A nuclear detonation unfolds in a rapid sequence of distinct phases, each with its own destructive reach. Within the first few seconds, an intensely bright flash of light appears, followed by a massive pressure wave, extreme heat, and radiation. What happens after that, from radioactive fallout to long-term climate effects, depends on the size of the weapon and whether it detonates in the air or at ground level. Here’s what each phase would actually look like.
The First Seconds: Flash, Heat, and Blast Wave
The detonation begins with a flash of light brighter than the sun, visible from hundreds of miles away. This “thermal pulse” lasts several seconds and is intense enough to ignite clothing, paper, and dry vegetation miles from the blast center. Anyone looking directly at the flash within dozens of miles could suffer temporary or permanent blindness, even on a clear day.
Immediately after, a fireball forms at the detonation point. For a weapon in the range of several hundred kilotons (a common size for modern warheads), this fireball can stretch over a mile wide. Everything inside it is vaporized. Surrounding the fireball, a shockwave of compressed air expands outward at faster than the speed of sound. This overpressure wave is what levels buildings. At 5 to 7 pounds per square inch (psi) of overpressure, wood-frame houses are nearly completely destroyed. Non-reinforced concrete walls shatter at just 2 to 3 psi. At 10 psi, virtually all buildings are flattened. For context, a large modern warhead can produce 5 psi of overpressure several miles from the detonation point, meaning total residential destruction across an area the size of a small city.
Following the initial outward push, a reverse wind rushes back toward the blast center as the vacuum fills. This creates a second wave of destruction, pulling debris inward. Between the two waves, the blast zone becomes a landscape of collapsed structures, shattered glass traveling at hundreds of miles per hour, and fires ignited by the thermal pulse merging into larger blazes.
Electromagnetic Pulse
A nuclear weapon detonated at high altitude (above about 3 miles) generates a powerful electromagnetic pulse, or EMP. This burst of energy doesn’t harm people directly, but it can disable electronics across an enormous area. A single high-altitude detonation could potentially knock out power grids, communication systems, vehicles with electronic ignition, and hospital equipment across a region spanning hundreds of miles or more. For a ground-level attack on a city, the EMP effect is more localized but still capable of disabling electronics within and well beyond the blast zone. The loss of communication and electrical infrastructure would compound every other challenge in the aftermath.
Radiation Exposure Near the Blast
The detonation itself releases an intense burst of ionizing radiation, primarily gamma rays and neutrons. For people close enough to receive a high dose but far enough to survive the blast and heat, radiation sickness becomes the immediate threat. This is called acute radiation syndrome, and its timeline is grimly predictable.
At high doses (roughly 6 to 8 gray, a unit measuring absorbed radiation), symptoms begin within minutes. Virtually everyone exposed at this level vomits within 30 minutes. Heavy diarrhea follows within one to three hours, severe headaches within three to four hours, and high fever within the first hour. What makes radiation sickness deceptive is what comes next: a “latent phase” where symptoms seem to improve for hours or even days, creating a false sense of recovery. The manifest illness phase then sets in, with bone marrow failure, immune system collapse, and in many cases, death within weeks.
At lower doses, the timeline stretches out. Nausea may not appear for hours, and the latent period can last weeks before symptoms return. Survival depends heavily on the dose received and whether medical care is available, which in a nuclear attack scenario it likely would not be at scale.
Fallout: Ground Burst vs. Air Burst
Whether a nuclear weapon creates dangerous local fallout depends almost entirely on one factor: did the fireball touch the ground? In a ground burst, the explosion scoops up enormous quantities of soil, rock, and debris, pulls it into the fireball, and irradiates it. These heavy particles then fall back to earth downwind of the blast, creating a plume of radioactive dust that can extend dozens to hundreds of miles depending on wind patterns and weapon size. About half the radioactivity from a ground burst settles in this local fallout. The other half attaches to finer particles that stay aloft and contribute to global fallout over weeks and months.
An air burst, where the fireball never contacts the ground, produces virtually no local fallout. All the radioactive material stays on tiny particles that drift through the upper atmosphere and disperse globally at very low concentrations. Military planners sometimes choose air bursts to maximize blast damage over a wider area while minimizing fallout. Ground bursts are used to destroy hardened targets like missile silos or underground bunkers.
For anyone downwind of a ground burst, fallout arrives as a visible layer of ash-like dust, often within an hour. This material is extremely radioactive at first but decays quickly. The “7-10 rule” describes this decay: for every sevenfold increase in time after detonation, radiation levels drop by a factor of ten. After 7 hours, radiation is at 10% of its initial level. After 49 hours (7 times 7), it drops to just 1%. This is why sheltering in place for the first 24 to 48 hours is considered critical.
How Shelter Changes Survival Odds
Not all shelter is equal. The key measurement is how much material sits between you and the radioactive particles outside. Gamma rays, the most penetrating form of fallout radiation, are reduced by half for every 5.6 centimeters (about 2.2 inches) of concrete or 8.4 centimeters (about 3.3 inches) of packed earth between you and the source. This means a basement with concrete walls and a few feet of earth overhead can reduce your radiation exposure by 90% or more compared to standing outside.
A wood-frame house with no basement offers minimal protection. A large concrete or brick building offers much better shielding, especially interior rooms on lower floors. Underground parking garages, subway stations, and the central corridors of large office buildings all provide meaningful protection. The goal during the first 48 hours is to maximize the mass of material between yourself and the outside air, and to avoid tracking fallout dust indoors.
What a Larger Exchange Would Mean
A single weapon detonated over a city would be catastrophic at a local and regional level. A full-scale nuclear exchange between major powers would create effects that extend globally. Climate modeling from researchers at Rutgers and other institutions, published in the Journal of Geophysical Research, projects that a large-scale nuclear war between the United States and Russia could inject enough soot from burning cities into the upper atmosphere to block sunlight and drop global average surface temperatures by more than 8 degrees Celsius (about 15 degrees Fahrenheit). This “nuclear winter” scenario would persist for years, devastating agriculture worldwide and threatening famine on a scale far exceeding the direct casualties from the weapons themselves.
Even a more limited regional exchange, such as a conflict involving 100 weapons of modest size, has been modeled to produce several degrees of cooling and significant disruption to global food production. The climate effects stem not from radiation but from the massive fires that nuclear weapons ignite in urban areas, which loft black carbon particles into the stratosphere where rain cannot wash them out.
The Timeline From Seconds to Months
Putting it all together, the sequence of a nuclear attack on a city would unfold roughly like this. In the first fraction of a second, the thermal flash appears. Within seconds, the blast wave reaches out several miles, collapsing structures and generating hurricane-force winds. Within minutes, fires are burning across the blast zone and merging. Within 15 to 30 minutes, the mushroom cloud has risen and, if a ground burst, fallout begins drifting downwind. Over the next several hours, the heaviest and most radioactive fallout settles, making unsheltered areas downwind extremely dangerous.
Over the first two days, radiation levels drop sharply thanks to the rapid decay of short-lived isotopes. By two weeks, outdoor radiation in fallout zones has dropped to levels that allow limited movement, though contamination remains a concern. Longer-lived isotopes in soil and water continue to pose health risks for months to years, increasing cancer rates across affected populations. For the broader region, disrupted infrastructure, contaminated water supplies, overwhelmed medical systems, and displaced populations would define the months and years that follow.