Why isn't the Sky a Rainbow When It's Foggy? Understanding Rainbows and Their Formation

Have you ever wondered why the sky doesn’t turn into a rainbow when the atmosphere is filled with mist? Or why the rainbow you see is incomplete? In this article, we’ll explore the science behind rainbows and why they appear the way they do.

Understanding Rainbows - A Breakdown of Their Formation

At its core, a rainbow is a beautiful optical phenomenon that occurs when light refracts, disperses, and reflects within water droplets. The primary reason for a rainbow's appearance is the interaction of light with water droplets in the atmosphere, but several factors come into play that determine its visibility.

Why Isn't the Whole Sky a Rainbow When It's Foggy?

Observers Position

A rainbow is a conspicuous circular arc that arises due to the specific angle at which light enters a water droplet, gets refracted and reflected, and then exits the droplet. This light then reaches an observer's eyes at a specific angle. Let's break this down further:

The typical angle for the primary rainbow is 42 degrees. The angle is highly specific and requires precise conditions to form. Only a portion of the sky appears as a rainbow due to the observer's position relative to the sun and water droplets. This principle applies to both misty conditions and full rain.

Droplet Size and Distribution

The size and distribution of water droplets significantly impact the visibility of a rainbow. Here's why the whole sky doesn’t turn into a rainbow when it’s misty:

In misty or foggy conditions, water droplets are often too tiny to create the necessary angular conditions for a rainbow. Even if there are many droplets, their spread-out nature prevents a consistent and visible rainbow.

Brightness and Contrast

For rainbows to be visible, there needs to be a significant contrast between the light and the background. In misty conditions, this contrast is often lost:

A foggy atmosphere can create a uniform light condition, making the colorful spectrum of a rainbow difficult to discern. Light and visibility conditions need to be favorable for a full rainbow to be perceived.

Why Does a Rainbow Appear as a Bow Shape?

The bow shape of a rainbow is a fascinating consequence of how light interacts with water droplets. Here’s a deeper look:

Refraction Angles

Light enters a water droplet and bends or refracts at specific angles based on its wavelength and color. This process separates light into its spectral components:

Rainbows display a spectrum of colors because of the different wavelengths being refracted at distinct angles.

Circular Arc

From a bird’s-eye view, rainbows would appear as complete circles. However, on the ground, you only see the upper half of this circle:

The lower half of the circle is usually obstructed by the ground, thus only the upper arc is visible to observers.

Geometry of Light

The geometry of how light interacts with spherical droplets creates the bow shape:

Each droplet contributes to the formation of the rainbow arc. When viewed collectively from a certain angle, the contributions of all droplets form the characteristic arc.

Summary

In summary, the visibility of a rainbow depends on a variety of factors including the observer's position, the size and distribution of water droplets, and the specific conditions of light refraction. The bow shape is a result of the physics of light interaction with spherical droplets, and only the upper half of the circle is visible from ground level.

Understanding these principles can help you appreciate the beauty and consistency of rainbows, no matter the weather conditions.