Hot objects appear to twinkle in the distance due to the effect of atmospheric turbulence on the propagation of light, causing rapid variations in the refractive index and deviations in the trajectory of light rays.
When observing a hot object from a distance, the air around it becomes a kind of turbulent lens. Why? Because hot air and cold air do not have the same density. As a result, the light rays passing through these different layers change direction. These small disturbances create a random distortion that makes the object appear to tremble or shimmer. Imagine looking at a hot road in the summer: you will see a kind of wavering in the distance. It’s exactly the same visual effect, caused by the air heated by the asphalt. The farther away the object is, the more pronounced the effect becomes, simply because the light passes through more turbulent layers before reaching our eyes.
When the air temperature changes, its density varies as well: warm air is less dense than cold air. However, light does not pass through these differences as easily; it is forced to bend slightly at each thermal change. As a result, it zigzags a bit like a stick appears broken in water. This phenomenon is what we call light refraction, and it explains why that hot object you see from a distance seems to move or shimmer. The alternating layers of hot and cold air act like many small invisible lenses, each bending the light just enough to visually distort what you see. The greater the temperature difference, the more visible the effect becomes. All of this creates unstable, twisted, and blurry images, giving that impression of constant shimmer.
The warm air rises, but it never does so in an orderly manner: its movement is agitated, irregular, brief, turbulent. This constant agitation forms what are called warm air vortices, which change position, speed, and size all the time. And it is precisely these unpredictable movements that disturb the usual trajectory of light, constantly causing slight deviations in light rays. The result: the image observed in the distance seems to dance, shimmer, and even slightly distort, creating that impression of visual vibration as above a hot road in the middle of summer. The more intense the turbulent movement, the more pronounced this effect becomes.
This shimmering mainly comes from the way light travels through an unstable atmosphere. When an object is hot, it heats the surrounding air, causing it to rise and create invisible ripples in the air. These ripples are formed by different air densities, constantly altering the path taken by the light rays coming from the object to us. As a result, the object seems to slightly change its position and brightness continuously. The eye translates all these small chaotic variations into visual shimmering. It's a bit like observing something through a glass of heated water: the internal movements produce an unstable, blurry vision that disrupts the normal, clear, and constant perception of what we are observing.
Hot air, having a lower density than cold air, causes light to change its path when it passes through layers of air at different temperatures, thus creating the effect of shimmering or sparkling.
The atmospheric turbulence responsible for the twinkling of hot objects is also the cause of the twinkling of stars, known as stellar scintillation.
Did you know that thermal shimmering can influence the accuracy of visual measurements taken from a distance, both for amateur photographers and professional astronomers?
The shimmering effect is more noticeable during hot summer days, particularly over paved roads or deserts, due to the large temperature variations.
The air located near a heated road expands and becomes less dense, rising and creating turbulent flows. These turbulent movements cause rapid variations in the refraction of light passing through these areas, thus producing the optical effect of shimmer that is frequently observed.
Yes, the twinkling is often more pronounced in hot and sunny weather, where strong thermal variations prevail near the ground. In contrast, during stable atmospheric conditions such as a cool, windless evening, atmospheric disturbances decrease and the twinkling is less noticeable.
Absolutely! The twinkling of stars is caused by atmospheric fluctuations in the upper layers of the atmosphere, while the shimmering of hot terrestrial objects, such as a distant fire or a heated surface, is directly related to the turbulent movements of the hot air in the immediate vicinity of those objects.
Indirectly. Even though a cold object itself does not generate disruptive warm air, the light reflected by that object still passes through varying layers of air. However, the phenomenon will be much less pronounced than with hot objects or surfaces, as the thermal variations in the immediate vicinity are limited.
When light passes through a thicker layer of atmosphere near the horizon, it encounters more disturbances and temperature variations. These increased fluctuations lead to frequent changes in light refraction, significantly intensifying the phenomenon of twinkling.
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