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Neat Red Skies Article

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PostPosted: Sat Feb 23, 2013 2:35 pm    Post subject: Neat Red Skies Article Reply with quote

Pulled this from surfline. Of course a little different for windsurfers.

Weather and waves!

Captain Ahab asks: What is the meaning of the old adage "Red sky at morning, surfers take warning. Red sky at night, surfer's delight."?
Surfers have a love/hate relationship with the wind, usually without the love part. Wind creates waves, so if there is no wind there can be no waves. But when blowing the wrong way at the wrong place, wind destroys waves by rendering them unridable. In a way, such ruined waves are worse than no waves at all since you're able to see surf you can't surf and only wonder about what might have been if it weren't for the local wind.

Due to: A. Unequal solar heating of the Earth's surface, and B. The planet's eastward rotation, the general atmospheric circulation in mid latitudes, (30-60 degrees), is from west to east. Weather systems, (highs,lows,fronts), get swept along in that prevailing river of air so they track eastward too. Lows are usually associated with stormy weather, i.e. clouds, winds and precipitation. Highs often correspond to clear/calm skies. So in general, high pressure systems bring mild weather, while foul weather comes along with the lows.

During sunrise and sunset, sunlight passes through the maximum thickness of atmosphere. All that air scatters/filters out the shorter wavelengths of light like purple and blue, while light with the longest wavelength, red, shines through. So anything floating in the sky when the sun is close to the horizon will take on a reddish hue.

The sun rises in the east and sets in the west. In order to have "red sky at morning" there must be clouds but the eastern skies must be clear enough to let the sun shine through. That implies that the bulk of the cloud cover is in the western skies. Since air flows from west to east, whatever is in the western skies is headed your way. If those red colored clouds are being set up by a storm, then that stormy weather is going to move over you, and you should take warning! In contrast, "red sky at night", suggests that most cloud cover is concentrated in the east. Since that's downwind of you, that means the bad weather has passed and better conditions are on the way, so you should be delighted!

Of course the "red skies" axiom is just a rule of thumb and can't be applied to all cases. Smoke, dust and other pollutants can also redden the atmosphere. Strong high pressure following right behind a low can kick up stiff winds for sloppy surf. That's happening in Southern California right now. But those breezy onshores in So Cal will drop off by tomorrow as waves steadily clean up, so the 24+ hour red skies forecast will validate there. Fact is, that old proverb has a proven track record of over 2,000 years. Because it has a base in atmospheric science and a catchy rhyme, the "red skies" adage appears in Shakespeare's play, "Venus and Adonis", and even gets mentioned in the Bible, Matthew XVI: 2-3.

In the tropics, the prevailing winds are Easterly Trades, which move from east to west. Since that air flow is in the opposite direction of the mid latitudes, the atmospheric axiom must be modified to: Red skies at dawn, surfers surf on. Red skies at evening, surfers start leaving.

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