“Hey, have you ever noticed how sometimes you can’t see the shadow of a plane flying really high up in the sky? What’s behind this phenomenon?”

When airplanes fly high in the sky, it’s often observed that they don’t cast a noticeable shadow on the ground. This phenomenon might seem puzzling at first, but it’s actually due to a combination of factors related to the nature of shadows, the altitude of the airplane, and the characteristics of sunlight.
Understanding Shadows
A shadow is formed when an object blocks light from shining on a surface. The shape and size of the shadow depend on the object’s shape, size, and its distance from the surface. In the case of an airplane flying high, several factors contribute to the shadow being either very faint or not visible at all.
Factors Contributing to the Lack of Visible Shadows
Altitude: Airplanes cruise at altitudes ranging from about 30,000 to 40,000 feet (around 9 to 12 kilometers) or even higher. At such great heights, the shadow of the airplane on the ground becomes extremely diffuse and spread out due to the angle of the sun and the distance the light has to travel through the atmosphere. This diffusion makes the shadow very faint and difficult to discern.
Angle of the Sun: The position of the sun in the sky plays a significant role in shadow formation. When the sun is high in the sky, shadows are shorter and more defined. However, when the sun is lower on the horizon, shadows stretch out and become less distinct. For high-flying airplanes, the angle of the sun often results in a shadow that is either too stretched out or falls in a way that makes it hard to see on the ground.
Atmospheric Conditions: The Earth’s atmosphere scatters sunlight, which affects the sharpness and visibility of shadows. At high altitudes, the airplane’s shadow has to travel through more of the Earth’s atmosphere to reach the ground, which scatters the light even more. This scattering effect further diffuses the shadow, making it even less visible.
Size and Speed of the Airplane: The size of the airplane and its speed also play roles. While larger airplanes might cast slightly more defined shadows, their speed means the shadow would be moving rapidly across the ground, making it even harder to spot. Smaller airplanes would cast even less noticeable shadows due to their size.
Ground Surface: The type of ground surface can also affect the visibility of shadows. Shadows are more easily seen on uniform surfaces like fields or deserts. However, urban areas with buildings, trees, and other structures can make it difficult to distinguish a faint shadow of an airplane.
Conclusion
The combination of high altitude, the angle of the sun, atmospheric conditions, the size and speed of the airplane, and the nature of the ground surface all contribute to why airplanes flying at very high altitudes do not cast visible shadows on the ground. While technically, an airplane would cast a shadow, these factors make it practically impossible to observe from the ground or even from the airplane itself. This phenomenon is not unique to airplanes but applies to any object flying at such high altitudes.