Sunsets showcase a stunning range of colors across the sky as the sun dips below the horizon each evening. The specific colors that comprise a sunset depend on a variety of factors, including atmospheric conditions, location, time of year, and composition of the landscape. By understanding what causes sunset colors, we can better appreciate the beautiful, varied sunsets nature paints for us.
Rayleigh Scattering
The primary reason sunsets appear colorful is due to a phenomenon called Rayleigh scattering. This effect describes how light waves interact with molecules and particles in Earth’s atmosphere. Shorter, blue wavelengths are scattered more widely by these particles, while longer, redder wavelengths pass straight through. During the day when the sun is high, the entire sky appears blue because blue light scatters everywhere through the atmosphere. But as the sun descends to the horizon, sunlight must pass through more atmosphere, causing more blue light to scatter away from your line of sight. With less blue making it to your eyes at sunset, more vibrant reds, oranges, and yellows are revealed.
Particle Size
The specific colors emerging through the evening sky depend heavily on the size of particles in the air. Different sized particles preferentially scatter different wavelengths of light. Large particles like dust and pollution tend to scatter longer red and orange wavelengths. Smaller particles such as molecules, smoke, and haze scatter the shorter blues and purples more effectively. Locations with lots of large particle pollutants will likely see more striking red sunsets. Cleaner environments with smaller particles produce more purple and pink-hued sunsets as the blues and reds mix in the scattered light.
Cloud Conditions
Clouds add another dimension to the sunset palette. Low clouds on the horizon can pick up bright reds and oranges as the sun’s light passes through them. High cirrus clouds also gleam with these warm sunset colors. Different cloud types and distributions lead to all kinds of sunset varieties. Wispy cirrus clouds may produce a spectacle of color streaks. Thicker cumulus clouds can form bright reds on their western edges. A clear sky allows the full spectrum of oranges, pinks, purples, and blues to shine through unimpeded. And a cloudy overcast sunset can glow with a diffused wash of red throughout the sky.
Time of Year
The position of the sun in the sky changes with the seasons, and this affects the path sunlight takes through the atmosphere at sunset. During summer, the sun sets farther north and its light hits the atmosphere at a lower angle. With a longer path through the atmosphere, more short blue light scatters out, yielding more fiery reddish sunsets. In winter, when the sun follows a more direct southerly path, its light passes through less atmosphere, allowing vivid oranges to dominate sunsets. Airborne particle concentrations also tend to be higher in summer, accentuating these seasonal sunset color differences.
Location
Latitude is another major factor determining sunset colors. At low tropical latitudes around the equator, the sun passes almost directly overhead and sinks quickly during sunsets without much color change. But farther from the equator, the sun follows a longer slanted path through the atmosphere, enhancing scattering and colors. Higher latitude locations like Alaska and Scandinavia are treated to remarkably prolonged, colorful sunsets in summer when the sun barely dips below the horizon. Pollution and particles also vary by location, with pristine areas boasting more vivid sunsets than hazy, polluted metropolises.
Composition of the Landscape
The reflection of sunlight off the landscape toward the observer provides another source of sunset color. Features like bodies of water, snow, and desert sand intensify and add their own colors to the sunset palette. For example, a red sunset reflected off the ocean surface appears even more intensely red. Different types of particulate matter over cities and in the countryside also impact the sunset colors seen. Even the green chlorophyll in trees and plants bounces more reddish light toward your eyes to enhance a colorful sunset.
Rare Sunset Phenomena
Under the right conditions, sunsets can produce some particularly remarkable displays beyond the typical range of oranges, reds, and purples:
- Green flashes – In very clear skies, a spot of green may briefly flash as the upper edge of the sun disappears below the horizon. This occurs when only the middle green wavelengths of the visible light spectrum bend enough to reach your eyes.
- Purple light rays – When the sun is relatively clear and particles are small in size, spiky beams of purple and violet light can shine and scatter through the atmosphere.
- Red sprites – Ethereal red electrical discharges high above some large thunderstorms are sometimes visible at sunset, triggered by the storm’s electrical activity.
Conclusion
Sunset colors vary day-to-day based on an intricate combination of atmospheric conditions, location, time of year, and landscape. The next time you observe a sunset, consider what factors are producing its palette of colors. And remember that we only enjoy colorful sunsets thanks to our planet’s unique atmosphere scattering light in beautiful ways.