The secret behind the blue sky in winter: A scientific explanation from Jeddah Astronomy
The head of the Jeddah Astronomical Society, Engineer Majid Abu Zahra, revealed the precise scientific reasons behind the widespread observation that the sky becomes noticeably bluer and clearer during winter compared to the rest of the year. He explained that this phenomenon is not merely an optical impression, but rather the result of complex physical interactions between sunlight and the components of Earth's atmosphere.
Rayleigh scattering phenomenon: The physical explanation
Abu Zahra explained that sunlight, which appears white, is actually composed of a spectrum of colors (the colors of the rainbow). When this light passes through the atmosphere, it undergoes a physical process known as Rayleigh scattering. This process occurs when light strikes very small air molecules, such as nitrogen and oxygen.
He added that physical laws govern this process, as scattering increases with shorter wavelengths of light. Since blue light has a short wavelength, it scatters in all directions much more than red light, which has a longer wavelength; this is why the sky appears so blue during clear daylight hours.
Historical background on the discovery of the phenomenon
This phenomenon is named after the British physicist Lord Rayleigh (John William Strutt), who in the 19th century provided a precise scientific explanation for why the sky is blue. His research contributed significantly to our understanding of optics and the behavior of light waves, which explains why we see the sky as blue during the day and red at sunset. At sunset, light travels a longer distance through the atmosphere, causing the blue light to scatter away, leaving only the red and orange hues that reach us.
The role of low humidity and the angle of the sun in winter
The head of the Jeddah Astronomical Society pointed to additional factors that enhance the blueness of the sky in winter, most notably the lower angle of the sun's path across the sky due to the tilt of the Earth's axis. This lower angle increases the distance sunlight travels within the atmosphere, thus increasing the likelihood of blue light scattering and its dominance in the sky.
He also explained that lower humidity levels in winter play a crucial role, as there is less water vapor and water droplets suspended in the air. These relatively large particles usually cause another type of scattering known as Mie scattering, which scatters all colors of the spectrum almost equally, making the sky appear white or hazy, as happens in humid summers. With the absence of this type of scattering in winter, Rayleigh scattering remains the only option, giving us a clear blue sky.
The human eye and color perception
Abu Zahra pointed out a subtle scientific detail concerning the physiology of the human eye, explaining that violet has a shorter wavelength than blue, and theoretically, we should perceive the sky as violet. However, the human retina is far more sensitive to blue light than to violet, and the sun emits more blue light than violet, making blue the dominant color in our visual perception.
The importance of clear skies for astronomical observation
Abu Zahra concluded his remarks by emphasizing that the combination of these factors (low humidity, reduced dust, and the angle of the sun) makes winter an ideal season not only for enjoying the beauty of the sky during the day, but also for observing stars and astronomical phenomena at night, especially in temperate regions located between latitudes 30 and 60 degrees, where light and air pollution is reduced, reflecting a clear picture of the subtle interactions in our atmosphere.
