Many people have noticed that the sky can take on a yellowish hue before a storm. This is a commonly observed phenomenon, especially before thunderstorms. But what causes the sky to turn yellow? And does a yellow sky always mean a storm is coming? Let’s take a closer look at the science behind yellow skies and what they signify.
What Causes the Sky to Turn Yellow?
There are a few factors that can contribute to a yellow tinge in the sky prior to storms:
Increase in Atmospheric Moisture
As a storm approaches, the atmosphere becomes more humid and moist. The added moisture enhances the scattering of sunlight, especially the shorter blue light waves. With less blue light passing through, the remaining sunlight appears more yellow.
Increase in Air Pollution
Higher humidity before a storm often traps air pollution like smog near the surface. The pollutants scatter sunlight, filtering out blue light and making the sky appear yellowish.
Increase in Cloud Cover
Clouds contain many water droplets and ice crystals. These water particles preferentially scatter blue light, allowing more yellow light to pass through. Thicker clouds before a storm lead to more scattering and a yellow tint.
Drop in Air Pressure
Falling barometric pressure typically precedes stormy weather. Lower air pressure results in fewer air molecules for sunlight to scatter off. With less scattering overall, the sky shifts toward yellow.
So in summary, the combination of higher humidity, changing air pressure, increased clouds and pollutants all contribute to create yellow-hued skies before storms. The thickening and darkening clouds also make the yellow color more noticeable.
Does a Yellow Sky Always Mean Rain is Coming?
While a yellow sky often precedes storms, it does not guarantee imminent rain. Here are some key points:
– A yellow sky caused by distant wildfire smoke may not lead to rain, especially if no storm systems are approaching.
– Temporary bursts of pollution like from traffic could briefly tint the sky yellow without signalling real changes in the weather.
– High cirrus clouds very high in the atmosphere can create a yellowish sky. But these thin, high clouds may not develop into storm clouds.
– The sky over desert regions can take on yellow daytime hues due to natural dust and sand particles, not because of an incoming storm.
So while a yellow sky is a classic sign of an approaching thunderstorm, especially across humid regions, other factors can also produce short-term yellowish hues. Looking for additional visual cues like building cloud cover can help confirm if rain is actually on the way.
What Other Colors Can the Sky Turn Before a Storm?
While yellow is the most common, the sky can display an array of colors before turbulent weather:
Greenish Hues
When a yellow sky mixes with the blue sky in swirling storm clouds, it can create an eerie greenish color. This effect is caused by the way our eyes perceive the complementary colors blue and yellow next to each other.
Purplish Shades
The contrast between yellow light being filtered through the atmosphere and the dark storm clouds below can make the sky take on deep purple and orange shades. These vivid colors are more likely near sunrise or sunset when the sun is low on the horizon.
Red Sky at Night
The famous red sky at night saying refers to vibrant red colors resulting from dust particles scattering the setting sun’s light before an approaching storm system. The red light passes more easily through the humidity and cloud layers than blue or yellow.
Gray Cast
Thick, dense storm clouds can block light and give the sky a dark, ominous gray appearance before unleashing rain and thunderstorms. The thick clouds absorb light instead of scattering it, muting all color.
So while yellow may be the archetypal pre-storm color, the sky can transform into a wide palette of colors before storms depending on moisture, clouds, time of day and other factors. Reading the full sky provides helpful context clues about incoming weather.
What Types of Storms Can Lead to a Yellow Sky?
Here are the most common storm types that create yellow skies:
Thunderstorms
Classic thunderstorms often churn humid air upward, scattering blue light and causing a yellow or greenish cast to the sky. Summer thunderstorms are a prime example.
Hurricanes
The vast spinning clouds and air masses of hurricanes contain lots of water droplets that can scatter light and create yellow, green and even purple-colored skies.
Tornadoes
The turbulent rotation of air before tornadoes leads to dramatic cloud formations. These clouds filter the spectrum of sunlight, frequently making the sky turn yellow, orange, or green.
Cold Fronts
The turbulent transition zone where cold dense and warm lighter air masses meet can generate towering storm clouds. The thickening clouds produce yellow or gray skies.
Nor’easters
These intense storms along the East Coast gather moisture from the Atlantic Ocean. The humid air mass filters light, often giving the sky a bright yellow daytime hue as nor’easters approach.
So most major storm systems produce the trademark pre-storm yellow sky, especially if humidity and cloud cover are increasing. However, the color depends on the time of day and the types of clouds forming.
What Conditions Make a Yellow Sky Most Likely?
Certain weather and sky conditions make a yellow color more probable:
– High humidity – Moisture droplets readily scatter blue light.
– Approaching cold front – Signals turbulent mixing and increasing clouds.
– Cumulonimbus clouds – Anvil-shaped thunderstorm clouds contain dense moisture.
– Late afternoon light – Sun angle enhances yellow and orange hues.
– Dusty or polluted air – Adds particles that filter light colors.
– Lower air pressure – Fewer air molecules to scatter light overall.
So the classic yellow sky arises from clouds, humidity and approaching storms interacting with the sun’s spectrum of light. The most vivid yellows occur in humid climates preceding thunderstorms when sunlight highlights the turbulent clouds at mid-day or late afternoon.
Are There Any Dangers Associated with a Yellow Storm Sky?
While a yellow storm sky is usually just an atmospheric effect, there are some hazards to be aware of:
– Lightning – Thunderstorms that cause yellow skies often produce dangerous lightning. Seek shelter.
– Flash floods – Heavy downpours can lead to rapid flooding, especially in low-lying areas.
– Hail – Strong updrafts indicate large hailstones that can damage property or injure people outdoors.
– High winds – Approaching storms often generate dangerous wind gusts that can topple trees and power lines.
– Tornadoes – Yellow-green skies coupled with a rotating storm can foreshadow tornado formation. Take shelter immediately.
So while simply seeing a yellow sky is not hazardous in itself, many of the accompanying storm conditions can pose threats. Heeding warnings and taking precautions is advisable whenever skies signal deteriorating weather.
How Long Before a Storm Arrives After the Sky Turns Yellow?
The time between a yellow sky appearing and the storm arriving can vary substantially:
– Within minutes or hours – Fast-moving thunderstorms can quickly follow a yellow sky.
– 12-24 hours – Cold fronts often make the sky yellow a day before the rain arrives.
– 48 hours or more – Large hurricane or tropical storm systems can take 2 days to impact an area after the sky changes color.
– Weeks away – Cirrus clouds or distant wildfires can tint the sky yellow without an imminent storm.
So the color change alone does not reveal the exact storm timing. Checking weather radar and forecasts for additional context can give more precise arrival estimates whenever yellow storm skies appear unexpectedly.
Conclusion
In summary, yellow skies result from greater scattering of blue light and fewer air molecules to scatter sunlight overall as storm systems approach. The thickening clouds and increasing humidity filter the sun’s rays. While yellow skies often portend thunderstorms, especially in humid climates, other factors can also temporarily tint the sky yellow without rain falling. But when those golden daytime hues appear because of rising moisture and gathering storm clouds, it is a good idea to confirm storm forecasts and seek shelter as needed before the rains arrive. So while not an exact science, paying attention to the sky’s color can provide helpful clues about changing weather patterns.
References
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[2] Dunlop, S. (2013). Why Does the Sky Sometimes Turn Yellow or Orange Before a Storm? Scientific American. https://www.scientificamerican.com/article/why-does-the-sky-sometimes-tur/
[3] RDP Staff. (2022). Yellow Sky – Sign of Wet Weather Ahead. https://www.netweather.tv/learn/weatherlore/yellow-sky–sign-of-wet-weather-ahead
[4] Kelsey, R. (2018). What makes the sky change color before a storm? ABC15 Arizona. https://www.abc15.com/news/region-phoenix-metro/what-makes-the-sky-change-color-before-a-storm
[5] NOAA National Severe Storms Laboratory. (n.d.). Severe Weather Primer: Questions and Answers About Yellow Sky and Storms. https://www.nssl.noaa.gov/education/svrwx101/thunderstorms/faq/
[6] Corfidi, S. F. (2015). Colors of the Sky. NOAA/NWS Storm Prediction Center. https://www.spc.noaa.gov/publications/corfidi/colors.html
