Last week, we got a chance to talk about why the sky is red toward sunset and sunrise but blue everywhere else: the atmosphere. Red light passes through the atmosphere pretty well, but blue light gets scattered more easily. When you look to the horizon at sunset/sunrise, the light passes through a tremendous amount of atmosphere, scattering the blue light all over the sky, but allowing the red light through to your eyes.
But then, I got a very good follow-up question:
Sometimes, either at night or in the morning, not just the horizon but nearly the entire sky turns red! Why does this happen, and is the old saying, ‘Red sky at night, sailors’ delight; red sky at morning, sailors take warning’ true?
First off, this is something that really happens. Take a look at a typical “red sky at night”, and you’ll immediately recognize the difference from what’s typical:
So the sky really does turn red all over, either at dawn or dusk, some of the time. But, what could cause so much of the sky to turn red, and what does it have to do with predicting fair or stormy weather?
It turns out that, while air (mostly Nitrogen and Oxygen) will scatter light a little bit, it takes about a hundred kilometers of dense atmosphere to scatter the blue light away. But you know what intensifies light scattering? Dust.
If you have lots of tiny impurities in your air, they can assist with the scattering. Even more of the blue light gets scattered away, reducing the amount of blue you have to see. But this dust will also scatter more red light, sending lots of red light to other parts of the sky. As long as there’s something else for the light to bounce off of (like more dust or clouds), you’re going to see a spectacular, red sky!
So what causes dust to just stay in the air like this, allowing the sky to turn red? Stagnant, unmoving air. This happens in highly polluted cities, but it also happens naturally over the ocean. All you need is a stable, high pressure weather pattern! High pressure areas mean low chances of rain and storms; generally, they indicate fair weather. When air to the west is stable and high pressure, that means there is good weather coming up, since weather patterns move from west to east.
However, when air to the east is stable and high pressure, that means another weather system, farther west, has pushed that stable, high pressure air off to the east, meaning that lower pressures (and greater chances of storms) are in your future. Additionally, you’ll only see the red sky if there is a high content of water vapor in the atmosphere, a further indicator of foul weather ahead:
Amazingly, this old adage has a fair bit of truth behind it, and now you know the science explaining it! Thanks to the Library of Congress (?!) for having some great information on this, as well.
And please, for your own safety, remember that the saying is no excuse for going out on the water unprepared. Not every red sky at night leads to fair weather the next day, and not every red sky in the morning brings storms with it!