Yellowstone’s air is almost always pure and dry. The high Yellowstone plateau has little dust in the air, yet it is cool or even cold at night in the spring so the humidity stays low. Light often stays clear and neutral colored throughout the day. The quality of the light varies but maintains a clarity that is rare in our industrialized world. When there is smoke in the air from a forest fire we see saturated oranges in the sky, yet usually the overall sense is that the air is clean. Color seems to glow from the clouds. The moon is usually a clean white in the sky near richly colored clouds. Reflections of a crystal blue sky are saturated and enhanced by the stillness and purity of the Yellowstone River’s quiet surface. The thin wispy cirrus clouds at sunset glow with color from a prismatic effect on the high, icy moisture. Rainbow colors appear at the edge of these clouds.