Warmer air rising up from the bottom of the canyon, meets cooler, moister air and creates the clouds you see in this National Park Service video by M. Quinn.
Winter conditions on the South Rim can be extreme. Be prepared for snow, icy roads and trails, and possible road closures. Winter weather typically begins by November and becomes well entrenched by December and January, with frequent light to moderate snows and increasingly colder weather.
Low temperatures are generally in the teens along the Rim; however afternoon high temperatures still average in the 40s (4-9°C), due to the amount of sunshine the area receives.
Along the river, cold air typically becomes trapped in the canyon leading to high temperatures only in the 40s and 50s (4-15°C) and low temperatures in the 30s and 40s (-1C-+9°C). Even with all of the winter sunshine, significant snowfall can be expected during the winter with an average snowfall of 50 to over 100 inches (1.3-2.5m) per year on the Rim, and occasionally snow will make it even to the river.