[Fran]

Fran asked the Naked Scientists:

Sometimes on a hot summer day you can see "heat" radiating from really hot surfaces it sort of shimmers. Since you can't see air move, what exactly are we seeing?

thanks and I love your show

What do you think?

[DoctorBeaver]

The surface is hotter than the air. This means the air heats up & consequently rises (air - the same as all gases - expands as it is heated, causing the pressure to drop to a level lower than the air above it & that makes it rise). The rising air plays havoc with lightwaves & the shimmering you see is due to lightwaves being distorted.

Even a very short distance above the surface the air is cooler. Hotter air rising into it causes heat transference and resultant turbulence and this can also add to the effect.

I dare say 1 of our physics whizzos can explain the mechanism of heat transference better than me so I shall leave it to them.

[chris]

Hi Fran

this is the "mirage" effect and it's caused by light altering its speed as it passes through patches of more and less dense air. Contrary to what most people think, the speed of light is not fixed and will change according to the substance through which it passes. It travels fastest in a vacuum, slightly slower in air and slower still in water. As a rule of thumb, the denser the medium  (e.g. water is denser than air) the slower light will travel in it.

But when light makes this transition, from one medium to another and so changes its speed, this also causes its path to bend slightly, which is known as refraction.

So when you look at a patch of road in the distance and there appears to be a lake hovering over the road surface, what has happened is that heat from the road surface has warmed the air above the road, making it less dense. Light hitting that patch of air speeds up and bends as it does so. Now, instead of looking at the road through that patch of air, you are actually seeing light rays that have come from the sky, so they look brighter, and bluer.

Exactly the same process makes stars appear to twinkle in the night sky as the light from them passes through denser and thinner regions of the Earth's atmosphere on it's way to reach you. If you saw the same star from space, however, it wouldn't twinkle at all.

Chris

[DoctorBeaver]

Show off!  [!]

[lyner]

air - the same as all gases - expands as it is heated, causing the pressure to drop to a level lower than the air above it & that makes it rise)

Sorry about this Dr B but-
Convection:
The air is heated up and expands - pushing outwards against the surrounding air. As it is now less dense, it is pushed upwards by the more dense, cool, air around it (floats). This, of course, give the 'effect' of hot air rising but it is actually 'pushed up' because "there is no such thing as suck". Fluids only move when  pushed. (Except when gravity pulls, natch)
God, I'm such a pedant!

[DoctorBeaver]

OI... I'm the pedant around here!

[lyner]

It really does look as though I am gunning for you at the mo.
It will pass.
Look upon me as a sort of Science - obsessed Robin Hood.
I Sher Would like to get things right.

[DoctorBeaver]

I Sher Would like to get things right.

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!