The color of the sky before a tornado can provide important clues about impending severe weather. As a powerful storm system approaches, the atmosphere goes through noticeable visual changes that can serve as a warning for residents in the path of a potential tornado. Being able to recognize these signs in the sky can give people precious time to take shelter before a twister touches down.
In this article, we will examine how the sky typically appears prior to tornado formation and what accounts for its unique coloration and appearance. We will also look at some of the common folklore and misconceptions about the pre-tornado sky and what people should really be looking for when assessing if tornado conditions are possible. Being tornado-sky savvy can literally be the difference between life and death.
Normal Sky Color and Light Scattering
Under normal meteorological conditions, the sky appears blue during the day because molecules in the air scatter blue light from the sun more than they scatter red light. This is known as Rayleigh scattering. The scattered blue light comes to us from all regions of the sky instead of just coming straight from the sun, making the sky look blue.
As the sun gets lower in the sky near sunset and sunrise, even more of the blue light is scattered away, allowing more red light to pass straight through to our eyes. This makes the sky appear red, orange, and yellow close to the horizon during these times of day.
So in typical daytime conditions with the sun overhead, the sky appears its familiar light blue color due to the physics of light interacting with molecules in the atmosphere. But this normal color can change dramatically as a severe thunderstorm starts to build.
Approaching Thunderstorms and Cloud Cover
The first signs a tornado-producing storm may be on the way are usually high clouds moving in to cover the sky. These high-level clouds are composed of ice crystals instead of water droplets and appear whitish or light gray as they blanket the sky.
Altostratus and cirrostratus clouds often herald the arrival of a big thunderstorm system. As the storm gets closer, thickening cumulonimbus clouds accumulate. The dark, dense cumulonimbus clouds bring heavy rain and can spawn tornadoes, hail, and lightning. The influx of clouds blocks sunlight from reaching the ground and causes the sky to become much darker during the day.
As the clouds thicken and lower before a storm, the sky can take on a greenish or yellowish cast. This eerie shade is largely due to the cloud cover blocking normal blue light from penetrating through the atmosphere. Without as much blue light coming down, the atmosphere scatters and transmits more green and yellow wavelengths to our eyes instead.
Greenish Sky Before Tornadoes
One of the most commonly reported sky colors before tornadoes and severe thunderstorms is green. But why does the sky sometimes turn green prior to dangerous weather? There are a few processes that can contribute to pre-storm greenish skies:
– **Sunlight interacting with hail**: Light reflecting off hailstones in a storm cloud can scatter green light to an observer below as multiple reflections occur between the ice crystals and cloud droplets. This scattering effect is more likely with smaller hailstones that allow light to bounce around more before exiting the cloud.
– **Green flash from sunsets**: If a tornado-producing storm is approaching around sunset time, the atmosphere can produce rare green flashes near the horizon as the green part of the light spectrum is refracted most strongly through the thick, turbulent atmosphere. This green splash of light can presage a nasty storm and tornadoes at dusk.
– **Cloud height and thickness**: Low, dense storm clouds that block a lot of sunlight create a dark backdrop that can make normal yellowish and brownish sky colors appear more green by contrast and visual perception. The eerie illumination can be exaggerated by the observer’s eyes and brain under strange storm light conditions.
So while many report a greenish sky before tornadoes, the exact causes are still not fully agreed upon within the meteorological community. But it likely involves tricks of light and color perception as an ominous storm approaches.
Yellow and Orange Skies
In addition to green, Yellow and orange skies are also common precursors to tornado activity as a severe thunderstorm bears down:
– As thick storm clouds block blue light from scattering down, more red and yellow light can shine through, causing the sky to take on a yellowish or orange cast.
– Dust and debris picked up by storm inflow winds can also scatter more reddish-yellow light, causing yellowish sky conditions.
– Near sunset, light passing through the turbulent atmosphere may appear more yellow and orange as the green and blue light is shifted around or blocked.
So both storm cloud dynamics and the position of the sun near the horizon can contribute to yellow or orange tinted skies prior to tornado development.
| Sky Color | Causes |
|---|---|
| Greenish | – Light scattering off hailstones – Green flashes at sunset – Visual contrast from thick, low clouds |
| Yellowish | – Loss of blue light scattering – Dust and debris scattering – Sunset light passing through atmosphere |
| Orange | – Increased red/yellow light passing through clouds – Sunset light scattering |
Other Visible Clues in the Sky
Beyond the eerie coloration, there are other visible signs to look for in the sky that may mean tornadoes are imminent:
– **Lowering cloud bases**: The cloud deck will appear much lower and closer to the ground as a powerful thunderstorm approaches. Dark, ominous storm clouds seeming to bear down on the landscape are a bad sign.
– **Greenish-black clouds**: Some report an ominous dark green coloration to the storm clouds themselves in addition to the overall sky color as tornado conditions come together.
– **Cloud striations**: Horizontally oriented striations in the clouds caused by high winds at different altitudes show dramatically increasing rotation in the atmosphere.
– **Wall clouds**: These lowered, rotating clouds appearing under the storm base are a tornado threat. Wall clouds moving in with a dark green sky call for immediate shelter.
– **Funnel clouds**: Once funnel clouds become visible extending down from the wall cloud, a tornado is likely imminent or already in progress. Seek best available shelter immediately.
Conclusions
In summary, the most common sky colors before a tornado are:
– Green – Caused by light scattering and visual perception effects
– Yellow/Orange – Due to loss of normal blue sky light scattering
These eerie skies are often accompanied by:
– Influx of thick, low storm clouds
– Rotating striated cloud structures
– Descending wall and funnel clouds
An experienced storm observer can get a sense when tornadic conditions are coming together by reading the sky. Looking for these visual cues before a tornado strikes could potentially save your life by giving you time to get to a safe place well in advance of the twister’s arrival. Having a plan and being prepared to take shelter quickly when an ominous tornado sky appears is critical for surviving in tornado prone regions.
So remain “sky aware” during severe weather season and learn to read the signs in the sky well before tornadoes develop. Your life may depend on it!
Summary
The color of the sky before a tornado is most typically greenish, yellowish, or orange as sunlight interacts with the turbulent, dense storm clouds and environment preceding a severe thunderstorm. These colors are caused by the scattering and filtering of different wavelengths of light through the stormy atmosphere as well as visual perception effects.
In addition to eerie colors, clear signs like lowering clouds, striations revealing rotation, wall clouds, and funnel clouds provide clues a tornado may be imminent with little time to spare before seeking the best available shelter. By remaining alert to the visually dramatic sky changes before tornado formation, those in harm’s way can potentially save lives by taking quick protective action. Heeding the warnings offered by observant “sky-watching” remains an important preparedness skill in tornado prone areas.