The old rhyme says “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning.” This refers to how the appearance of the sky, particularly at sunrise and sunset, can be used to predict weather conditions. But sometimes the sky appears more pink than red. So is the rhyme referring to red skies or pink skies? Let’s take a closer look.
What Causes Red and Pink Skies
The red and pink colors in the sky are caused by the scattering of sunlight by particles in the atmosphere. Here are the main factors that influence the color:
- Particles: Dust, smoke, pollution, water droplets, ice crystals
- Sun angle: Lower angle during sunrise/sunset causes light to travel farther through atmosphere
- Cloud cover: More clouds means more particles for scattering
When the sun is low on the horizon, its light has to pass through more atmosphere. This allows longer wavelengths like red and pink to be scattered. As the sun rises higher in the sky, shorter wavelengths like blue are scattered instead, making the sky appear blue.
More particles in the air, from sources like dust storms, forest fire smoke, or air pollution, increase the scattering effect. Having more clouds also adds water droplets or ice crystals that scatter light. Different particle sizes and compositions will also affect the hue of the scattered light.
Red Sky at Night
A red sky at sunset is primarily caused by dust particles in the atmosphere. Dust scatters the long red wavelengths of light coming from the setting sun to produce vivid red colors.
Dust particles come from dry, arid conditions. When a dry cold front moves in, bringing dry air, it sweeps up dust into the atmosphere. The dry, stable air causes the dust to linger longer. The more dust that’s present, the redder the sunset will appear.
Thus, a very red sunset sky indicates an atmosphere with lots of dust and dry air. Since dry air is associated with fair weather, while wet air accompanies storms, a red sky at night is a sign of dry weather to come.
Pink Sky at Night
A pink sky at sunset is caused by high-altitude cirrus clouds that contain ice crystals. The ice crystals refract and scatter the long red rays of the setting sun to create soft pastel pink colors.
Cirrus clouds indicate moisture and instability in the upper levels of the atmosphere. This often means a storm system or frontal boundary is approaching from the west. The approaching storm is drawing warm, moist air up into the upper atmosphere where cirrus clouds form.
So unlike the red sky, a pink sky suggests wet weather is likely coming soon. A pink sky at night indicates unsettled weather may be on the way.
Red Sky in Morning
A red sunrise sky means the dust and dry air from the previous day are still present. However, instead of fair weather, the red morning sky is a sign that the dry, high pressure system is moving on. The approaching storm displaced the dry air.
This means the red sky in morning is a sign that wet weather is now to the west of you. Showers and possibly thunderstorms will soon arrive. So a red dawn sky is a sailor’s warning they should seek safe harbor before the storm hits.
Examples and Photos
Here are some examples of red and pink skies and what they indicate:
| Sky Color | Weather Indicated |
|---|---|
| Bright red sunset | Dry air, fair weather ahead |
| Peachy pink sunset | Incoming storm system, unsettled weather likely |
| Dark red sunrise | Storm and rain approaching from the west |
A vivid red sunset like this indicates stable dry air is present (Image credit: Pixabay)
A pink sunset sky means high cirrus clouds are incoming, signaling an approaching storm (Image credit: Flickr)
A dark red dawn sky is a warning that wet weather approaches from the west (Image credit: Pexels)
The Science Behind Red and Pink Skies
To understand why red and pink skies have different weather meanings, we need to explore some atmospheric optics principles. Here are key factors at play:
Rayleigh Scattering
Light scattering by particles much smaller than the wavelength of light is called Rayleigh scattering. It varies inversely with the 4th power of wavelength, so short blue wavelengths scatter much more strongly than long red wavelengths.
At sunrise/sunset, sunlight must travel farther through air, allowing the weakly scattered red light to be seen. The sun’s elevation angle determines the intensity of the colors.
Mie Scattering
Scattering by particles similar in size to the wavelength is called Mie scattering. It’s much less dependent on wavelength than Rayleigh scattering.
Dust particles cause Mie scattering, scattering red light almost equally, so sunsets appear more intensely red when dust is present.
Particle Size and Composition
The size distribution and composition of particles changes the scattering properties:
- Smaller particles like smoke favor blue/purple light.
- Larger particles like haze and dust favor red light.
- Water droplets and ice crystals refract and scatter light in ways that favor pink/orange hues.
Cirrus Clouds
Cirrus clouds at high altitudes (20,000 – 40,000 ft) contain ice crystals that effectively scatter light into pastel pink colors.
Their high elevation means cirrus clouds are often the first indication of an approaching frontal system or upper level disturbances.
Conclusion
In summary, a red sky at night is caused by dust particles scattering red sunset light, signaling dry stable air and fair weather ahead. A pink sky at night is caused by high cirrus clouds scattering light, indicating an incoming storm.
A red sunrise indicates dry air still in place from the previous day, but with wet weather now approaching from the west. The key difference lies in the air mass bringing the dust versus the clouds.
So while similar in appearance, red and pink skies can have very different weather meanings. Observing these tell-tale signs can help mariners, meteorologists, and amateur weather enthusiasts predict conditions.
