With the arrival of winter and the U.S. coronavirus outbreak in full swing, the restaurant industry — expected to lose more than $230 billion in 2020 — is clinging to techniques for sustaining outdoor dining even through the cold and vagaries of a U.S. winter.Yurts, greenhouses, igloos, tents and all kinds of partly open outdoor structures have popped up at restaurants around the country. Owners have turned to these as a lifeline that can help fill some tables by offering the possibility at least of a safer dining experience."We're trying to do everything we can to expand the outdoor dining season for as long as possible," says Mike Whatley with the National Restaurant Association.Dire times have forced the industry to find ways to survive. Whatley says more than 100,000 restaurants are either "completely closed or not open for business in any capacity." A recent survey by his organization found that the number of restaurants offering outdoor dining fell from 74% in early September to 52% in late November."It's going to be a hard and tough winter," Whatley says. "As you see outdoor dining not being feasible from a cold-weather perspective, or, unfortunately, from a government regulations perspective, you are going to see more operators going out of business."In recent months, many cities and states have imposed a raft of restrictions on indoor dining, given the high-risk of spreading the virus in these crowded settings.Many have capped occupancy for dine-in restaurants. Some halted indoor dining altogether, including Michigan and Illinois. Others have gone even further. Los Angeles and Baltimore have halted indoor and outdoor dining. Only carry-out is allowed.Those who can serve customers outdoors, on patios or sidewalks, are coming up with creative adaptations that can make dining possible in the frigid depths of winter.Urging diners and servers to embrace the 'yurtiness' Washington state shut down indoor dining in mid-November and has kept that ban in place as coronavirus cases continue to surge.On a blustery December evening, servers at the high-end Seattle restaurant Canlis are huddled together in the parking lot, clad in flannel and puffy vests, while their boss Mark Canlis gives a pep talk ahead of a busy night."The hospitality out here is exactly the same as it is in there," says Canlis, gesturing to his restaurant, which overlooks Lake Union. "But that looks really different, so try to invite them into the 'yurtiness' of what we are doing."Canlis has erected an elaborate yurt village in the parking lot next to his family's storied restaurant.It includes an outdoor fireplace and wood-paneled walkways winding between small pine trees and the circular tents. The assemblage of yurts, with their open window flaps, is the Canlis family's best effort to keep fine dining alive during the pandemic and a typically long and wet Seattle winter (referred to locally as the "Big Dark.")Arriving guests are greeted with a forehead thermometer to take their temperature and a cup of hot cider."It gives us an excuse to think differently," says Canlis of the outdoor dining restrictions.The yurts not only shield diners from the elements, but also from infectious airborne particles that might otherwise spread among different tables of guests.Dining inside such structures is not risk-free: Guests could still catch the virus from a dining companion as they sit near each other, without masks, for a prolonged period of time. But Canlis says there is no easy way to determine whether every member of a dining group is from the same household."I'm not the governor or the CDC," he says. "I'm assuming if you are there at the table, you're taking your health into your own hands."There are new rules, crafted during the pandemic, for outdoor dining structures in Washington that have required Canlis to consider issues like how to properly ventilate the yurts and sanitize the expensive furniture."What is the square inch of yurt volume space? What is the size of the door and the windows? How many minutes will we allow the yurt to 'breathe?' " says Canlis.The structures get cleaned after each dining party finishes their meal and leaves; during the meal service the waiters enter and leave quickly, wearing N95 masks.Igloos, domes and tents: Just how safe are they? Another, more modern-looking take on outdoor dining involves transparent "igloos" and other dome-like structures that have become popular with restaurant owners all over the country.Tim Baker, who owns the Italian restaurant San Fermo in Seattle, had to order his igloos from Lithuania and assemble them by hand with the help of his son.His restaurant's policy is that only two people are allowed in an igloo at a time, to cut down on the risk of people from different households gathering together."You're completely enclosed in your own space with somebody in your own household. These domes protect you from all the people walking by, on the sidewalk, and the server doesn't go in with you," he says.Baker says he consulted with experts in air flow and decided to use an industrial hot air cannon after each party of diners leaves the igloo and before the next set enters — aiming to clear the air inside the structure of any lingering infectious particles."You fire this cannon up and it just pushes the air through really aggressively," quickly dispersing the particles, Baker says.His restaurant's igloos have become a big attraction."I'm particularly proud of anything that we can do to get people excited right now, because we need it," he says. "We're all getting crushed by this emotionally."Not all outdoor dining structures are created equally, says Richard Corsi, an air quality expert and dean of engineering at Portland State University."There's a wide spectrum," says Corsi. "The safest that we're talking about is no walls — a roof. And then the worst is fully enclosed — which is essentially an indoor tent — especially if it doesn't have really good ventilation and good physical distancing."In fact, Corsi says some outdoor dining structures that are enclosed and have lots of tables near each other end up being more dangerous than being indoors, because the ventilation is worse.Dining that is truly outdoors, with no temporary shelter at all, is much safer because there are "higher air speeds, more dispersion, and more mixing than indoors," Corsi says, which means that respiratory droplets harboring the virus don't accumulate and are less concentrated when people are close to each other."If they have heaters, then you're going to actually have pretty good ventilation," says Corsi, "The air will rise up when it's heated and then cool air will come in."He says private "pods" or "domes" can be fairly safe if they are properly ventilated and cleaned between diners. That also assumes that everyone eating inside the structure lives together, so have already been exposed to each other's germs.But Corsi says he is still not going out for a meal in one of the many new outdoor dining creations — "even though I know they've got a much lower risk" of spreading COVID-19 than most indoor alternatives.This story comes from NPR's health reporting partnership with Kaiser Health News. Copyright 2020 NPR. To see more, visit https://www.npr.org.