There are a few potential reasons why the sky may appear orange or reddish in color at night. The most common causes are wildfires, dust storms, and air pollution. The scattering of light by particles in the atmosphere gives the sky these vivid hues. Let’s explore the science behind orange skies and what’s causing the strange color where you are observing it.
Wildfires
Wildfires produce huge plumes of smoke that rise into the sky. This smoke contains tiny particles and aerosols that can scatter sunlight and moonlight, causing vibrant orange and red colors to dominate the skyline.
The particles act as filters, allowing more red and orange wavelengths to pass through. Bluer light is more easily scattered away. So when dense smoke fills the air at sunrise or sunset, these long reddish wavelengths prevail, causing the orange glow.
Many of the worst wildfire seasons lead to weeks of orange skies over large regions. The western United States, Australia, and Mediterranean climates are most prone to massive wildfires that color whole horizons. The orange tones can linger even thousands of miles downwind of the fires.
Dust Storms
Dust storms also loft tons of fine particulate matter into the air that can filter the colors of sunlight. Airborne desert dust is a common cause of hazy orange skies in arid regions like the southwestern United States and northern Africa. Giant plumes blowing off the Sahara Desert occasionally reach as far as North America.
The drier the conditions, the more easily the winds can pick up dusty soil. Like smoke, dust clouds preferentially scatter blue and violet light. This leaves the longer orange and red wavelengths to shine through. The dust particles themselves also impart an orange tint when illuminated by the sun.
Haboobs are intense walls of dust that can arrive in thunderstorm gust fronts. These can spread long streams of orange, blowing dust over cities and landscapes in desert regions. The arrival of a thick haboob makes it seem like another planet.
Air Pollution
Poor air quality from urban smog and vehicle exhaust can also give sunrises and sunsets an orange cast. Pollutants like nitrogen dioxide, ozone, and sulfate aerosols scatter light in similar ways to smoke and dust. Tiny solid and liquid particles filter the natural spectrum of sunlight.
Cities like Los Angeles and New Delhi often have a dingy orange tone during periods of heavy pollution. In urban areas, the air itself takes on a brown or orange hue when smog levels spike. Layered pollution trapped near the ground filters each ray of sunlight, leading to otherworldly skies.
Wildfire smoke, dust storms, and air pollution all share a similar chemical composition. They contain miniscule particles and liquid droplets able to interact with light waves. These aerosols and particulates enable Mie scattering, the phenomenon that gives orange sunrises and sunsets.
The Science of Scattering
When people see orange looking skies, they are observing a natural demonstration of Mie scattering theory. This light scattering effect was named after German physicist Gustav Mie.
In the early 20th century, Mie solved Maxwell’s electromagnetic equations for how light interacts with spherical particles. He calculated that smaller particles scatter blue light more easily, while allowing longer orange and red wavelengths to pass through unaffected.
This scattering effect applies to particles around 0.05 to 10 micrometers in diameter, spanning the size of many smoke, dust, and pollution particles. Aerosols and particulates within this range selectively filter sunlight, dominating sunsets with vivid oranges and reds.
Here’s a more detailed look at how this works:
Rayleigh Scattering
The bluer sky we see during the day results from a separate light scattering effect known as Rayleigh scattering. It describes the interaction of sunlight with tiny molecules in the air like nitrogen and oxygen.
These tiny molecules scatter higher frequencies of blue light more readily than long red wavelengths. Rayleigh scattering gives the sky its familiar blue tone, as the red, orange, and yellow colors are sifted out. It occurs consistently during the day regardless of fires, storms, or pollution.
Mie Scattering
But Mie scattering becomes prominent when higher concentrations of particles and aerosols are present. Smoke, dust, and pollution provide millions of surfaces for light to bounce off and filter through.
The ideal size for Mie scattering is around 1/10th the wavelength of visible light, from 0.05 to 0.5 micrometers. Smaller nanoparticles cause Rayleigh scattering while larger particles over 10 micrometers tend to just block light.
In this sweet spot between nanoparticle and large particle is where orange and red colors shine through. The wide area coverage of events like wildfires, dust storms, and pollution waves makes for vivid sunsets across entire regions.
Tyndall Scattering
There is also an intermediate form of light scattering called Tyndall scattering that applies to larger colloidal particles around 0.5 to 10 micrometers. Many smoky, dusty, or polluted conditions feature a mix of Tyndall and Mie scattering.
Together, these two regimes explain why orange is such a prominent color when small particulates dominate the air. Blue subtractive Tyndall scattering and orange/red additive Mie scattering combine to give vivid sunrises and sunsets.
Where in the World?
Certain parts of the world are more prone to orange looking skies based on their fire, dust, and pollution patterns. Here are some hotspots:
Western North America
Wildfires in the western U.S. and Canada are increasing in severity with climate change. Smoke blows hundreds of miles across the continent at times, bringing orange skies with it. Prolonged drought makes fires worse.
Urban California also has some of the worst air pollution in the U.S., trapping smog in valleys and basins. Pollution levels spike on hot, windless days.
Sahara Desert and Sahel Region
The Sahara is the dustiest place on Earth, sending seasonal plumes of dust out over the Atlantic. Africa’s Sahel region south of the Sahara also generates massive dust storms that blow north adding haze to desert skies.
East Asia
China and other growing Asian nations continue to struggle with controlling air pollution amid rapid development. Beijing and other Chinese cities routinely suffer orange smog from coal plants, factories, and vehicle emissions.
India
Indian cities like New Delhi and Mumbai have some of the highest levels of air pollution worldwide. Vehicle exhaust, industry, construction, and crop burning all choke the air regularly. Orange skies often signify dangerous breathing conditions.
Middle East
The arid Middle East is prone to giant dust storms that originate in Iraq and Saudi Arabia. These dust events, called shamals, spread a veil of sand and orange haze over cities downwind.
Australia
Bushfires in Australia have been growing more extreme as climate change worsens droughts. Thick smoke has filled the skies over cities like Sydney and Melbourne during past severe fire seasons.
When Should I Worry?
Orange looking skies are often nothing to worry about if they last a short time. But prolonged periods of orange haze, dull sunsets, and reduced visibility can signify unhealthy air. Here are some indicators the air may have hazardous levels of particulates:
– An orange or brown cast persists for multiple days
– Nights are brighter and moon is orange
– Distant mountains or horizon cannot be seen
– Ash or grit accumulates on surfaces
– Smell of smoke or chemicals
– Reports of air quality alerts
In these cases, sensitive groups like children, seniors, and those with respiratory conditions should limit time outdoors. Everyone should avoid exercise outside when the sky takes on a post-apocalyptic orange glow for long periods.
Consult local air quality agencies or indexes to be sure the particles causing orange skies are not reaching dangerous levels. If pollution is elevated, wearing an effective respirator like an N95 mask can help filter out smoky or dusty air when going outside is necessary.
Will the Sky Clear Up?
Orange skies caused by extreme events usually persist from a few days to around a week, but can last longer if the emission source continues. Weather and winds driving the smoke, dust, or pollution play a big role in duration. Rain can help settle pollution particles from the air.
Smoke from distant wildfires can get trapped in atmospheric inversions and oscillate through an affected region for weeks. Dust storms occasionally resonate in cycles through desert basins. And urban pollution often follows weekly traffic patterns.
Consult air quality reports and weather forecasts to gauge when particle levels may decline. Often the return of blue skies day and night indicates the temporary haze has abated. But as wildfires and poor air quality episodes worsen with climate change, orange skies will likely become more frequent.
Appreciating the Beauty
While problematic air quality lies behind orange skies, they can still display an eerie beauty. Some photographers even venture out intentionally during intense wildfires or dust storms to capture images.
Particle-scattered sunsets and sunrises often glow more intensely with vivid reds, purples, and yellows. The unusual light can create an almost alien vibe, especially when street lights, the Moon, and stars also turn orange. But appreciate the sight from indoors if air quality is poor.
The orange color itself also holds meaning, reminding us of wildfire damage, climate issues, and the impacts of air pollution. Take in the strange beauty but also reflect on what natural and manmade forces collaborate to color the horizons. We don’t often slow down to admire sky colors, but orange skies demand our attention.
Conclusion
Orange looking skies derive from specific patterns in how particles interact with light. Wildfire smoke, blowing dust, and air pollution all contain the right sized aerosols and particulates to filter sunlight and moonlight. This produces vivid sunrises and sunsets dominated by long reddish wavelengths.
While momentarily stunning, prolonged orange horizons imply poor air quality that can threaten health. Check local air indexes and weather forecasts to see when unusual orange skies may dissipate. Their increasing frequency in our warming world reminds us of the global issues influencing our very atmosphere. But they also connect us to optical wonders and celestial motions far beyond our daily lives.