Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 17/11/2009 17:50:59
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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?
Asked by Jade, Leamington Spa
Hear this Question on our Podcast (http://www.thenakedscientists.com/HTML/podcasts/show/2009.11.15/)
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We posed this question to John Hammond at the Met Office...
[img float=right]http://www.thenakedscientists.com/forum/copies/RTEmagicC_Redskyatnight.jpg.jpg[/img]John[/b] - 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.
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I think it has something to do with light diffraction and possibly air pollution increases the effect.
I use the old farmers saying, "Red sky at night shepherds delight, red sky in the morning, barn's on fire".
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A red sky at sunset occurs because there is a cloud free area well to the West of the observer, so the setting sun is able to illuminate any clouds (or dust) in the general vicinity of the observer.
In the UK, the weather mainly moves in from the West, so it tends to indicate there are no imminent storms because there is a high pressure system coming in from the Atlantic.
The absense of a red sky indicates that light from the setting sun is being blocked by heavy cloud cover to the West, which probably means there is rain coming.
The sun appears more red when rising and setting because the light we are receiving from it has to travel through a lot more of the Earth's atmosphere to reach us than when the Sun is high in the sky. This filters out the higher frequecies in the spectrum of light received from the sun.
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What proportion of countries have prevailing winds from the west, so as to fit this explanation? i.e. are there any countries where the reverse is known to be true?
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I don't think you can tell the weather by looking at the colour of the sky, unless its blue, with perhaps some white fluffy bits, or as black as your hat, those two might give some indication.
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BTW, I confess I made up this explanation! It might be plausible, but perhaps there is a much better one. I'm not sure why a red sky in the morning would mean much at all.
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expanding on the weather thing. red sky at night could mean a warmer night cloud cover in effect insulating the ground, which is crucial in the lambing season when it is genrally very cold. red sky in the morning, the same thing but loads of rain so the farmer gets wet
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This is one of the oldest and more accurate of all weather related folklore. As with everything else there are conflicting accounts and reasons behind the saying, so, when in doubt I turn to my dad for the answer, and he said something like:
If we take the first part of the rhyme Red sky at night, Shepherds delight (or sailor if you like)
There are more than one reasons for this. The first is that the eastwards passage of a cold front will allow the sun that is setting in the west to reflect off the top of the clouds. The passing of a cold front causes a rise in atmospheric pressure and (generally) gives decent or better weather.
High pressure will usually give us finer weather. It also gives us inversions. Inversions trap pollutants such as dust, this dust causes light from the sun to be scattered which leaves the longest wavelength, which is red, to illuminate the evening sky. So a red sky at night can be an indication that there is an night time temperature inversion and that high pressure is dominant for a day or so giving more settled, finer, weather.
Red sky in the morning, Shepherds warning
Warm fronts have a forward facing slope and the first signs of an approaching warm front are the high cirrus clouds. These are composed of ice crystals. It's the sunlight from the rising sun (in the east) reflecting off these cloud tops that turns the sky red by scattering the light (as above). So a red sky in the morning can mean that there is a front approaching and rain can be expected during the next 12 hours.
A weather forecasters take can be found here (with pretty pictures):
http://www.alabamawx.com/?p=17896
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Like a red morn, that ever yet betoken'd Wreck to the seaman‥Sorrow to shepherds.
[1592-3 Shakespeare Venus & Adonis l. 453]
[;D]
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I have been hearing this saying for as long as I can remember, it started with my grandparents and now I say it 32 years later lol. Although it's just a saying and theres no scientific proof it actually means anything, in my experience I've found it actually is true. Every time there is red skies at night the following day is a sunny day with hardly any clouds in the sky, and the same with red skies in the morning you can guarantee a few hours later heavy clouds appear and rain then follows not long after. It's strange how it works but I do believe it works.