The color of the sky is often seen as blue on a sunny day. But the actual color can vary quite a bit depending on factors like weather, time of day, location and more. Understanding what makes the sky appear different colors helps explain the science behind this optical phenomenon.
What Makes the Sky Blue?
On a clear sunny day, the sky appears blue because molecules in the air scatter blue light from the sun more than they scatter red light. When white sunlight enters Earth’s atmosphere, the shorter wavelength blue light is scattered in all directions by gas molecules and airborne particles. The longer wavelength red, orange, and yellow light pass through this atmospheric filter more easily. So when we look up at the sky, we see the cooler, scattered blue light.
The main gas molecules responsible for scattering blue light are nitrogen and oxygen. Nitrogen makes up about 78% of the Earth’s atmosphere, while oxygen makes up 21%. These colorless, odorless gases absorb almost no light in the visible spectrum, but do scatter shorter wavelength light like blue and violet more strongly due to Rayleigh scattering. This scattering effect is inversely proportional to the 4th power of wavelength, so blue and violet light scatter far more than other colors. This is why the sky looks blue from the surface of the Earth.
Why Isn’t the Sky Violet?
If Rayleigh scattering scatters blue light more than other colors, then why isn’t the sky violet instead of blue? Violet light has an even shorter wavelength than blue light. The reason is that our eyes are more sensitive to blue light than they are to violet light. So even though more violet light is scattered, our eyes perceive the sky as blue because they can detect the blue light better.
The sun also emits more light in the blue and green wavelengths than in the violet. So the predominance of blue light scattered down through the atmosphere appears blue to our eyes. The violet light scattered gets drowned out by the other wavelengths. If our eyes were more sensitive to violet light, or if the sun emitted more violet frequencies, then the sky would appear violet instead of blue.
Why is the Sky Sometimes Red or Orange?
At sunrise or sunset when the sun is low on the horizon, the sky can take on orange, red or yellow hues. This is because sunlight has to pass through more atmosphere to reach our eyes when the sun is lower in the sky. More of the blue light gets scattered away, allowing more long wavelength red, orange and yellow light to get through. The sky transitions to these warmer colors when the sun is within about 15 degrees of the horizon.
The amount of dust, pollution or water vapor in the atmosphere can also affect how much reddening occurs at sunrise and sunset. More particles in the air leads to more scattering of short wavelength blue light, allowing more long wavelength red and orange light to prevail. Different types of cloud formations in the sky near the horizon can also amplify these red and orange hues.
Why is the Sky White or Gray Sometimes?
On an overcast or foggy day, the sky can lose its typical blue color and appear white or gray instead. This is because the water droplets or ice crystals that make up clouds and fog are larger than individual air molecules. These larger particles scatter light of all visible wavelengths more evenly, so that the full spectrum of colors reaches our eyes. With no color dominating, the sky appears white or gray when it’s overcast.
The density of the cloud or fog layer also matters. Thin, wispy cirrus clouds high in the atmosphere may have little effect on the blue sky. But thick storm clouds or fog banks can scatter enough of the spectrum to wash out the blue completely, creating a dull gray backdrop. So the whiter or grayer the overcast sky is, the denser the cloud cover blocking the sun typically is.
How Does Altitude Affect the Sky’s Color?
The color of the sky also shifts with increasing altitude and distance from the horizon. Overhead around midday, the sky generally looks the deepest blue because sunlight passes through the least amount of atmosphere before reaching our eyes. At higher elevations there are fewer air molecules and less airborne dust to scatter light, so less blue light is filtered out. This makes the sky even darker blue when viewed from mountaintops or from an aircraft.
Near the horizon around sunrise and sunset, the sky transitions to yellow and orange hues because more blue light has been scattered away. At high altitudes the sunlight has passed through less atmosphere so the colors appear less vivid. The sky around the horizon retains a darker blue color rather than transitioning fully to red because less scattering has occurred.
What Makes the Sky Look Different Colors on Other Planets?
The compositions of planetary atmospheres can create skies with very different colors than Earth’s blue. For example, the atmosphere on Mars is about 100 times less dense than Earth’s atmosphere. It contains about 95% carbon dioxide, which scatters red light. This makes the Martian sky appear reddish rather than blue. Dust storms common on Mars can further intensify the red color.
The thick atmospheres of gas giants like Jupiter and Saturn scatter blue light. But they also contain trace chemicals like phosphine gas and hydrocarbons that absorb red light, leading to more blue hues. Neptune appears blue for a different reason – the methane gas in its atmosphere absorbs red light. So the blue color comes from the remaining hues not being absorbed. The range of colorful skies on other worlds shows how atmospheric composition can influence color.
The Variable Nature of Daytime Skies
While the sky usually appears blue on clear days, changing conditions can create more white, gray, orange or reddish skies. The blueness we typically see results from preferential scattering of short wavelength blue light by gas molecules in the air. But the shade of blue shifts based on factors like cloud cover, time of day, dust and altitude. Understanding Rayleigh scattering, light absorption and the role our eyes play helps explain the science behind different sky colors.
Conclusion
The commonly perceived blue color of a clear mid-day sky is an optical effect caused by Rayleigh scattering of sunlight off gas molecules and other microscopic particles in the atmosphere. But the sky can take on a range of hues from white and gray to orange, red and even violet depending on conditions like weather, altitude, time of day and position of the sun. The composition of the atmosphere and sensitivities of the human eye also influence which colors are perceived most strongly. Examining all the variables that determine sky color gives insight into this atmospheric phenomenon that impacts the shades we see overhead.
