“Red Sky at Morning” Explained….

For years we’ve blindly repeated the old adage “Red Sky at Night, Sailors Delight; Red Sky at Morning, Sailors Take Warning”.  When this premise was challenged – and the morning’s red sky brought rain and winds 20 gusting to 24 from the south rocking and rolling our boat – I googled the adage to see what I could find.

First thing up was Scientific American (full link here) which states that the adage originated in the Bible.

Red Sky at Night: Overnight to Vivorillos Cayes

Red Sky at Night: Overnight to Vivorillos Cayes

“(Matthew 16:2-3), the following quote is attributed to Jesus: “When it is evening, ye say, fair weather: for the heaven is red. And in the morning, foul weather today for the heaven is red and lowering.” There are also versions of this saying that refer to shepherds instead of sailors.”

I had no idea.  Then NOAA and UK MET (the United Kingdom weather service) explained the why of the weather phenonamon as only being applicable in the middle latitudes where weather systems move west to east.  Here’s the UK MET explanation (full link here):

Why does red sky appear at sunrise and sunset?

The saying is most reliable when weather systems predominantly come from the west as they do in the UK. “Red sky at night, shepherds delight” can often be proven true, since red sky at night means fair weather is generally headed towards you.

Red Sky at Night, Sailor's Delight

Red Sky at Night, Sailor’s Delight

A red sky appears when dust and small particles are trapped in the atmosphere by high pressure. This scatters blue light and leaving only red light to give the sky its notable appearance.

A red sky at sunset means high pressure is moving in from the west so therefore the next day will usually be dry and pleasant. “Red sky in the morning, shepherds warning” means a red sky appears due to the high pressure weather system having already moved east meaning the good weather has passed, most likely making way for a wet and windy low pressure system.

Click here for the NOAA version, even though I’m not particularly enamored with their expertise at forecasting the weather these days.  NOAA goes into a much more technical discussion with illustrations showing light refracted and why the red skies happen – click the NOAA link above for the technical stuff, or just read their summary, here:

Now you have the science behind the adage. Picture yourself on a ship in the middle of a mid-latitude ocean. Sailboat There the wind (and thus storm paths) is from west to east. It is morning and you are watching the sunrise. It is red. Since it is morning you are looking east, and the red sky indicates that there is high pressure there. Because you are in the mid-latitudes, the high is moving eastward–away from you. That could only mean that a low, and very likely an associated storm, is moving toward you from the west.  Sailor take warning! Now picture yourself watching the sunset from the ship, and the western sky is red.  That means that an area of high pressure is to your west, the westerlies are moving it toward you, and good weather is on the way–sailor’s delight!

Note that this only works in the belt of westerlies, from about 30 degrees to 60 degrees latitude in both the northern and southern hemispheres. The region 30 degrees on either side of the equator is characterized by easterlies (the trade winds). This adage would be opposite in that region.

We’ll wait and see what tonight’s sunset and tomorrow’s sunrise bring!   In the meantime, it’s raining after our red sky at morning which is why I had time to do this google research.  Windy conditions and the rain pretty much erase our hopes of going spearfishing this morning.  Cheers!  Jan


  1. I was also curious about this wonderful phenomenon, so i did my research. What i found is that when the sun is up in the Zenit, the light goes through only one width of atmosphere, so all the colors of light spectrum reach the Earth, and we see it from white to yellow.
    When the sun is down near the horizon, the light goes through about seven times the width of the atmosphere (due to the angle) which works as a “light filter”. The most powerful color of the spectrum is red, or at least powerful enough to reach the Earth across the atmosphere, and thats why we see red sunsets.
    Also, during night, the dust and moist floating in the air falls to Earth, so during the morning the light from the Sun is whiter. Along the day, the air gets warmer, so dust and moist elevates, creating again a “light filter”. Thats why during the afternoon the light from the Sun looks more orange.
    Sorry for my English, it’s a little bit rusty.
    Enjoy your sunsets.
    Felipe Sens

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