Why do gasoline fumes cast a shadow on a cold, sunny day?

This week's question comes to us from Heleny Psaltis from Fort Steele, British Columbia, who asks: 

I was filling my truck with gasoline on a bright sunny, but cold day, and saw shadows of fumes. Really cool, but why?

Shadi Dalili, an associate professor of chemistry at the University of Toronto, Scarborough, says this is an interesting phenomena that she has observed herself. You can see a shadow of gasoline vapours and fumes when pumping gas because the vapour has a different density than the air around it.

Shadows occur as light passes through the vapour and hanges directions slightly because of those different densities of the air and the gasoline fumes.  This is seen as a visible shimmer or the shadow.

The phenomenon is known as refraction, and is the same reason that a stick appears to be bent when you look at it under water because the light is passing between different media that have different refractive indexes. 

This effect may be magnified in cold air as it is denser than warm air. Cold air has a slightly higher refractive index than that of warm air. As a result, in cold air, because of the bigger difference in refractive index between the air and the vapour, you're more likely to see the shadows in the shimmering of the gas fumes. Even if the fumes  are not visible, you still see that shadow. 

It may also be easier to see the shadow on sunny day because of the simple contrast difference. Diffraction differences in the air can be harder to see if you are looking at them through a background that doesn't provide much contrast.