Atmospheric perspective

Why it happens

Air molecules scatter short wavelengths far more than long ones (Rayleigh scattering, roughly proportional to 1/λ⁴), which is why a clear sky and the veil of air in front of distant hills are blue. Larger particles such as water droplets, dust and smoke scatter all wavelengths about equally (Mie scattering), so humid air, haze and fog turn distance grey or white. When the sun is low, its light crosses a long path of air, loses most of its blue, and the atmosphere glows warm.

Two things happen at once. Light from the object is attenuated, and the sunlit air between you and it adds airlight in the atmosphere’s colour. Koschmieder’s model (1924) describes the result:

seen = t × object + (1 − t) × atmosphere,   t = e^−βd

Here t is the transmittance: 1 up close, falling towards 0 with distance d at a rate β set by the density of the air. Every colour therefore slides along a straight line towards the atmosphere colour. Dark objects get lighter and bluer; very light objects, such as snow in sunlight, can get slightly darker and warmer, because blue is scattered out of the light they send you. The builder below uses exactly this model.

A short history

Pale, bluish distances already appear in Roman wall paintings, such as the first-century BC landscapes of the Odyssey frieze found on the Esquiline Hill. In Chinese landscape painting, Guo Xi’s Early Spring (1072) separates its planes with graded ink washes and bands of mist, and his writings on the “three distances” treat depth as a matter of tone and clarity as much as of placement.

Leonardo da Vinci gave the idea its European name. In the notes later collected as the Treatise on Painting he wrote of prospettiva aerea (aerial perspective), alongside a perspective of colour and a “perspective of disappearance”, in which outlines and details are lost with distance. He advised painting a more distant building bluer and less distinct, and the blue-grey mountains behind the Mona Lisa and the Virgin of the Rocks put this into practice, together with the soft transitions of his sfumato.

Joachim Patinir’s “world landscapes” (early sixteenth century) turned it into a formula of brown foreground, green middle ground and blue distance. Claude Lorrain painted luminous, warm hazes looking into a low sun; J. M. W. Turner dissolved whole scenes into atmosphere; Caspar David Friedrich’s Wanderer above the Sea of Fog (c. 1818) stacks ridges in fog; and the Hudson River School painters, such as Albert Bierstadt and Frederic Church, built vast American distances from layered, glowing haze.