During some mornings in Yosemite Valley, when the weather is below freezing, you can see a rare phenomenon called "frazil ice" at the base of the waterfalls. These are small ice crystals of frozen mist that float down the creek and form a moving stream of slush that looks like frozen lava! It usually occurs in the morning, before 9 am.
Frazil ice is often accompanied by another phenomenon called the snow cone which occurs when falling water from the falls freezes before it hits the ground and piles into a large cone of ice.
You can often witness frazil ice in spring, especially in April, but sometimes in March and May as well.