Jeddah Astronomy Society reveals the secret behind the blue skies and clear skies in winter

Engineer Majed Abu Zahra, head of the Jeddah Astronomical Society, 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 emphasized that this phenomenon is not merely an optical impression, but rather the result of specific physical interactions between sunlight and the components of Earth's atmosphere.

Physical explanation: Rayleigh scattering

Abu Zahra explained that sunlight undergoes a physical process known scientifically as Rayleigh scattering as it passes through the atmosphere. This process occurs when light collides with very small air molecules such as oxygen and nitrogen. Because blue light has a shorter wavelength, it is scattered much more than red light, which has a longer wavelength, resulting in the sky appearing its characteristic blue color during clear daylight hours.

Historical and scientific context

This phenomenon is named after Lord Rayleigh, the 19th-century British physicist who discovered the relationship between light scattering and wavelength. This discovery is a cornerstone of our understanding of atmospheric optics, not only on Earth but also in the study of other planetary atmospheres, where the colors of their skies vary depending on the composition of their gases and how they scatter starlight.

The role of sun angle and humidity in winter

The head of the Jeddah Astronomical Society pointed to two additional factors that enhance the blueness of the sky in winter:

• The sun's path: Winter is characterized by a lower path of the sun across the sky due to the tilt of the Earth's axis. This lower path forces sunlight to travel a longer distance through the atmosphere, increasing the chances of blue light scattering and dominating the scene, while other colors fade.
• Low humidity: Winter is a season of relative dryness in the upper atmosphere, where water droplets and water vapor are less abundant. The presence of these large molecules in summer causes a phenomenon known as Mie scattering, which disperses all colors of the spectrum equally, resulting in a white or hazy appearance. The absence of this type of scattering in winter allows the sky to appear clearer and bluer.

Environmental and touristic importance

The impact of this phenomenon is not limited to the aesthetic aspect alone, but also carries important environmental and touristic implications:

• Air Quality Index: Dark blue skies are a natural indicator of clean air and low levels of pollutants and suspended dust, reflecting high environmental quality during this season.
• Astronomical and desert tourism: In Saudi Arabia and the Arab region, winter is the golden season for desert trips (camping) and astronomical observation. Clear, blue skies during the day often mean dark, starry nights, providing ideal conditions for astrophotography enthusiasts and nature lovers, especially in areas between latitudes 30 and 60 degrees north and south.

Abu Zahra concluded his speech by emphasizing that the human eye plays a role in this perception. Although the color violet has a shorter wavelength than blue, the eye's sensitivity to the color blue makes it dominant in our vision of the dome of the sky, giving us this captivating winter scene.