The sight of a green sky during a powerful thunderstorm is a striking and often unsettling phenomenon. This unusual coloration results from specific meteorological conditions and how sunlight interacts with dense storm clouds. Understanding this phenomenon provides insight into the intensity of the weather event.
The Unique Conditions of Severe Thunderstorms
Severe thunderstorms, often associated with tornadoes, involve immense cumulonimbus clouds. These clouds exhibit significant vertical development, towering high into the atmosphere. They contain an exceptionally high concentration of liquid water droplets and ice crystals. This density and composition influence how light is scattered by the cloud.
Light Interaction with Storm Clouds
Sunlight is composed of various colors, each with a different wavelength. When light encounters water droplets, all wavelengths are scattered equally, which is why clouds often appear white. As a cloud grows thicker and denser, it blocks more sunlight, causing its appearance to shift towards gray or dark. Within deep, moisture-laden storm clouds, the large volume of water and ice particles preferentially scatters shorter blue wavelengths of light.
The “Green” Effect: Blue Light + Yellow Light
The green color observed during these storms results from a unique combination of light and cloud properties. Blue light, scattered by dense water droplets and ice crystals within the storm cloud, plays a role. This blue light then mixes with yellow or reddish light from the sun, especially when the sun is low on the horizon. As sunlight travels through more of Earth’s atmosphere at these low angles, shorter blue wavelengths are scattered away, leaving the longer yellow and red wavelengths to dominate. When this yellowish light illuminates the blue-tinted storm cloud, the combination creates the green hue, similar to how blue and yellow pigments mix to form green.
What a Green Sky Indicates
Observing a green sky indicates the presence of a severe thunderstorm. This color signifies that the storm clouds contain a substantial amount of water and ice, often suggesting heavy rainfall or large hail. While a green sky is a strong indicator of an intense storm, it does not guarantee that a tornado is forming or imminent. However, the atmospheric conditions necessary to produce such a deep, moisture-rich storm are also the conditions that can lead to tornadoes or significant hail. Therefore, a green sky serves as a visual warning, prompting individuals to seek shelter and monitor local weather alerts.