Andrew McDonald, a student of High Sierra Workshops, took this photo at the falls, amid a sea of photographers. For a few weeks in February if the conditions are just right, for about 10 minutes around sunset, one waterfall in Yosemite National Park looks more like its opposite — a firefall. Visitors who flocked to the California park last week, many with cameras in tow, have not been disappointed by the glowing transformation of Horsetail Fall, which flows from El Capitan. “In the over 20 years I have been photographing the firefall and leading workshops there in Yosemite, I have never seen a more spectacular one,” said Michael Mariant, a photographer from Morro Bay, Calif., who leads teaching trips to Yosemite. The phenomenon occurs if there has been enough snow and rain in the Sierra Mountains to fuel the waterfall, if the skies are clear and if the setting sun strikes the water at an angle that creates the illusion of lava. Mike Gauthier, the park’s chief of staf...