THE NUCLEAR BLAST RADIUS EXPLAINED

The first time I really understood a nuclear blast radius, I was sitting at my kitchen table with a paper map of my city and a compass, the geometry kind, and I was trying to draw a 5 psi overpressure ring for a 475 kiloton warhead. I had the numbers in a notebook next to me. I had Glasstone and Dolan's The Effects of Nuclear Weapons open on a second tab, scanned PDF, and I was swearing at the compass because it kept slipping. The ring came out to about 4.3 miles from ground zero. My apartment was inside it. My grocery store was inside it. The bus stop I used every day was inside it. That number, 4.3, did more to rewire my understanding of what a nuclear weapon actually does than any documentary I had ever watched.

Because here is the thing about blast radii. Everyone kind of knows they exist. You have seen the circles on the news, the diagrams in textbooks, the interactive maps. You have a vague sense that the bomb goes off and stuff gets blown up in some area around it. But the word radius is doing an enormous amount of work in that sentence, and most people never actually interrogate what any specific circle represents. Which circle melts steel. Which circle flattens houses. Which circle just sets your curtains on fire. They all look the same from the outside. They are not the same.

I want to walk through what a nuclear blast radius actually is, layer by layer, from the fireball out to the horizon. Because once you understand the physics of each ring, every nuclear map you ever look at stops being a vague threat diagram and becomes a very specific, very readable document.

The fireball is not a metaphor

The innermost ring on any nuclear effects map is the fireball, and the first thing to understand is that the fireball is a literal ball of plasma. Not a figure of speech. Not a visual shorthand for "really hot explosion." When a nuclear weapon detonates, the X-rays released in the first microsecond of the chain reaction superheat the surrounding air to something on the order of 100 million degrees Celsius. That is about six times hotter than the core of the sun, concentrated in a sphere that starts out smaller than a basketball and expands from there.

Inside the fireball, matter does not burn. Matter dissociates. Atoms separate into their nuclei and electrons and slosh around as a soup of charged particles. Steel, concrete, bone, water, glass, air, they all become the same thing, which is luminous plasma glowing in the radio, infrared, visible, ultraviolet, and X-ray bands all at once. For a Hiroshima-sized weapon, the fireball reaches a maximum radius of around 180 meters. For a 1 megaton modern warhead, the fireball hits about 1 kilometer across. For the Tsar Bomba at 50 megatons, the fireball was something like 4.6 kilometers in diameter, visible from over 600 miles away.

Inside that zone, survival is not a question. There is nothing left to survive in. Which is why on a nuclear map the fireball is usually the smallest circle and also the one nobody talks about. There is nothing to say about it. It is the part where the word "casualty" stops applying.

Overpressure is the real killer

Here is the ring that actually defines the damage footprint of a nuclear weapon, and it is the one most people understand the least. Overpressure is the increase in atmospheric pressure above normal, measured in pounds per square inch, produced by the shockwave expanding out from ground zero. A normal day at sea level is 14.7 psi of atmospheric pressure all around you, which your body is built to handle. Overpressure from a blast adds to that in a sudden, violent spike that then drops below normal in a phase called the negative pressure, which is what actually tears a lot of structures apart as air rushes back toward the low-pressure zone.

The standard threshold used on most blast maps is 5 psi. That is the radius where overpressure is strong enough to collapse most residential buildings. Wood-frame houses buckle. Unreinforced brick walls fail. Windows shatter into shrapnel traveling at hundreds of feet per second. People inside buildings that collapse tend to die from crushing injuries. People outside in the open are thrown against hard surfaces by winds that, at the 5 psi line, are blowing at around 160 mph.

The 20 psi ring, which sits closer to ground zero, is the heavy damage zone. Reinforced concrete structures collapse. Highway overpasses fail. Essentially no civilian building stays standing. Fatalities at 20 psi approach 100 percent, whether from the structural collapse itself or from the 500 mph winds that come with the shockfront.

The 1 psi ring, which sits way out at the edge of the blast map, is where windows break. That sounds minor until you remember that a single pane of window glass turned into shrapnel at 100 mph does an enormous amount of damage to human bodies, and a 1 megaton airburst breaks windows out to a radius of about 8 miles. That is a very large area of cities full of people getting cut up by their own architecture.

Here is what matters about overpressure rings. They scale. A 10 kiloton Hiroshima-style weapon produces a 5 psi radius of about 1.1 miles. A 1 megaton modern warhead produces a 5 psi radius of about 4.3 miles. That is a 4x increase in radius for a 100x increase in yield, which is the cube-root scaling law that governs nuclear blast effects. Doubling the yield does not double the damage radius. You have to increase yield by a factor of 8 to double the radius of a given overpressure ring. That is why modern arsenals moved away from the multi-megaton giants of the 1960s toward smaller, more numerous warheads delivered by MIRVed missiles. Twenty 100 kiloton warheads cover substantially more ground than a single 2 megaton device, even though the total yield is the same.

Thermal radiation is the big circle

The largest visible ring on most nuclear maps is the thermal radiation radius, and it catches almost everyone off guard the first time they see it. The thermal flash is exactly what it sounds like. In the moment of detonation, an enormous amount of the weapon's energy is released as light and heat in all wavelengths at once, and that radiation travels at light speed in straight lines out to the horizon.

For a 1 megaton airburst, the third-degree burn radius, where exposed skin gets burned through all its layers and requires skin grafts or becomes fatal, extends to about 8 miles from ground zero. Second-degree burns extend further still. The thermal pulse lasts several seconds, not just an instant, which is long enough for clothing to ignite, for dry vegetation to catch fire, and for any flammable material in line of sight of the fireball to start burning simultaneously across an enormous area.

This is where the firestorm problem enters the picture. After Hiroshima and especially after the Tokyo firebombing, researchers documented what happens when thousands of fires start simultaneously across a dense urban area. The fires pull air inward from all directions, creating a self-sustaining convective column that reaches temperatures high enough to ignite materials that would not normally burn, draws oxygen out of shelters, and generates winds strong enough to knock down anyone still trying to move on the ground. The thermal radius is not just a burn zone. It is the ignition zone for a fire event that outlasts the explosion itself by hours.

What makes thermal radiation interesting from a mapping standpoint is that it is line of sight. A hill between you and the detonation blocks thermal radiation completely on the far side. A thick wall does the same. Overpressure wraps around terrain. Thermal radiation does not. Which is why you see some of the weirdest survival stories out of Hiroshima involving people who happened to be inside a concrete stairwell facing the wrong direction, or behind a stone wall that cast a shadow during the flash.

The prompt gamma pulse

Here is a ring that does not show up on most consumer-facing blast maps, which is a problem because for smaller yields it matters a lot. The prompt gamma radiation pulse is the burst of ionizing radiation released in the first minute after detonation, traveling out from the fireball and ionizing everything it passes through.

For weapons in the 10 to 20 kiloton range, the lethal dose radius for prompt radiation, meaning 500 rem or more of acute exposure, is actually larger than the 5 psi blast radius. A Hiroshima-yield weapon kills more people with prompt radiation near ground zero than with overpressure, even though most maps only show you the blast rings. For weapons in the megaton range, this inverts. The blast and thermal effects extend much further than the prompt radiation lethal range, because the fireball itself absorbs and scatters a lot of the gamma flux before it can reach the ground.

This is why tactical nuclear weapons, the small-yield stuff in the 0.3 to 10 kiloton range, are often described in radiation terms rather than blast terms. A neutron bomb, the old enhanced-radiation weapon concept, was basically a design that maximized the prompt radiation lethal radius while minimizing blast and thermal effects. The idea was to kill tank crews through their armor without flattening the city they were driving through. Strategically questionable. Physically real.

Delayed fallout is the wind problem

The fifth ring, which I talked about in more depth in a separate piece about nuclear fallout maps, is the delayed radioactive fallout. This is fundamentally different from the other four effects because it is not a ring. It is a plume. Fallout is the radioactive dust and debris that gets lofted into the stratosphere by a surface burst or low airburst, then settles back to earth over the following hours and days wherever the wind happens to carry it.

A surface detonation of a 500 kiloton warhead in moderate winds produces a contamination footprint that extends 100 to 200 miles downwind in an elongated cigar shape, with the heaviest doses clustered in the first 20 miles and a long tail of lower-dose contamination stretching beyond. An airburst, which is how most weapons are targeted against cities, produces far less fallout because the fireball never touches the ground and there is no surface material to vaporize and contaminate. This is one of the reasons Hiroshima was an airburst. Immediate blast and thermal casualties are maximized. Long-term radiological contamination is minimized.

Yield scaling and why Hiroshima looks small

If you want to calibrate your intuition, here is the comparison that broke my brain the first time I ran it through NUKEMAP. Hiroshima was 15 kilotons. The modern W87 warhead that sits on top of a Minuteman III ICBM is about 300 kilotons, which is 20 times more powerful. The cube-root scaling says the radius ratio should be the cube root of 20, which is about 2.7. So a W87 produces a blast radius roughly 2.7 times the size of the Hiroshima bomb's.

Hiroshima's 5 psi ring was about 1.1 miles. W87's 5 psi ring is about 3 miles. Hiroshima killed 70,000 to 80,000 people in a city of 350,000. A W87 dropped on central Los Angeles, according to published modeling, would produce casualties in the hundreds of thousands in the first day, with injuries pushing past a million. And the W87 is not the biggest weapon in the US arsenal. It is not the biggest weapon in the Russian arsenal. The active Russian SS-18 warheads run to 800 kilotons. Legacy Soviet megaton-class weapons are still sitting in inventories somewhere.

The Tsar Bomba at 50 megatons, which is the largest device ever detonated, had a fireball that reached the ground from 2.5 miles up and a thermal radiation radius large enough to cause third-degree burns from 60 miles away. The weapon was tested at half its design yield. At full yield it would have been 100 megatons. The thermal pulse from a full-yield Tsar Bomba would have started fires in Finland from a detonation over Novaya Zemlya.

These numbers are not theoretical. They were measured, in test after test, by the US, Russia, UK, France, and China during the years of atmospheric testing that ran from 1945 to 1980. Everything we model today is grounded in that measured data, calibrated against hundreds of actual detonations across a range of yields.

Why map-based thinking matters

If you have ever played with a tool like NUKEMAP or watched a nuclear war simulator run a regional exchange scenario, you have already seen the way a blast radius turns abstraction into geography. That translation is the entire point. A kiloton is a number. A 5 psi ring that covers your zip code is a place you know, with a Target and a school and a bus line you take. The bomb becomes local, which is the only mode in which humans actually process risk.

I am not going to tell you this kind of thinking is comfortable. It is not comfortable. It is the opposite of comfortable. What it is, in my experience, is honest. The weapons exist. The physics is measured and public. The maps are free. The only thing that varies, from person to person, is whether you have ever looked hard enough to know what circle your house is in under various scenarios, and what specifically would happen to you in each one.

For me the answer was a bad hour in 2014 with a paper map and a compass, and a permanent recalibration of how I think about everything from foreign policy to the real estate market to what "basement" means as a word. The numbers stay with you. That is the point of the numbers. Blast maps work because they refuse to let you keep the abstraction. Once you have drawn a 5 psi ring around your life, you do not get to un-draw it.

Which is maybe the most useful thing the physics of a nuclear blast radius can do. It is not a game. It is not a horror movie. It is a piece of cold, well-documented, measurable reality that has been sitting in the world for 80 years, and the maps just let you see it clearly enough to stop pretending it is not there.