Re: Why does hot air tremble? It is not because of moving air, but why then?

Hello Joris,

You see the 'trembling' of air over a hot object because the index of
refraction (a measure of how much light is bent by a medium) of air
changes with temperature. 

When an object is hotter than the surrounding air, it will heat the air by
convection. This causes currents of air to flow around the object,
with warm air rising and cooler air falling. Light coming from objects
behind this 'convection cell' will appear to tremble as it is
refracted (bent) differently through the different temperatures of air.
Light passing through hot air all at the same temperature would not appear
to tremble - you need the different temperatures.

I'm an astronomer, and this effect is very important to us. The images
of stars wobble around and are made blurry (we call this 'seeing') as the
starlight passes through the different layers of air in the atmosphere. A
lot of effort goes into finding telescope sites that have good atmospheric
seeing, and we also try to reduce 'dome seeing' by keeping the telescope
dome the same teperature as the outside. We can also try to compensate for
the atmosphere with a fancy technology called 'adaptive optics'.

Pauline Barmby
(presently in Arizona, where there is a lot of hot air!)