Every year in late February, hundreds of viewers gather in Yosemite to witness this amazing event. But the Yosemite Firefall can be fastidious. Although Horsetail Fall is visible from multiple viewpoints in Yosemite Valley, some factors must converge to trigger the Firefall. If conditions are not perfect, the Yosemite Firefall will not glow.
First and foremost, Horsetail Fall must be flowing. If there’s not enough snowpack in February, there will not be enough snowmelt to feed the waterfall, which tumbles 1,570 feet (480 meters) down the east face of El Capitan. similarly, temperatures must be warm enough during the day to melt the snowpack. If temperatures are too cold, the snow will stay frozen and Horsetail Fall won’t flow. Lack of runoff is why there is no Firefall in October when the sun hits Yosemite Valley at around the same angle. By October the runoff that feeds Horsetail Fall has long since dried up.
Second, the western sky must be clear at sunset. If situation are cloudy the sun’s rays will be blocked, and Horsetail Fall will not light up. Winter weather is extremely variable in Yosemite, however, and days that start off cloudy can clear up by sunset.
If everything comes together and conditions are just right, the Yosemite Firefall will light up for about ten minutes. To see Horsetail Fall glowing blood red is an almost supernatural experience.