Media playback is unsupported on your device Media caption Urban skiers must play cat-and-mouse with the police

Slopestyle skiers made a successful debut at the Winter Olympics in Sochi, eliciting gasps with their stunts and jumps. Many miles away in northern parts of the US, a growing number of skiers use city settings to perform and film similar tricks.

They scope out urban landscapes to find rails to skid down and walls to slam their skis against. They wait for schools and libraries to close, sometimes emerging only under cover of darkness. They play cat and mouse with the police.

On one recent day in St Paul, Minnesota, a group of three skiers and a cameraman checked for onlookers, swiftly unloaded a car, and hurried up a staircase onto a balcony at the front of the Central High School.

A bungee was attached to the balcony railing, and two skiers pulled the third back before releasing him towards a pre-built snow ramp. From there he slid across a railing, before dropping backwards onto a ledge, then forwards again to the ground.

Freeskiing categories Park: Skiing in a mountain area with built-in obstacles such as rails and boxes

Urban: Using city architecture to perform ski tricks

Backcountry: Skiing off piste

It took about a dozen attempts to get the trick right, just before a private security guard drove up and - after watching the jump played back on a video camera and proclaiming it "cool" - politely but firmly asked the skiers to leave.

This is urban or street skiing, an often stealthy pastime that has gradually spread since first surfacing about 15 years ago. That was when, inspired by skateboarders, people first started filming each other sliding down handrails on snowboards or skis.

The skiers say that cities allow them to be more creative, and offer greater variety than resorts.

"Skiing was born in the mountains and it always will be a mountain sport, but it's cool to bring it into the streets and just put a different spin on it," says Sandy Boville, one of the skiers in St Paul.

At the top end are crews who spend much of the winter following the city snow.

Their skiers and snowboarders have become increasingly ambitious, using tools such as mechanised winches and bungees to perform ever more spectacular stunts.

They star in slick videos made each year by a handful of specialist production companies. One recent film included a segment shot among the derelict industrial buildings of Detroit, which has been viewed on YouTube more than 600,000 times.

"Anywhere it snows in the whole world is your canvas," says Karl Fostvedt, who had the main role in the Detroit segment and spent last month skiing in Japan.

Image copyright Poor Boyz Productions

Outside North America, the region in which urban skiing is most common is Scandinavia.

In Norway, film company Field Productions has started promoting skiing videos on the mass market. Tens of thousands of people went to watch Supervention, a film that features urban skiing, after it was released in cinemas across the country late last year.

Street skier's lexicon Gnarly: Big, scary, dangerous

Tranny: Angled landing

Wall ride: sliding bases of skis along vertical surface

Mashed potatoes: Wet and thick snow

Butter: Rotating on only edges, noses, or tails of skis

Wu-Tang: Gradient of a jump (e.g. "this jump has a lot of wu-tang")

Burly: See "gnarly"

The job may seem glamorous, but it can also be demanding.

First, the skiers have to scout out features on which they can perform an original trick.

"We usually start with high schools and parks and that sort of area because the architecture's usually conducive - there are stairs and railings and balconies," says Logan Imlach, another of the skiers in St Paul. "But you branch out to really anywhere in cities - wherever there's concrete and metal there's a good chance that you can ski on it."

Snow ramps are often constructed in advance, to limit the time spent on site. When the skiers return, water and salt is sprinkled over the snow to prevent them from crumbling, or in urban skiing slang, turning into "mashed potatoes".

Image copyright BBC Sport

Protocol demands that snow be shovelled from concrete steps under railings, so those who slip off don't get away with a soft landing.

Tricks are then repeated again and again, until skier and cameraman are both happy.

"It's not going on a sunny day to the mountains and skiing with your friends, it's putting in hard work," said Khai Krepela, the third of the trio of skiers in St Paul, who declined to ski one day because his "mood" wasn't right. "It's stressful but it's fun."

One hazard is being kicked off sites by the police or property owners, either of whom could be worried about damage or liability.

Officials in the US are seen as being stricter about urban skiing than their counterparts in Scandinavia - though the police in St Paul seemed unusually relaxed. At one site, they responded to a complaint from a resident only to suggest an alternative spot to the skiers. When the skiers returned a couple of days later, another police car arrived, but the policeman sat in his car watching for more than 20 minutes before finally encouraging the skiers to move on.

Another hazard is injuries. The skiers have broken bones, torn ligaments, and suffered concussions on a regular basis.

Image copyright BBC Sport

It is also hard to make a living. Whilst freeskiers who compete in big competitions can strike more lucrative sponsorship deals, the urban specialists say they struggle to make it through the winter.

The crew of four in St Paul were sharing two double beds in a single motel room and making their own sandwiches for lunch.

Boville, a 23-year-old Canadian, and Krepela, a 22-year-old from Utah, both have some sponsorship and are spending the winter filming with Level 1 Productions. Imlach, an Alaskan and at 26 a relative veteran, works three-week stints on an oil platform in his home state to help fund skiing trips.

Many urban skiers are already experts on the mountain. But they say the fact that you can ski streets without travelling to a resort and buying a ski pass can make the activity more accessible and help it spread further.

"That is a huge part of it," says Imlach, "you don't have to buy a lift ticket. It's growing in popularity." You hear about a lot more kids nowadays trying it out, he adds.

Sig Tveit, a Norwegian urban skier currently on tour in Sweden, says he's seen many more people skiing and snowboarding in Scandinavian cities recently - "just friends going out in the streets and filming each other. I think there's a lot of that."

Tveit also says that some urban skiers who learnt to ski in cities without ever going to a resort have been emerging from Russia and eastern European countries.

The urban skiers take some pride in the underground, amateur nature of what they do. To borrow a word from their phrasebook, it can be more "gnarly" (scary and dangerous) than skiing on a mountain, and more authentic than going to a ski park with built-in rails and jumps.

Even so, a little more attention - and financial backing - wouldn't hurt.

"It would just be nice for these young guys to think 'I could be a professional urban skier someday and that could be my job'," says Imlach, "but unfortunately right now that's just not the reality."

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Additional footage in video courtesy of Level 1 Productions