Every year when winter finally loosens its grip on northern Japan, Tomoko Kobayashi begins what has become an annual rite for her and a small band of collaborators. They head out with measuring devices to keep tabs on an invisible threat that still pollutes the mountains and forests around their homes: radioactivity.

In her car, Ms. Kobayashi follows a route that she now knows by heart, making regular stops to probe the air with a survey meter, a box with a silver wand that looks and acts like a Geiger counter. She uses it to detect gamma rays, a telltale sign of the radioactive particles that escaped when three reactors melted down at the Fukushima Daiichi nuclear plant in March 2011 after an undersea earthquake sent a towering tsunami crashing into the coastline.

She and a group of fellow residents of Odaka, a small community 10 miles north of the plant, spend days collecting readings at hundreds of points, which they use to create color-coded maps of radioactivity levels emanating from reactor particles still scattered across the countryside. Ms. Kobayashi posts them on the wall of her small inn for guests to see, making up for a lack of government maps detailed enough to reveal potentially hazardous spots.

“The government wants to proclaim that the accident is over, but it isn’t,” said Ms. Kobayashi, 72, who reopened her inn, Futabaya, seven years ago, after the evacuation order in Odaka was lifted. The inn has been in her family for four generations and she grew up here, never imagining she would one day have to master an arcane knowledge of microsieverts and atomic half-lives.

“I choose to live here, but is it safe? Can I pick these nuts or eat those fruit? The only way to know for sure is do the measuring ourselves,” she said.

Ms. Kobayashi is one of Fukushima’s citizen scientists, residents around the plant who responded to official coverups and silences by acquiring their own measuring devices and teaching themselves how to use them. They defied a government that at first tried to prohibit nonprofessionals from measuring radiation and later just ignored them.

Almost 14 years after the meltdowns, the citizen scientists persist, fueled by smoldering distrust of authority. While their numbers have dwindled as some grew old or moved away, many like Ms. Kobayashi remain vigilant, eager to make their voices heard or simply reclaim control of lives shattered when towns around the plant were evacuated or contaminated.

They have created new communities with their networks of like-minded people. By filling gaps left by government inaction, they have grown proficient at measuring and mapping invisible radiation, leading to what experts have called a democratization of expertise. This grass-roots embrace of science is an enduring legacy of the Fukushima disaster and a path to self empowerment.

“Around the world, we have seen a growing contempt for expertise, but these citizen scientists are going against that trend,” said Kyle Cleveland, a sociologist at Temple University in Tokyo who has researched perceptions of radiation during the Fukushima crisis. “They are using knowledge to understand their environment and claim legitimacy for their grievances.”

While the citizen scientists were often the only source of radiation numbers in the months after the meltdowns, these days they play watchdog, verifying the government’s figures and providing a level of detail that officials still won’t. After falling for several years, radiation outside the plant has plateaued at levels often still many times higher than before the accident.

Some groups have achieved considerable expertise in detecting these invisible particles. One is the Mothers’ Radiation Lab Fukushima — Tarachine, started by a group of mothers in the city of Iwaki, an hour’s drive south of the plant, to protect their children.

Begun in a single room with three donated measuring machines, Tarachine now occupies almost the entire floor of its building, with 13 salaried staff, a health clinic and a laboratory filled with equipment. Its self-taught technicians, most of them mothers, can measure even tough-to-detect types of radiation. They publish their findings on the group’s website.

When the nuclear power plant’s reactor buildings started to explode, the group’s founder, Kaori Suzuki, was a homemaker whose only outside work had been a brief stint in the fashion industry. Anxious for her teenage daughter, Ms. Suzuki joined protests against the lack of official information before concluding that the best response was to learn to measure radiation herself. When other mothers joined, they chose the name Tarachine (pronounced tah-rah-chee-nay), a term from ancient Japanese poetry used to describe a strong mother figure.

They faced enormous resistance from official scientists dismissive of their efforts and social pressure from fellow residents fearful of radiation-related discrimination similar to that faced by the survivors of the atomic bombing of Hiroshima and Nagasaki. Ms. Suzuki learned to use the machines by deciphering English-language manuals. Once Tarachine’s doors opened, demand was overwhelming, as parents brought food from supermarkets and farmers handed over their own produce to be measured.

“Within one month, we had a three-month waiting list,” she recalled.

Worries about food declined as radiation levels dropped, but Ms. Suzuki, 59, has taken on other concerns. One is the decision by the Fukushima plant’s operator, Tokyo Electric Power Co., to begin releasing into the Pacific Ocean more than a million tons of water that has been treated but remains contaminated. Tarachine now sends out boats.

“We still have to keep verifying the company’s claims,” Ms. Suzuki said.

In Tsushima, a small village nestled in a narrow valley surrounded by dark peaks, only the area along the main street has been decontaminated. The rest, 98.4 percent of the village’s land, remains off-limits with radiation levels that can still reach hundreds of times above normal.

At the height of the accident, a plume from the plant reached Tsushima during a snowstorm, lacing the falling flakes with dangerous isotopes. These soaked into the ground, heavily contaminating the village despite its location 18 miles from the reactors.

While the small central area was reopened two years ago, only five people have returned from a previous population of 1,400. One hoping to restart his life here is Hidenori Konno, 77, who was born and raised in Tsushima. He makes frequent trips back to fix the century-old ryokan inn that has been in his family for generations.

During those visits, Mr. Konno uses a handheld device to map radiation readings in the village. By identifying places to avoid, he hopes to convince former neighbors that it is safe to come back.

“If we can see where the hot spots are, and know how much risk we’re actually taking, then I don’t feel as frightened about returning,” Mr. Konno said, sitting on a tatami mat in his inn, which sat empty for 12 years while the village was evacuated.

Helping him is Shinzo Kimura, a radiation scientist who is setting up a small lab in an old clay storehouse behind the inn. During the disaster, Dr. Kimura quit his job at a government research institute near Tokyo, which tried to block him from taking measurements around the plant. He moved to Fukushima, where he has taught locals like Mr. Konno how to make radiation-hazard maps.

“Science gives them a way to visualize a radioactive danger that they cannot see, smell or taste,” Dr. Kimura said. “It restores what the accident robbed from them, which is an agency over their own lives.”

For Ms. Kobayashi, owner of the reopened inn in Odaka, it was her own maps that reassured her about moving back. She said citizen scientists must stay on the lookout for new leaks, with the cleanup expected to take several more decades.

“The radiation is not gone,” she said, “nor is the need to protect ourselves.”

Kiuko Notoya contributed reporting.

More than a billion people across the world, from China to the Philippines to diaspora communities in the United States, began celebrating the Lunar New Year on Tuesday with fireworks, family time and feasts.

On Wednesday, the first new moon of the Year of the Snake will mark the imminent arrival of spring.

Known as Seollal in South Korea and Tet in Vietnam, the beginning of the lunisolar year is the most important holiday in many Asian countries. In China, it prompts the world’s largest annual migration. Hundreds of millions of people brave jammed roads, train stations and airports, many making the exodus from major cities to their hometowns.

Lunar New Year traditions vary across and within countries, but similar threads run throughout: family time, rituals for prosperity and to honor ancestors, and marathon feasts. Many flock to temples to place offerings of traditional food, and light incense at altars for ancestors and elders.

In China, children receive red envelopes as blessings from their relatives. In Southeast Asia, dragon dances, believed to bring good luck, prosperity and rain, are held in the streets — and sometimes underwater.

Here’s how people said goodbye to the Year of the Dragon and welcomed the Year of the Snake:

People were hurt and deaths were feared after millions of Hindu pilgrims at the Maha Kumbh Mela, a huge festival in the Indian city of Prayagraj, rushed to bathe in holy river waters on what is considered one of the most auspicious dates in the Hindu calendar.

As pilgrims rushed to the confluence of the Ganges and Yamuna rivers, which Hindus consider sacred, some people who were sleeping on the ground were trampled and a barrier broke, government officials said.

The Kumbh Mela at Prayagraj occurs every 12 years. Hindus believe that bathing at the spot where the two holy rivers meet, along with a third mythical river called Sarasvati, will purge them of all sins and help them attain salvation. Because of certain favorable celestial alignments, millions more were expected this year at the event, named the “Maha” or Great Kumbh.

The government of the state of Uttar Pradesh, where Prayagraj is, has estimated that around 400 million people will attend the six-week festival from all corners of India. To house them all, the government built a temporary city on the banks of the Ganges, with tents, toilets, streets, pontoon bridges and waste management facilities.

The danger posed by huge crowds has been a frequent problem at the Kumbh Mela. In 2013, 42 people were killed and 45 injured in a crowd crush on a train platform.

Government officials became much more organized and focused on the safety and security of pilgrims after the 2013 incident. This year, the Uttar Pradesh government has employed sophisticated technology to monitor the inflow and outflow of people so that police personnel on the ground can redirect crowds.

Despite the precautions, festival employees and others were encouraging people to go toward the confluence of the rivers, with some even using the public address system to do so. Police officials were unable to clear the bathing areas before more pilgrims rushed in.

This is a developing story that will be updated.

In Hanoi and other Vietnamese cities at this time of year, potted kumquat trees fastened to motorbike seats dodge and weave through traffic in a haze of orange. Families buy them as symbols of luck and good fortune for the new Lunar New Year, which started on Wednesday.

This year a typhoon and extreme heat dented the harvest, scrambling prices for kumquats and other ornamental plants associated with the holiday, which is known as Tet in Vietnam. Some people bought smaller kumquats or switched to less expensive options, like orchids or persimmon branches.

Ornamental plant farmers are now stuck with unsold inventory after months of price swings in the market. In the case of kumquats, wholesale prices initially rose because of limited supply. Then they cratered for a lack of demand linked to consumer jitters and a perception that this year’s golf-ball-size kumquat fruits do not look very pretty.

“We’re all in a sad mood,” Nguyen Thi Hoa, 39, who grows kumquat trees near Hanoi’s Red River, said of the ornamental plant farmers in her corner of the capital. Unsold kumquat trees stood beside her, each selling for about 600,000 Vietnamese dong, or $24. That is at least 40 percent less than in a typical year.

It would be hard to overstate how important the Lunar New Year is to Vietnamese people — imagine Christmas and Thanksgiving combined — or how ubiquitous kumquat trees are across Vietnam and parts of neighboring China as the holiday approaches. The squat citrus plants are a regular presence in living rooms, shops and office lobbies.

In September, Typhoon Yagi flooded farmland and damaged crops across northern Vietnam during a critical growing period for kumquats and other ornamental staples of Lunar New Year. Ms. Hoa said floodwaters from the storm killed about half of the 500 kumquat trees she had planted.

Higher-than-average temperatures and a shortage of rainfall last year also hurt the harvest, said Pham Thi Thanh Nga, the director of Vietnam’s Institute of Meteorology, Hydrology and Climate Change.

The extreme weather translated into steep price swings at the markets and sidewalk stalls where people buy Lunar New Year kumquats, peach blossoms and bananas. The lack of rain also made kumquat trees weaker and their fruit less attractive, farmers say.

“This tree is much less beautiful than what I expected,” said Nguyen Thi Nguyet, 39, as she inspected a potted kumquat at a Lunar New Year market in Hanoi this week. The fruits looked smaller and thinner than usual.

The tree still cost the equivalent of about $80, or roughly double her budget. So Ms. Nguyet, who works at the Education Department in Hanoi, instead paid about $13 for a bouquet of orchids imported from China.

Nguyen Thi Loan, a retired teacher, was stunned to see the price on a bunch of 21 green bananas lying on a plastic tarp: about $28. She usually pays a little over $1.

“These are the most expensive bananas I have ever touched in my life,” Ms. Loan, 64, said as flowers and pork sausages poked out of her shopping bag. Bananas, the go-to fruit for placing on family altars to honor ancestors, are usually the cheapest item to buy for the holiday, but this year they are more expensive than meat, she added.

“It’s unheard-of,” she said. “It’s crazy!”

The banana vendor, Tran Van Huy, 50, didn’t budge on the price. So Ms. Loan bought one bunch instead of the three she had planned for. She said she would add other fruit to the family altar this year.

The price sensitivity to ornamental plants is partly a function of general economic malaise in Vietnam, Ngo Tri Long, a retired Finance Ministry official, told the news site VnExpress this week. Even though Vietnam’s economy grew by about 7 percent last year, Mr. Long said that it hadn’t fully recovered from the pandemic and natural disasters.

Consumers can adapt to a volatile market for kumquats and other ornamentals by changing what they buy, but farmers are still dealing with the effects.

One kumquat farmer on the outskirts of Hanoi, Nguyen Duc Vinh, said he had lost 40 percent of 3,000 trees to flooding and high winds from Typhoon Yagi. That was especially painful because it happened at a time of year when wholesale traders start inspecting kumquat farms and making orders for Lunar New Year.

As the holiday approached, Mr. Vinh, 51, raised his wholesale kumquat prices by about 50 percent to cover his labor costs, he said. But traders didn’t bite so he reduced them to the normal price of about $10.

“This craft has become more precarious than ever,” he said.

Nguyen Van Loi, a kumquat vendor in Hanoi who bought 1,000 trees from Mr. Vinh, said on Monday that he still had about 400 left to sell, even after cutting the price by half.

“One of the worst years in my 10 years of trading,” said Mr. Loi, 44, as his wife watered kumquat trees to keep them fresh.

A couple on a motorbike stopped to check the tree prices, then drove off without buying anything.

Judson Jones contributed reporting.