The sight of a green sky before a storm can be both unusual and unsettling. This striking atmospheric phenomenon is a natural event that holds a scientific explanation. Understanding the reasons behind this green color can offer insights into atmospheric processes.
The Physics of a Green Sky
The green hue observed in storm skies results from an interaction between sunlight and the composition of dense storm clouds. As the sun dips lower in the sky, during late afternoon or evening, its light travels through more of the atmosphere, causing more blue light to scatter away, leaving a reddish or yellowish tinge. This phenomenon is similar to the vibrant reds and oranges seen during sunsets.
When this reddish or yellowish sunlight penetrates a very deep and thick thunderstorm cloud, the conditions within the cloud modify the light. These large cumulonimbus clouds are packed with abundant water droplets and ice particles. These hydrometeors within the cloud are effective at scattering blue light, making the light under the thundercloud appear primarily blue.
The blue light scattered by the water and ice particles inside the cloud then mixes with the reddish-yellow light from the low-angle sun. This combination of blue light from within the cloud and the yellow-red light from the sun creates green. This process is analogous to mixing blue and yellow paints to produce green. Therefore, the timing of the storm, when the sun is low on the horizon, plays a crucial part in the sky’s green appearance.
Why Green Skies Signal Severe Weather
The atmospheric conditions necessary to create a green sky are linked to severe weather. Such a sky indicates deep, dense thunderstorm clouds, often characteristic of supercells. These powerful storms feature strong, rotating updrafts which lift large quantities of water and ice high into the atmosphere.
This high concentration of water and ice, especially large hail, is particularly effective at scattering light, contributing to the green appearance. Thunderclouds capable of producing hailstones frequently exhibit this green coloration. While a green sky does not directly mean a tornado is imminent, it strongly suggests the storm is severe and capable of producing large hail, damaging winds, and potentially tornadoes.
Not all severe storms will display a green sky, and conversely, the absence of green does not mean a storm is not dangerous. However, when a green sky is observed, it serves as a visual indicator of an energetic weather system. The phenomenon is relatively uncommon, even in areas accustomed to severe weather, making its appearance notable.
When to Take Action: Safety Measures
Observing a green sky should prompt immediate attention and a proactive approach to safety. This striking color serves as a strong natural warning sign that severe weather conditions are either present or rapidly approaching the area. It is important to treat a green sky as an alert that heavy rain, large hail, and strong winds are likely imminent, rather than a mere visual spectacle.
Upon seeing a green sky, individuals should immediately check local weather forecasts, radar, and official severe weather alerts from meteorological services. These alerts, such as severe thunderstorm warnings or tornado watches, provide detailed information about the specific threats and affected areas. Seeking sturdy shelter indoors is recommended, moving to an interior room on the lowest floor or a basement if available.
It is also advisable to stay away from windows and doors during severe weather, as these can pose a hazard from flying debris or shattered glass. If caught outdoors with no immediate shelter, finding a low spot away from trees or poles can offer some protection. Staying informed through reliable weather sources and being prepared to act on official guidance are crucial steps when the sky signals impending severe conditions.