Red Sky at Night

I've always wondered about the old saying 'Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning.' Why do we get red sky? How does it determine the weather for the next day, just as the saying goes?

Answer
We posed this question to John Hammond at the Met Office...

John - Well, we get red skies, well of course here in East Anglia the skies are pretty impressive because of the lack of hills or mountains mean we see so much more of them as well. But we usually get these red skies because of how light is reflected and bounces around, basically, in the atmosphere. Now we get them mostly, of course, first thing in the morning or last thing in the evening, especially because the Sun is so low. All the other elements of the spectrum if you like, right the way down towards those blues and indigos are being bounced around the atmosphere, leaving behind those that are the oranges and reds. You need, of course, the angle of the sun to shine from underneath the cloud up towards the base. So you can see it from the ground. But when you’ve got higher cloud, then you’ve got more chance to do that. Now, when you get higher clouds, what often can be the case is that you’ve got clouds called cirrus clouds, more cirrusstratus or altocumulus clouds which are fairly high in the atmosphere. So they’re anything from, say for example, 10,000 feet right up towards 30,000 feet. Now, these clouds themselves can be the forerunner to a weather system coming in off the Atlantic Ocean and then overnight of course that cloud lowers at the front, this weather system, moves in from the Atlantic Ocean and brings us a spell of rain. Or even perhaps snow during the winter, of course, as well. And of course, you get the converse effect during the course of the morning, when you have “red sky in the morning-shepherds’ warning”, which I suppose is not very good news if you got sheep, and you want them to keep dry during the day because, of course, that means red sky on the morning but the cloud’s going to lower so you’ll be getting rain but during daylight hours rather from darkness.

Diana - So when you have higher cloud coupled with the sun at a low angle you’re much more likely to get red skies. And that high cloud can often be indicative of impending rain. So if you get red sky in the evening, rain might come during the night and leave you with a dry, sunny day afterwards, whereas red sky in the morning means rain during the day.