The phenomenon of a green sky
The sky turning green is a rare but fascinating weather phenomenon. A green sky most often occurs right before storms, especially severe thunderstorms. But what causes the sky to look green instead of its usual blue color? The green tint is caused by light scattering. Here’s a quick overview of how it works:
- Normally, sunlight reaches us relatively unimpeded through clear air and appears white or yellowish.
- But before storms, the air contains more water droplets from humidity and precipitation.
- These water droplets in the atmosphere scatter the longer wavelengths of sunlight (yellow, orange, red) leaving mostly shorter wavelength blue light to reach our eyes.
- However, when storm clouds become very tall, with high water content, they scatter even more of the spectrum of sunlight.
- This extra scattering also eliminates the blues, leaving mostly green light to be transmitted through the atmosphere. And our eyes see the sky as green.
So in essence, a green sky requires extra scattering of longer wavelength light by exceptionally tall storm clouds high in liquid water content. Let’s explore the science and causes behind this phenomenon in more detail.
How light scattering produces color
To understand why stormy skies sometimes turn green, we need to first understand a bit about the physics of light and how it interacts with particles in its path.
Light can be thought of as electromagnetic radiation that travels in waves. The wavelength of light determines its color. Shorter wavelengths appear blue or violet, while longer wavelengths appear red. The wavelengths of visible light range from about 400 nanometers (purple) to about 700 nm (red).
| Color | Wavelength range (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-750 |
Sunlight contains light of all visible wavelengths. But when light encounters particles in its path, like molecules in the air, the particles can redirect the light – a process called scattering. Shorter wavelength blue and violet light is scattered most readily by particles in air. Under normal conditions, this scattering of shorter wavelengths leaves direct sunlight passing through the atmosphere to appear yellowish.
But when there are more particles suspended in the air, such as from humidity or haze, even more blue light is scattered away from its original path. With less blue reaching our eyes directly from the sun, the sky appears blue as this scattered light reaches us from all angles of the sky.
So in clean, dry air the sky appears a deeper blue. In polluted or hazy air, blue light is scattered even more, sometimes leaving the sun to appear reddish and the sky milky white or gray.
Light scattering by storm clouds
Clouds contain an immense number of tiny water droplets and ice crystals that can interact with light. As storms rolls in, the clouds grow taller and their liquid water content increases.
With storm clouds, the scattering effects are much more dramatic and selective than clear air. In addition to water droplets, taller storm clouds have ice crystals at higher altitudes that also influence the light scatter.
The combination of water droplets and ice crystals results in *selective scattering* of sunlight. This means that some colors are scattered more than others, rather than scattering evenly across the spectrum.
In storm cloud scattering, the longest wavelengths of light are scattered most. The reds, oranges, and yellows are scattered away, leaving the blues and greens to pass through. At sufficient density, the blues also get removed leaving primarily green hues to be transmitted and produce a green-tinted sky.
So in summary, a green stormy sky requires:
– High, dense storm clouds
– Containing large amounts of water droplets and ice crystals
– That selectively scatter away longer visible wavelengths
– Leaving mainly green light to be seen
When does the sky turn green?
For a sky to take on a green color, a specific set of weather conditions are required:
– **Powerful thunderstorms:** The storm clouds need to reach high altitudes with dense water content to produce enough scattering. Only vigorous updrafts in major thunderstorms can generate these towering storm clouds.
– **Hail and haze:** Hail indicates violently rising air that forms tall clouds. Haziness also increases light scattering. Both make a green color more likely.
– **Moist low levels:** Abundant moisture in the lower atmosphere leads to large water droplets in the clouds that scatter light effectively. Dry low layers prevent green skies.
– **Late afternoon:** The sun has to be fairly low in the sky, but not totally setting, to cast light through the clouds at an angle good for illuminating greens.
– **No tornado:** An actual tornado destroys the vertical extent of the clouds and disrupts the scattering needed for a green sky. But green skies often precede or accompany tornadic storms.
So in summary, a green sky requires just the right combo of moist, turbulent atmosphere and sunlight angle to produce the unusual scattered light. While rare, this phenomenon can create vivid green clouds, sunsets, and shafts of light before severe weather.
When green skies happen
Green storm skies tend to occur most often in the following circumstances:
– Spring and summer late afternoons – prime convective storm season
– Along frontal boundaries primed for thunderstorm formation
– With supercell storms that have vigorous updrafts
– In the central and southern Plains and Midwest, tornado hotspots
– Anywhere severe thunderstorms are possible
So if you notice a green tint to the sky near active springtime storms, it could forewarn of possible hail, high winds, or tornadoes. While not a guarantee of severe weather, a green sky should prompt caution and preparation just in case.
Famous examples of green skies
Some particularly dramatic examples of green storm skies have been captured over the years:
Jarrell, Texas Tornado – May 27, 1997
This violent F5 tornado left a mile-wide damage path and killed 27 people. Photos show an ominous dark green sky ahead of the twister. Debris in the funnel also tinted it green temporarily.
El Reno, Oklahoma Tornado – May 31, 2013
Chaser videos captured vibrant green clouds swirling around this record 2.6 mile-wide EF5 tornado. The ominous green color appropriately foretold the storm’s severity.
Tuscaloosa-Birmingham Tornado – April 27, 2011
Many incredible photos show the sky illuminated in bright green hues before an EF4 tornado struck these Alabama cities, causing major damage and 65 deaths.
Dallas-Fort Worth Supercells – May 5, 1995
Vivid lime green scud clouds were photographed under the bases of several tornadic supercell storms traversing the Dallas-Fort Worth metro area on this day.
So some of the most violent and photogenic tornadoes have occurred underneath disturbingly green skies. The striking visuals demonstrate the immense power and danger signaled by this unique storm palette.
What causes the green color inside thunderstorms and tornadoes?
In addition to green-tinted skies, sometimes the clouds and funnels themselves glow green. What leads to these illuminated storms?
Sunlight scattering through hail
Light passing through concentrated hail can emerge green. Hail indicates strong rising air in a thunderstorm, boosting the chances of tornado formation.
Diffuse scattering in cloud layers
Scattering by ice crystals and water droplets within the storm internally scatters light to produce a “ball lightning” effect inside the turbulent cloud.
Light reflected off grass and trees
Tornadoes kicking up vegetation and debris can fill the funnel with green material that reflects its color.
So the same scattering physics that green the sky external to storms can light them up internally as well. This glowing green indicates robust updrafts and potential severity.
How rare are green storm skies?
Green thunderstorm skies occur relatively uncommonly, for a few reasons:
– Many elements have to align just right – moisture, instability,SUN ANGLE
– Tall, dense supercell storms are needed
– Most ordinary storms don’t have enough vertical development
It’s estimated green skies appear in only about 1% of thunderstorms. Specific numbers are hard to quantify since many go unreported if they occur in rural areas.
But scattered citizen reports and storm chaser photos confirm that while rare and fleeting, these vivid green storms continue to emerge each year, especially in Great Plains tornado hotspots. Their rarity makes them all the more a marvelous spectacle when captured on camera.
When green skies do happen
Green storm sightings seem to occur most frequently in these circumstances:
– 5-8 pm as the sun is lowering but still shining on storms
– With supercells that have strong rotation and updrafts
– Along fronts and boundaries triggering vigorous storm growth
– During significant tornado outbreaks
– In “Tornado Alley” from Texas to Kansas
So late day major tornado outbreaks in the Plains during springtime offer prime sky-greening ingredients. Being prepared pays off for those lucky enough to witness the surreal palette while avoiding any accompanying hazards.
Are green skies always followed by tornadoes?
While green thunderstorm skies often do precede tornado activity, they do not guarantee a twister will form. Tornado formation relies on additional ingredients like:
– Strong vertical wind shear
– A cap to incite explosive growth when broken
– Sufficient spin and updraft strength for sustained rotation
– Lack of external factors that undercut the storm
So green clouds signal conditions that favor strong to severe storms, but tornado genesis can be more nuanced. Green by itself means high water content, not necessarily rotation. Still, anytime the sky turns an emerald hue, caution is certainly warranted.
Sky green tornado probabilities
Statistics suggest the tornado probabilities following green storm skies are:
– Green clouds: ~25% chance of an eventual tornado
– Green precipitation shafts: ~45% tornado chance
– Green scud under cloud base: ~60% tornado chance
So while not an absolute determiner, green skies should prompt urgent protective action just in case a twister spins up. DO NOT try to observe the cool colors or photograph clouds if a warning is issued. Stay safe!
How to photograph green storm skies
Capturing images of rare green thunderstorms takes planning, equipment, and practice:
– Use a DSLR for manual settings adjustments
– Adjust white balance to real lighting
– Increase saturation moderately in post to compensate for color diminishing over distance
– Use a circular polarizer filter to cut haze and boost color
– Use stitching or panorama software to capture wide sky areas
– Frame clear horizons for reference and shoot periodically as green intensifies
– Photograph cloud bases, precipitation shafts, and illuminated fields
– Capture lightning if possible as it will be emerald-tinted
Experiment with angles and composition to showcase the full vibrant majesty of nature’s green palette against stormy canvases. But remain flexible and ready to take shelter, as observing angry skies always carries safety risks.
Conclusion
While uncommon, green thunderstorm skies remain one of nature’s most dazzling wonders. The emerald hues illuminate the immensity of the storms with an otherworldly glow unlike any other weather phenomenon. Photos of green supercells and tornadoes circulate widely across social media because their rarity makes them highly prized captures.
Yet as spectacular as green skies may be, they also serve as a reminder to fear the immense power of storms. Their unique scattered light signature comes from extreme heights and turbulence, signaling potential hazards ahead. Green clouds demand reverence for the atmosphere’s force and respect for its dangers. For veteran storm observers, a verdant sky is one of the most humbling vistas that nature can conjure.
