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How far does dangerous fallout travel after a ground burst vs airburst?
Executive summary
Airbursts generally produce much less local, early fallout than ground or surface bursts because the fireball does not entrain large amounts of soil and debris; that reduces heavy, early-falling radioactive particles but can increase wider, more dilute global dispersal [1] [2] [3]. Ground or surface bursts vaporize and loft contaminated earth and structures into the cloud, creating heavy, localized downwind fallout that can begin arriving within an hour and extend far downwind depending on winds and yield [2] [4].
1. How burst height controls whether the fireball “touches” the ground
Whether a detonation produces heavy local fallout depends first on whether the fireball touches and mixes with surface material. Airbursts that keep the fireball aloft avoid sucking up soil and urban debris into the cloud, so they are described as “fallout‑free” for local effects; surface or near‑surface detonations draw large volumes of irradiated material into the cloud and thus generate heavy local fallout [1] [5] [2]. Military and technical guides treat “fallout‑free” as a practical height threshold that varies with yield [4] [6].
2. Quantified thresholds and examples cited in the literature
Practical guidance gives approximate height thresholds below which appreciable early fallout occurs — for example, one historical summary says a 20‑kiloton device may need to be at least roughly 600 ft above ground and a 1‑megaton device roughly 3,000 ft to avoid appreciable early fallout; below those heights appreciable fallout is expected [6]. Hiroshima is often cited as an example of choosing an airburst height (about 550–610 m) to maximize blast effects while minimizing residual ground radiation [1].
3. Fallout pattern differences: local “heavy” versus global “dilute” fallout
A surface/ground burst produces a cloud containing vaporized soil and debris; large particles fall out quickly and heavily downwind, producing concentrated contamination near and extending many kilometers beyond ground zero depending on winds and particle sizes — some fallout can arrive within an hour [2]. In contrast, high airbursts leave fewer large particles; fission products remain aloft longer and dispersed over a wider area and longer timescales, contributing to more widespread but generally lower dose rates locally and potentially enhanced global deposition [4] [3].
4. How far “dangerous” fallout can travel — the meteorology and yield caveat
No single distance fits all cases: the extent of hazardous fallout depends on yield, fraction of fission yield, HOB/DOB, local surface type, particle sizes, and changing winds. The National Academies review emphasizes these parameters as critical in determining the fallout pattern and notes that atmospheric winds from the time of burst until particles reach ground (hours later) shape the ground track [4]. Wikipedia and other technical overviews reiterate that stronger winds spread fallout farther but also dilute it, changing the width and dose along the plume [2].
5. Military doctrine and practical guidance about airbursts vs groundbursts
U.S. doctrinal treatments state there is “essentially no local fallout from an air burst” for many operational concerns, though they also flag limited neutron‑induced activation near ground zero and stress that shallow or penetrating bursts can still produce heavy local fallout [7]. Analysts and commentators note the operational tradeoff: airbursts maximize blast damage and reduce immediate localized fallout, while groundbursts are chosen against hardened targets despite producing substantial fallout [8] [3].
6. What the public often wants to know — distance thresholds and timelines
Public summaries emphasize two practical points supported by the sources: [9] surface bursts create heavy, early fallout that can contaminate areas downwind starting within an hour and over distances set by weather and yield [2]; [10] airbursts reduce that heavy local fallout but can still contribute to wider, lower‑concentration contamination that follows atmospheric transport [4] [3]. Exact “dangerous” distances are not given in these sources because they are case‑specific and depend on the factors above [4].
7. Limits of available reporting and competing emphases
Available sources agree on the physical distinction between air‑ and ground‑burst fallout but differ in emphasis: technical guides and doctrine stress operational thresholds where local fallout is negligible [7] [6], while academic treatments emphasize the many variables that make simple distance numbers unreliable [4]. Available sources do not provide a single, universal downwind distance labeled “dangerous” because yield, design, height, surface, and meteorology control outcomes [4].
8. Bottom line for readers
If your concern is intense, localized radioactive contamination within hours, ground/surface bursts are the main driver — they produce heavy fallout that can travel many kilometers downwind depending on winds and yield [2] [4]. If local fallout avoidance is the objective, an appropriately high airburst can dramatically reduce early local fallout though it shifts the problem toward wider dispersion and longer‑term, lower‑intensity deposition [1] [3].