Sailors and doomsday prophets aren't that different, right? They both study the skies for signs of catastrophe. Sure, the sailor looks for pink or red clouds that indicate weather patterns, and your run-of-the-mill seer of doom watches for blood raining from the skies, but it's basically the same idea – should we carry an umbrella to work or not?
And you know what? It's not so crazy after all. (Clarification: Predicting an impending apocalypse may be crazy. In general, any person eagerly watching the sky for the end of the world might be a bit unhinged.) But watching the color of the sky for weather predictions? Not such a terrible idea – and we'll explain why. But first let's brush up a little on how weather works.
We hear terms like high and low pressure all the time on the 5 o'clock forecast, but it's time we figured out what they really mean. Low-pressure systems are generally indicative of bad weather, while high-pressure systems usually signal a calmer, more placid atmosphere. When air converges in low-pressure areas, it moves upward, creating clouds and precipitation. With a high-pressure system, the air tries to converge downward – suppressing clouds and bad weather but creating a "dirty" atmosphere, where particles like dust and smoke and whatever else is floating around are trapped at the surface.
Got that? Low-pressure systems equal clouds but a cleaner atmosphere; high-pressure systems equal clear skies but dirtier air [source: Augustine and Smith]. Now, here's where the "red" comes in. When the atmosphere is dirty, the light from the sun disperses differently. Clean air gives us calm blue light. However, dust and dirt molecules scatter longer wavelengths – which we see as red – toward us quite efficiently, particularly at sunrise and sunset when the sun's rays travel farther to reach us. We see reddish light at sunrise or sunset when a high-pressure system is in place, thanks to the light's longer journey and the dirt in the air.
Generally, wind and storms move from west to east. As a result, a red sky in the morning (when you're looking east) means the high-pressure system has passed and a low-pressure system is on its way. However, a red sky during the sunset (looking west) means that a nice high-pressure system is on its way [source: Met Office]. This is where the old saying "Red sky at night, sailor's delight; red sky in morning, sailor's warning" comes from.
One important note – this adage only works from about 30 to 60 degrees latitude in the Northern and Southern hemispheres [source: Augustine and Smith]. If you're 30 degrees on either side of the equator, you'd have to say, "Red sky at night, sailor's warning; red sky in morning, sailor's delight." But that doesn't quite rhyme, does it?