The picture above is Glacier Falls when embers from a man-made fire are sent over the edge of Glacier Point in the same area. It is a beautiful sight for a summer night.
But in February the sun and the earth are aligned in such a way that at just the right time the light from the sun will hit the granite behind the waterfall. The rays reflect through the water making it look like red-hot lava flowing down the mountain.
Horsetail Falls is the longest free-falling waterfall in Yosemite. It falls 1500 feet, then hits granite and spills over another 500 feet. The falls drain a small area atop El Capitan. It drains only in the winter and spring IF there has been enough snow and rain. This has been a dry year but hopefully there will be enough water to fall. It takes very little to allow the red effect.
Weather conditions have to be right too. The sky must be clear and this is the time of year when the cloud cover could cause problems.
So photographers are gathering. They are hoping for clear skies. Then if they are lucky they might capture the two minutes at sunset that will make Horsetail Falls look like a cascade of fire.