The blue haze blanketing the Blue Mountains in New South Wales is commonly attributed to the area’s eucalyptus forests.
A popular theory is that airborne droplets of eucalyptus oil combine with dust particles and water vapour, refracting rays of mainly blue light.
Another theory is they appear blue for the same reason the sky appears blue. Dust, water droplets and air particles scatter short wavelengths of blue light more than long wavelengths of red light. The air acts like a translucent plastic sheet, giving the mountains a blue tint. This explains why mountains without eucalyptus sometimes appear blue.