Extraordinary weather phenomenon: green storms

Cumulonimbus, these clouds ofthunderstorms responsible for phenomena weather report violent, are known to darken the sky as they pass. Sometimes they even plunge cities into darkness for a few minutes. The arrival of thunderstorms has always been associated with dark colors. But some observers have noticed a color very different: green! A greenish color that is sometimes dark and dull, and other times very bright, when a thunderstorm arrives.

This phenomenon remains rare, but is sometimes observed under American, Australian, and sometimes European thunderstorms. In France, it is rare to observe green thunderstorms, but it is not impossible. One of the best examples of green clouds was seen during the devastating Reims thunderstorm of June 19, 2021. Many witnesses described “a curtain green” under this supercell which then flooded the city, generating giant hailstones and gusts of wind over 120 km/h.

Described as “apocalyptic”, this historic storm for the region is typical of the situations encountered during the passage of a green cloud. Because even if there are not yet precise conclusions on the subject, green storms are always associated with violent phenomena, in particular hail. In the USA, some people think that green indicates the imminent formation of a tornado under the storm, but there is no correlation.

Green storms and hail: a very likely link

What is the phenomenon at the origin of the green color under certain storms? There is no certainty to date, but only several hypotheses. One of the hypotheses is based on the fact that the green would result from the mixture between the yellow of certain parts of the cloud and the blue of the sky behind. Another hypothesis, the most probable to date, would be that the hail under the cloud would decompose the light from sunset Sun, which would give this green, turquoise or yellowish color. Almost all the green thunderstorms observed were followed by heavy hailstorms, often even giant hailstonesand violent phenomena (torrential rains, strong electrical activity) in general.

Note that the green color is present under the cloud, and not on it: it is generally observed under thearch, ahead of the storm cloud, at the level of the precipitation curtain. It is also possible that the thickness of the storm cloud, and its ability to filter the sun’s rays, probably also plays a role, as well as the presence of dust or sand particles on which light can reflect. As for the skies of fire fiery reds, the more particles that light can reflect off of (water droplets and ice crystals that make up the cloud), the more vivid the colors. The cumulonimbus that generate hail are thicker, and this feature may also contribute to the coloring.

Broadly speaking, the color green is linked to a precise alignment between the Sun and the storm, with its curtain of precipitation at the right angle to the light, but exactly why green appears is still debated. .