City lights appear to shimmer or flicker across the horizon. This visual effect, often described as twinkling, is not inherent to the light source itself but is caused by interference from the Earth’s atmosphere. The atmospheric conditions between the city light and the viewer are the primary cause of this perceived instability.
The Atmosphere as a Moving Lens
The Earth’s atmosphere is a dynamic system filled with pockets of air that are constantly shifting due to temperature variations. This movement of air, known as atmospheric turbulence, creates an uneven density profile along the light’s path. Warm air is less dense than cool air, and these differences in density cause the air to act like a series of imperfect, moving lenses.
As light rays travel through these constantly changing air pockets, they are continuously refracted, or bent, in slight but rapid ways. Refraction occurs because light changes speed when moving between media of different densities. The path of light is not a straight line but a slightly zigzagging trajectory, analogous to the shimmering distortion seen over a hot road surface. Since the pockets of warmer and cooler air are constantly moving, driven by convection and wind, the light path is never stable. This continuous shifting of the light’s trajectory warps the light beam before it reaches the observer.
Scintillation and the Point Source Effect
The visual effect of light rapidly fluctuating in apparent brightness and position is scientifically termed scintillation. This perceived twinkling is a direct consequence of the atmospheric bending of light rays. When the distorted light path momentarily directs the beam away from the observer’s pupil, the light seems to dim or vanish; when the path bends it toward the eye, the light appears brighter.
Distant city lights, much like stars, are perceived as a highly concentrated “point source” of light. Despite a streetlamp being physically large, its immense distance reduces its apparent size to a single, tiny luminous point. Because the source is so small, a single turbulent air pocket can easily divert the entire light beam away from the viewer.
This is the main difference between distant city lights and closer objects, such as planets in the night sky. Planets do not twinkle noticeably because they are close enough to be seen as a small disc rather than a point. The light from a disc source is composed of many parallel rays, and while some rays are diverted by turbulence, many others still reach the eye, causing the light to appear steady and canceling out the flickering effect.
Viewing Conditions That Maximize the Twinkle
The specific conditions under which city lights are viewed significantly enhance the twinkling effect. When observing a distant city skyline, the light travels through a long, nearly horizontal path of the atmosphere, keeping the light within the densest and most turbulent layers of the air for the greatest amount of time.
Turbulence is particularly pronounced close to the ground, especially within an urban environment. City structures, such as buildings and pavement, absorb heat during the day and release it at night, creating localized pockets of warm, buoyant air. This localized urban heat maximizes the temperature gradients and the resulting atmospheric movement near the light source.
The combination of a long path length through the atmosphere and the high concentration of localized turbulence near the ground makes city lights particularly susceptible to scintillation. This is why distant city lights shimmer so noticeably, often more so than stars viewed high in the sky.