The sight of a pink sky after a storm is a natural display. This phenomenon, which transforms the blue expanse into shades of rose and crimson, results from an interplay of light, atmospheric composition, and conditions after a weather event. Understanding this involves how light interacts with the Earth’s atmosphere.
Understanding Light and Color
Visible light is composed of various colors, each corresponding to a different wavelength. For instance, violet light has the shortest wavelength, while red light has the longest. When white sunlight enters the Earth’s atmosphere, it encounters countless tiny particles, primarily nitrogen and oxygen molecules. This interaction causes light to scatter, meaning it is redirected in different directions.
A key process in this scattering is known as Rayleigh scattering, which occurs when light interacts with particles much smaller than its wavelength. This type of scattering is highly dependent on wavelength; shorter wavelengths, such as blue and violet light, are scattered far more efficiently than longer wavelengths like red and orange. This is why the daytime sky typically appears blue: blue light is scattered across the atmosphere in all directions. Conversely, red and orange light scatter less, passing through the atmosphere more directly.
The Post-Storm Atmosphere
After a storm passes, the atmosphere undergoes significant changes that influence how light interacts with it. Rain introduces a substantial amount of moisture into the air as water droplets and humidity. Storms also churn the air, leaving behind a higher concentration of dust and aerosols. These particles, unlike the tiny gas molecules responsible for typical blue sky scattering, are often larger and more varied in size.
The presence of these larger particles alters the scattering properties of light. While tiny air molecules primarily cause Rayleigh scattering, larger particles scatter light more uniformly across all wavelengths. They scatter all colors of light more equally, which is why dense clouds or fog often appear white or gray. The post-storm atmosphere presents a unique medium for sunlight, distinct from a clear, dry sky.
How These Elements Create Pink Skies
The phenomenon of pink skies after a storm synthesizes the principles of light scattering with specific atmospheric conditions and the angle of the sun. Pink and red skies are most commonly observed when the sun is low on the horizon, such as during sunrise or sunset. At these times, sunlight must travel through a much greater depth of the Earth’s atmosphere to reach an observer. This extended path means that a significant amount of the shorter-wavelength blue and violet light is scattered away by air molecules well before it reaches the viewer.
With the blue light largely removed, the longer wavelengths—red, orange, and yellow—are left to dominate the light that continues its path. When these reddish hues encounter the moisture and varied particles remaining in the post-storm atmosphere, they are then scattered and reflected by the water droplets and aerosols. This reflection and additional scattering by the larger particles cause these warm colors to become diffused and intensified, painting the clouds and sky in vivid shades of pink and red. The combination of low-angle sunlight and the particle-rich, moist air after a storm creates the perfect conditions for these striking pink displays.