What Happened When the Trinity Test Bomb Detonated

• Scientists set off the Trinity test atomic bomb on July 16, 1945.
• Though they chose a somewhat secluded area of the desert, people lived less than 20 miles away.
• It was impossible to hide the light, noise, shockwave, and fallout from the bomb. 

At approximately 5:30 a.m. on July 16, 1945, the world’s first atomic bomb exploded in the New Mexican desert. It was bright, hot, and loud. 

Scientists and military personnel crouched nearby in bunkers. Less than 20 miles away, residents awoke to a brilliant light, jolting sensation, or rumbling sound that officials later blamed on an ammo dump explosion.

Over the next days and weeks, the fallout cloud from the Manhattan Project’s Trinity test would drop radioactive debris on nearby ranches and farms before moving onto dozens of states and two other countries. 

Here’s what it was like to witness the Trinity test, from its dazzling light to its drifting, deadly radioactive flakes.

The heat was 10,000 times hotter than the sun’s surface

National Security Research Center

Ground zero: The heat generated when the plutonium device imploded was 10,000 times hotter than the sun’s surface, National Geographic reported.

Something that surprised physicist McAllister Hull was seeing in photographs the way the heat completely vaporized instrument-containing balloons that had been tethered to the tower. The tower itself disintegrated, too. 

Fusing the desert’s quartz and feldspar sandstone with bits of the bomb, the heat created a new material called trinitite, which is a glass-like substance that’s mostly shades of green but also sometimes red. Trinitite is radioactive and the Atomic Energy Commission buried most of it in the 1950s.

Bunkers (6 miles away): To the south of ground zero, a member of the Special Engineer Detachment (SED), Hans Courant, saw the flash, and then, “My hands got warm from the heat from the bomb, which just grew and grew,” he later said during a 2015 interview with the Atomic Heritage Foundation.

Base camp (9.5 miles away): Nobel Prize winner Enrico Fermi protected his face with a piece of welding glass inserted into a board. “My first impression of the explosion was the very intense flash of light and a sensation of heat on the parts of my body that were exposed,” he said. 

The light emitted shades of gold, purple, violet, blue, and more

Universal

Ground zero: Upon detonation, about one-third of the total energy from the bomb was in the form of ultraviolet, visible, and infrared light, according to physicist and Manhattan Project expert Bruce Cameron Reed.

Bunkers (6 miles away): For physicist Val Fitch, then part of the SED, his welding glass wasn’t enough to block out the “enormous flash of light” from the bomb, which he estimated took 30 microseconds to arrive. To Warren Nyer, another physicist, “it looked like a living thing with a blue glow,” creating a contrast between the mountains and sky. 

This was the location of many of the cameras that recorded the explosion. None, however, picked up the many colors Brigadier General Thomas F. Farrell said he witnessed, “golden, purple, violet, gray, and blue.” 

Base camp (9.5 miles away): Through his dark glass, Fermi had the impression the desert was suddenly brighter than day. He “could see something that looked like a conglomeration of flames that promptly started rising,” he wrote. “After a few seconds the rising flames lost their brightness and appeared as a huge pillar of smoke with an expanded head like a gigantic mushroom that rose rapidly beyond the clouds.” 

It was the brightest light physicist I. I. Rabi had ever seen: “It blasted; it pounced; it bored its way into you.” Though it only lasted a couple of seconds, he said he felt like it would never end. 

Campañia Hill (22 miles away): Before the blast, Edward Teller liberally applied sunscreen to protect his skin from the UV light. SED member William Spindel was instructed to keep his eyes closed for 10 seconds after the detonation so he wouldn’t be blinded. 

Hans Bethe compared the flash to a giant magnesium flare. He calculated the rising white ball of fire at 268 mph (120 meters per second). “After more than half a minute, the flame died down and the ball, which had been a brilliant white became a dull purple,” he wrote.

Alamogordo Air Base (60 miles away): One of General Leslie Groves’s officers reported the flash lit “the entire northwestern sky.” Not far off, a rancher awoke as though “somebody turned on a light bulb right in my face,” according to Janet Farrell Brodie’s book “The First Atomic Bomb: The Trinity Site in New Mexico.”

Sandia Mountains (110 miles away): After spending the night camping in the Sandia Mountains, Manhattan Project chemist Lilli Hornig saw boiling clouds and color — “vivid colors like violet, purple, orange, yellow, red.” 

Amarillo, Texas (280 miles away): Despite the distance, some residents saw the sky brighten when the bomb went off.

Sound from the explosion was heard as far as 95 miles away

National Security Research Center

Bunkers (6 miles away): After the flare of light and the fireball, it took a while for the sound, which travels slower than light, to catch up. When it did, Fitch remembered it coming “a long time afterwards, the sound. The rumble, thunder in the mountains.” 

Campañia Hill (22 miles away): Estimating the time it would take for the sound to travel over 20 miles, SED member Spindel waited through the most “intimidating minute I’ve ever spent” after seeing the fire rise into the sky. 

Albuquerque (95 miles away): Residents all over the area heard the blast. When Hornig stopped for breakfast near Albuquerque after her night on Sandia Mountain, she recalled that the man behind the counter asked if she knew anything about the explosion. 

The shock wave shattered the windows of homes 120 miles away

Los Alamos National Laboratory

Ground zero: The implosion device looked like a soccer ball, with 32 lenses made of explosives surrounding a plutonium core, about the size of a softball. The simultaneously detonated lenses created a shockwave that compressed the plutonium, triggering the explosion.  

The resulting blast was between 15 and 20 kilotons of force. When the blast wave reflected from the ground, it met up with the original wave and formed the stem of the mushroom cloud. 

Equipment bunker (½ mile away): Shelters containing seismographs to measure the bomb’s ground shock and other instruments were located about 2,400 feet from ground zero. The cables were strewn about, and the top of the bunker was bare, all the dirt the SED members had layered on top of it was gone, according to Brodie. 

Schmidt/McDonald Ranch House (2 miles away): The government took over the Schmidt/McDonald ranch in 1942, and scientists and military personnel later used it as the assembly site for the Trinity test bomb. Though the explosion blew out the windows and doors, there was little structural damage to the house. 

Bunkers (6 miles away): It took about 30 seconds for the shock wave to reach Fitch, by his calculations. 

Base camp (9.5 miles away): Some observers were knocked over by the force of the blast when it arrived, according to Reed. “Thirty seconds after the explosion came first, the air blast pressing hard against the people and things,” Farrell wrote. 

Fermi, who was a few miles farther than Fitch, put the time of the shock wave at 40 seconds.

15 miles away: The force caused sheepherder Jack Denton to fall off of his cot, Brodie reported. 

Silver City (120 miles away): Whole houses shook, windows shattered, and dishes and cabinets rattled when the shock wave reached nearby cities.  

Fallout rained down, burning cattle located 30 miles away

AP Photo

Ground zero: In the minutes after the blast, the mushroom cloud stretched 50,000 to 70,000 feet into the atmosphere, the New York Times reported. As the fireball cooled, vaporized fission products condensed into a cloud of particles while also sucking in water from the atmosphere.

This debris became radioactive fallout, according to Reed. In addition, 10 pounds of highly-radioactive plutonium never underwent fission and got caught up in the fallout cloud, according to National Geographic. 

Bunkers (6 miles away): Prior to the test, scientists set a limit of 5 roentgens, a legacy measure of radiation exposure, then bumped it up to 10 for evacuations. One bunker did evacuate for what may have been a false reading, according to Reed. Director of the health group at Los Alamos Louis Henry Hempelmann later called the numbers “just arbitrary,” according to the book “Atomic Doctors: Conscience and Complicity at the Dawn of the Nuclear Age” by James L. Nolan Jr.

“Exposure rates in public areas from the world’s first nuclear explosion were measured at levels 10,00 times higher than currently allowed,” a 2010 report for the Centers for Disease Control and Prevention found.

Author and journalist Jennet Connet noted that as close as the men were to ground zero, they had little protection. “Had the wind been blowing the wrong way, they all would’ve been showered in a fair amount of radioactive dust,” she said. 

A few days after the test, a technician found radiation had destroyed some of the camera film, according to Brodie. 

Bingham (12 miles away): About four miles outside the town of Bingham, some equipment measured 6.5 roentgens per hour, but the town’s residents weren’t evacuated, according to Brodie. 

Chupadera Mesa (30 miles away): Fallout rained on cattle near Chupadera Mesa, giving them serious beta burns, which appear similar to a sunburn. Their fur fell out, then grew back gray or white, according to Nolan. The government purchased some of the cows for testing.

Oscuro (45 miles away): Strange white flakes fell for days on a family farm, where later the chickens and family dog died. 

Ruidoso (50 miles): Some teenage girls at a dance camp fell out of their bunks and heard an explosion. Later, what felt like warm snow drifted down on them. They put on their bathing suits and rubbed the flakes on their faces, according to National Geographic. Only two of the girls lived past the age of 30, Nolan reported. 

Tularosa (51 miles away): Eleven-year-old Henry Herrera watched the fallout cloud drift away then return to Tularosa. Black ash covered the Herreras’ laundry on the clothesline, according to Brodie. 

Sébastien Philippe, Susan Alzner, Gilbert P. Compo, Mason Grimshaw, Megan Smith

Over 100 miles away: The cloud split into three, mostly drifting northeast, raining fallout over an area of about 100 miles long by 30 miles wide. 

A 2023 Princeton University study used weather data and modeling software to show how the cloud spread over northeast New Mexico, as well as to the south and west of ground zero.

Trinity test “downwinders” have been lobbying to receive compensation from the Radiation Exposure Compensation Act (RECA). Members of the  Tularosa Basin Downwinders Consortium say their families have unusually high rates of cancer, which they attribute to living close to the atomic bomb testing site.  

Over 1,000 miles away: In August 1945, Kodak customers complained that their X-ray film, sensitive to radiation, was ruined. Kodak physicist Julian Webb found that the culprit was the strawboard, a packaging material made of straw, from a mill in Indiana. The Trinity test fallout had reached the Midwest.

The Princeton study showed radioactive fallout reached as far as Canada and Mexico over the course of 10 days.