What is the scientific basis of the saying Red Sky at Night, Shepherds Delight?

30 October 2005



We'd like to thank James Smith, who got in touch via email and thinks he has the answer to the 'red sky at night, shepherd's delight' question we had the other week (is there any truth in the saying 'red sky at night, shepherd's delight?)


He says that in Matthew chapter 16 vs. 2-3, Jesus says to the Pharases 'when it's evening ye say, it will be fair weather for the sky is red. And in the morning it will be foul weather for the sky is red and lowering.' So that goes to show that for 2000 years or so, variants of this saying have been in use. But what's the theory behind it? Here in the Northern Hemisphere, the prevailing wind direction is usually going from the east towards the west. If you want there to be a red sky in the morning or the evening, there have to be clouds on the horizon where the sun is. If you think of red sky at night shepherd's delight, then the sun will be setting in the west, and the clouds must also be in the west for the sky to be red. Since the prevailing wind direction is going towards the west, that would suggest that the clouds are being blown away, leaving clear skies. If clouds are assumed to be associated with bad weather, then red sky at night would predict a good clear day the next day. That leaves us with red sky in the morning, shepherd's warning. Well the reason for that would be that the sun is coming up in the east, and so the clouds would also have to be in the east. Since they're likely to be blown towards you during the day, potentially bringing bad weather, that's why red sky in the morning might be a shepherd's warning. That seems like a reasonable explanation to me, so thanks very much to James for sending that in.


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