Yoga instructor Kathy Gabriel, 24, does not like sticking to one location for her classes, especially indoor ones.

Instead, she has held sessions on the rooftop of YMCA and at the boardwalk at Marina Bay Sands. On July 3, she will hold a session at the Haw Par Villa theme park.

Ms Gabriel says: "Yoga can be done anywhere, so why not practise in different outdoor environments versus in a studio?"

The founder of community yoga group Urban Yogis launched Pop Up Yoga two months ago. These are weekly classes held at outdoor locations across the island.

Attendance has grown steadily since the first event, from about 15 people to a maximum of 50 a session.

Each session costs $5 a person and those interested can register at online community marketplace Funzing (sg.funzing.com).

Ms Gabriel says outdoor yoga is an "organic experience" that dates back to the earliest practitioners. "The ancients did yoga outside all the time and there was no such thing as air-conditioned studios," she says.

The Pop Up Yoga sessions are part of a wave of yoga classes held in the open and practised by people who want to move the activity out of air-conditioned studios.

Beyond parks and beaches, classes are also held in urban spots such as building rooftops, waterfront spaces and obscure spots.

There are at least eight yoga centres and groups here that either hold outdoor classes regularly or ad-hoc pop-up events, The Straits Times found.

In a remote grass field in Little India, lifestyle consultant and certified yoga practitioner N. Nithiya ran a free yoga class on two Sundays last month.

Held at Hindoo Road and called Free Yoga In Little India, the sessions drew 10 to 15 people, who heard about her event via Facebook as well as migrant workers who were passing by and decided to join in.

Ms Nithiya, 32, wanted to use yoga to engage the community in Little India.

She chose yoga because it is a simple activity that costs almost nothing. "You don't need fancy equipment, just a yoga mat," she adds.

The next session will take place on July 3.

She also plans to add other activities, including zumba, for variety.

Yoga collective Outta Hatha, which does not have a physical studio, holds regular classes at the Customs House Terrace by Fullerton Bay Hotel.

Groups of five to 15 can be spotted doing the downward dog or balancing on their heads along the scenic waterfront space every Monday to Thursday evening.

Ms Wee Shu Ting, 32, marketing and operations director of Outta Hatha, says outdoor yoga is challenging yet liberating.

"Being outdoors challenges our students to block out distractions - be it occasional loud music from nearby restaurants or tourists taking pictures.

"But our downtown location also gives students an easy escape from the stress and frenzy of work and urban life."

Classes are priced at $20 a person for a walk-in session. A package of $150 for 10 sessions is also available. Pop-up events are held twice a month on weekends too.

Although organisers of outdoor yoga classes save on rental, their events are at the mercy of the weather. On rainy days, participants either wait for the rain to stop or the class is cancelled and they get a refund.

This is all part of the experience, says instructor Wendy Chan, 36, director of Yoga Seeds.

She holds outdoor yoga sessions at the monthly Car-Free Sunday SG initiative in the Civic and Central Business Districts, and sunset yoga session in East Coast Park.

She says: "Being outdoors and surrounded by nature, students breathe in fresh air and notice how beautiful their surroundings are."

Other organisers are also incorporating outdoor yoga into their events such as the Car-Free Sunday SG and DBS Marina Regatta.

Yoga enthusiasts such as lawyer Ooi Jian Yuan, 34, enjoys the alfresco option.

Although he is signed on with a conventional yoga studio, he tries to attend some Outta Hatha's weekend outdoor classes.

He says: "It's safer and more cloistered in the studio whereas I feel closer to nature outdoors. Also, there is a community vibe at the outdoor sessions as people are friendlier."

His only gripe is having to take along a yoga mat, but he adds that it is a "minor thing".