46 states. Only Washington and Oregon were spared. The map shows that the dust reached as far as the Midwest and the Great Lakes, consistent with historical accounts of radioactive contamination found in materials like strawboard packaging in Indiana and Iowa shortly after the blast.

To ensure these findings were not just a fluke of one specific weather scenario, the researchers ran the Trinity simulation ten times, each time using slightly different versions of the weather data to account for uncertainty. Despite the variations, the large-scale pattern remained stubbornly consistent: the fallout reached the same vast area in nearly every run. This robustness gives the team confidence that their map of the Trinity fallout is accurate, even though the weather observations from 1945 were far less dense than they are today.

Finally, the researchers combined the results from all 94 tests into a single, cumulative map of the United States. This comprehensive view shows the total accumulation of radioactive dust from every non-zero-yield atmospheric test in New Mexico and Nevada. The map highlights the intense local fallout near the test sites but also reveals a widespread, low-level layer of deposition that covers the entire nation. While the total amount of radiation varies greatly from place to place, the study confirms that no part of the country was untouched by the drift of these tests.

This work provides the most detailed, physics-based reconstruction of U.S. atmospheric nuclear fallout to date. It moves beyond the limitations of old, scattered measurements to offer a continuous, time-resolved picture of how radioactive material traveled across the continent. By correcting the assumptions about particle sizes and utilizing the most advanced weather data available, the study offers a clearer understanding of the environmental legacy of these tests. It establishes a solid foundation for future studies that might translate this physical deposition into specific health risks, bridging the gap between the physics of the explosion and the long-term reality of the fallout.