Atmospheric Optical Illusion

its a type of mirage called a Fata Morgana when it contains an objct that seems to float in water at a horizon

No, it isn't. A fata morgana is a kind of mirage. I asking about a shimmering effect caused by hot air where you are looking at an image that appears to be wobbling, but it isn't. It's also similar to the effect you sometimes see when hot air is blowing from a vent, creating a visual distortion of movement.

refraction

What do you call a person who lacks manners?

RoDaSm?

Unfortunately refraction is not the word I am looking for. If you can imagine lying on a field on a hot day, you would see this effect, and there is a specific word for it. It is the shimmering effect that separates it from refraction. It seems incredible to me that that this word seems to have completely vanished without trace as I have researched all kinds of avenues to try and find it, and I didn't know that words could do that.

Varying convection currents over hot surfaces cause variation in the refractive index of the air, hence the shimmering of objects viewed through such convection. I know of no special term for this other than optical distortion. Light rays from a nebulous object such as the sky can undergo sufficient refraction by those currents to give the mirage of "water on the ground" at some distance from an observer.

It's sometimes called heat haze.

http://www.merriam-webster.com/dictionary/heat%20haze

Hi, Farmerman,

My apologies. I think my reply was a bit rude. Thank you for taking the trouble to answer.

I've heard the term "heat shimmer" which sounds good.
Also it can be a very simple "mirage"

Are you thinking of the word "chimera"?