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Under shade of hemlock and pines, 300 nozzles emit a blanket of mist that cascades downhill. When it crosses the line of trees into direct sunlight, it bounces up and dissipates.

“Fujiko humbly insists, ‘Nature does 90 percent of the work and I just help it perform at its best,’” Mergel said.

But the artist’s 50 years’ experience with the medium and site-specific research shaped the Instagram-ready conditions, which a search of #fogxflo shows have been catching park visitors by surprise. Nakaya tailored the sculptures to the terrain of each site: Jamaica Pond, Franklin Park, the Fens, Leverett Pond, and the Arnold Arboretum of Harvard University.

“What Nakaya can control to produce her sculptures are the exact position and number of nozzles placed, and the duration and pressure that the vapor is released,” Mergel says. “For example, she may decide to make a sculpture with 260 nozzles or as many as 900 and locate these on the ground or up high to interact with the ground or wind.”

If the bones of the Arboretum sculpture, called “Fog x Hill,” are Nakaya’s patented stainless-steel nozzles that emit 17-micron droplets, the heart is a motor pump sitting in a nearby shipping container. It forces filtered water from park sources through the nozzles at high pressure, more than 1,000 pounds per square inch (for comparison, the average kitchen faucet flow is around 50 psi).

Karen Mauney-Brodek, president of the conservancy, said that when she sought Mergel’s help finding an artist to capture the spirit of the parks’ architect, Frederick Law Olmsted, the Visual and Environmental Studies alumna nailed it with Nakaya.

Like Olmsted, Nakaya defers to “genius of place,” seeking to enhance rather than compete with nature. Mergel said that while selecting sites for her Boston sculptures Nakaya consulted maps and wind rose diagrams, even Olmsted’s notes, and was as interested in what he left alone in the landscape as in what he changed.

Mergel pointed to another similarity between Olmsted and Nakaya: Both “sculpt with water.” And according to Mauney-Brodek, both embody democratic ideals, Olmsted in his passion for making public spaces accessible to all and Nakaya in creating multisensory art that transcends language. Both even had a hand in a world’s fair, Olmsted in 1893 and Nakaya in 1970.

The Arboretum has planned a series of events to coincide with the exhibit, including a performance of “Macbeth” on Oct. 21, cosponsored by Harvard University Committee on the Arts. The conservancy expects all five sites to draw up to a million visitors before the fog rolls out for the last time on Halloween.

Though ephemeral in nature, Nakaya’s work has achieved some permanence at the Arboretum visitor center, where a staffer affectionately marked its hillside spot in the diorama of the nature reserve with a piece of cotton.

“Fog x FLO: Fujiko Nakaya on the Emerald Necklace” is on view from dawn to dusk through Oct. 31. Explore via mobile device at fogxFLO.info.