Ilmavärähtymän

Ilmavärähtymän, known in English as air shimmer or heat haze, is an optical phenomenon that occurs when light passes through layers of air with different temperatures. These temperature differences create variations in the air's density, which in turn affect the refractive index. As light rays travel through these unevenly heated air layers, they are refracted, or bent, in different directions. This constant bending and shifting of light causes distant objects to appear distorted or to "shimmer."

The most common cause of ilmavärähtymän is the heating of the ground surface by the sun. The ground radiates heat upwards, warming the air immediately above it. This creates a layer of hot, less dense air near the surface and cooler, denser air above. When light from an object passes through this gradient, it bends, and the observer's eye perceives the light as coming from slightly different positions, leading to the shimmering effect. This phenomenon is often observed on hot summer days, especially over roads, deserts, or other surfaces that absorb and radiate significant amounts of heat. It can also be seen over bodies of water, though the effect might be less pronounced due to the moderating influence of the water temperature. The term "ilmavärähtymän" is Finnish and directly translates to "air vibration" or "air wavering."