Every year about this time, the trees all seem to lose their leaves at once, almost always with a rain and strong winds. I always thought there should be a name for it.

Today I asked our weatherman Davis Nolan if there was a name for this since it seems like it happens the exact same way every year. He said I’m not crazy to have noticed it. He said it’s called Fall.

Listen to this awesome Robert Krulwich segment I heard today on WPLN called Why Leaves Really Fall Off Trees. It’s only 4 minutes.

Around this time of year in the Northern Hemisphere, as the days grow shorter and colder, those changes trigger a hormone in leaf-dropping trees that sends a chemical message to every leaf that says, in essence, “Time to go! Let’s part company!”

Once the message is received, says Raven, little cells appear at the place where the leaf stem meets the branch. They are called “abscission” cells. They have the same root as the word scissors, meaning they are designed, like scissors, to make a cut.

And within a few days or weeks, every leaf on these deciduous trees develops a thin bumpy line of cells that push the leaf, bit by bit, away from the stem. You can’t see this without a microscope, but if you looked through one, you’d see those scissors cells lined right up.