What causes the haziness and blueness of distant objects?

Leonardo wanted to answer this question. He applied his observations and reasoning to come up with various ideas which he tested. His writings show that he approached the questions much as a modern scientist would, using what he knew about optics and the makeup of the atmosphere to formulate his hypotheses.

Leonardo recognized that the atmosphere surrounding the earth contained moisture and dust as well as air. The further away an object lay he reasoned, the more the moisture and dust would reflect light from it blurring its true colors and details.

He also reasoned that the air itself was not blue in color. If it was, the sky overhead would appear bluer from lower elevations where the earth's atmosphere is thicker than from a mountain top where it is thinner. As Leonardo observed, the opposite is true.

Leonardo made observations of smoke and water vapor viewed against dark and light backgrounds. He concluded that the blue color of distant mountains results from warm water vapor in the air illuminated by sunlight in front of the dark background of the mountains.

Was Leonardo correct in his scientific understanding of aerial perspective? Not completely, but considering the limited scientific tools and knowledge of his day, his ideas were remarkably insightful.