Green sky before a storm
A green sky before a storm usually signals a very deep thunderstorm cloud lit by low, reddish sunlight. Clouds that tall are the kind that can produce hail or tornadoes, though green alone does not guarantee severe weather.
What it may mean
A green sky is honest about one thing only. Ordinary rain clouds look gray; only a storm cloud tall enough, often towering miles high and heavy with water and ice, filters the light into that eerie green, especially toward sunset, when the sun is low and reddish. So the color tells you the cloud is very deep. It does not tell you what the cloud will do. Some green-sky storms drop hail or worse, and many end in nothing but a hard rain, yet the mind tends to jump straight from big to bad. Many signals in daily life work the same way: a meeting added to your calendar with no subject line, a message that says only "call me when you can," a sudden quiet at the dinner table. Each one says that something is sizable. It says much less about whether it is bad.
Depending on how you saw it
Green-gray in late afternoon
Thunderstorms often build in the afternoon heat and can still be raging as daylight fades and the low sun shines reddish light into the storm. What looks strange in your days simply because of the hour at which you see it?
A green sky and then only rain
A green tint does not promise hail or a tornado, and many green-sky storms deliver nothing worse than a downpour. People remember the green skies that were followed by trouble. How do you treat a warning that turns out to be a false alarm?
Out on the open plains
On the plains, where a storm can be watched building for an hour across open country, people grow used to reading cloud colors from far away. Which signs from a distance have you learned to read in the people you know best?
Why it happens in nature
Air scatters blue light, and a very deep thunderstorm cloud full of water droplets and ice lets chiefly blue light through from beneath. When the sun is low and its light reddish, as it often is when storms build late in the day, the mix can make the cloud look green. Clouds that deep are the kind that may produce hail and tornadoes, but a green sky does not guarantee severe weather.
To reflect on
Which signal lately told you that something was big, and what did you assume about whether it was bad?