“Red skies at night, sailor’s delight. Red skies at dawn, sailors be warned.” This bit of meteorological advice for predicting the weather has been true from ancient times to today. So how does it work?
To understand the wisdom behind the rhyme we need to understand the basics of two fields: how weather patterns form and the visible portion of the electromagnetic spectrum (light).
At a high-level (literally) the jet stream is moving air from west to east at 30,000-50,000+ feet above sea level. When jet streams come together they force other air down towards Earth’s surface. This creates areas of high pressure. High pressure areas also force down particulate matter, dust, “stuff” that is moving around in the air, to gather closer to Earth’s surface.
When streams eventually diverge from one another they create space for the air closer to Earth’s surface to rise back up and occupy. This divergence of jet streams creates a low pressure zone because the air you had compacted closer to Earth’s surface can spread out and fill more space. This also spreads out the particulate matter in the air. It’s in and around these low pressure areas that most storms are formed because as the air is rising up it cools, some of it condenses into water, and storm clouds are formed. Incidentally this is how a barometer works: measuring pressure changes to predict the weather.
I Saw the Light
Anyone who has seen the cover of Pink Floyd’s The Dark Side of the Moon knows that visible light is a combination of light at different wavelengths that, on their own, are different pure colors (red, green, blue, etc). When they come together they make white light. Violets and blues have the shortest wavelengths while oranges and reds have the longest.
So what about the rhyme? Weather moves from the west to the east while the sun moves the other way around, from east to west. At sunrise and sunset the the sun is just over the horizon and at such a wide angle its light has to travel through the most amount of Earth’s atmosphere to reach you. When you see a red sky at night it means that the light of the setting sun is passing through a high pressure zone. High pressure zones, with lots of particulate matter closer to Earth’s surface, block the shorter wave length light colors from traveling great distances to reach you. This leaves only the long waves of red light. Seeing red light at night means that the weather traveling east to you will be a high pressure zone, and with high pressure comes clear weather and smooth sailing.
Red skies in the morning are red for the same reason as at night – high pressure zones are blocking out bluer light. However, with the rising sun in the east the high pressure area of good weather giving you that red light is already past you or is passing you. On the heals of high pressure is low pressure, and with low pressure comes the potential for stormy weather.