“The sky is blue because. . .”: 5 summer weather lessons

Ursula and Virgil have been asking a lot of weather questions lately. This makes sense: It’s been a particularly “weathersome” summer, and they’ve been outside a lot to notice.

One recent wet afternoon, I went through our bookshelves to try to answer one of Virgil’s questions. As is often the case with rainy-day research, I found many other interesting questions along with the answer I was seeking, including the mother of all weather queries, Why is the sky blue?

Here, then, is the science behind one classic question, two weather sayings, and two weather smells.

Why is the sky blue? When sunlight enters the earth’s atmosphere, it comes into contact with gases, water vapor, and dust particles. Passing through the atmosphere splits the light into the colors of the rainbow. The seven colors, each with its own wavelength and intensity, scatter in every direction like billions of colored billiard balls. Blue and violet rays bounce around more because they have the shortest wavelengths . Blue light is also very bright — 8 times brighter than red light. Those extra-bright, extra-bouncy blue “balls” of light give the sky its tint.

When the wind is in the west / The weather is at its very best. In most of the United States and Europe, the weather comes to us on a west wind and leaves to the east. That is, we live within the latitudes of prevailing westerlies. High pressure systems maintain the flow of weather from west to east, bringing fair weather. But winds that shift announce a change in the weather, often for the worse.

Red sky at night, sailors’ delight / red sky at morning, sailors take warning. At sunrise and sunset, the sun is near the horizon, so its light passes through more of the earth’s heavier atmosphere than at midday. The shorter blue wavelengths are scattered beyond our line of sight, letting the longer red wavelengths reach our eyes and creating those beautiful red skies at dawn and at dusk.

How can a red sky mean fair weather when it comes before sleep, yet mean rain when it comes after sleep? We should really be saying red sky at night, red sun in morning. Brilliant red sunsets often occur during warm, dry weather, when there are more dust particles in the air. So a reddish hue to the sky in the evening means good-weather air coming toward us. But when the air contains moisture, those dust particles absorb the water. The larger particles make it even harder for those short blue billiard balls to reach us, turning the sun a fiery red. Seeing a red sun in the east signals the presence of water vapor in the atmosphere and tells us that rain is on the way.

What’s that smell before it rains? The smell before rain is caused by ozone, a gas that is formed by high-voltage electrical discharges in thunderclouds. Wind carries the ozone out ahead of the rain, bringing us that smell of rain on the way.

What’s that smell after it rains? I’ve seen Ursula and Virgil out in the yard after it rains, just smelling . . . I’ve done it, too. Now I have a word for that smell: petrichor, a word coined in 1964 by two Australian geochemists. Plants produce oils during dry periods. When it rains, this oil is released into the atmosphere, producing the distinctive musty odor that we call “the smell after rain.” My grandmother used to say, “My, that smells fresh!”

This weather information came from Born to Explore: How To Be a Backyard Adventurer, by Richard Wiese; How Come? Planet Earth, by Kathy Wollard and Debra Solomon; and The Old Farmer’s Almanac Book of Weather Lore.

I’ll say more about Born to Explore in my next post.

Learn more
Kathy Wollard writes the "How Come?" column for Newsday. She and illustrator Debra Solomon have compiled the column into several books. The How Come? website contains answers to the newest questions and encourages people to submit their own questions.

Read a detailed description of the ozone smell of approaching rain by an MIT grad student.

“Great Kids, Great Outdoors” is an AMC Outdoors blog, written by Kristen Laine.

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