Wednesday, February 02, 2011

Punxsutawney Phil and Atmospheric Lore

Punxsutawney Phil: The immortal prophet of spring's annual arrival. Though I question Phil's meteorological expertise, I can't help hoping he is right. He didn't see his shadow this morning, and according to legend, that means we'll have an early spring. Though I can't explain the shadow thing, I'll take it.


So, Phil didn't see his shadow. That must mean it was overcast in Punxsutawney, Pa., this morning. To me that says more nasty winter weather is due - now. In fact, it was cloudy and cold in Punxsutawney this morning. So, I ask, how do those weather conditions foretell of an early spring? It seems to me like if it were bright and sunny, and Phil saw his shadow, that would be an indication that spring is on its way. No one can explain to me how the weather on this day has anything to do with the weather over the next few weeks. But traditions are traditions. And some aren't so bad.

Weather folklore has been around since Biblical times, and likely longer. Though I question Phil's abilities, some of lyrical atmospheric lore has some merit to it. Here's a look at some of the common lore that people have developed to interpret the signs of the skies:

First is one of the most common sayings:

Red sky at night, sailor's delight.
Red sky at morning, sailors take warning.
Weather in the Northern Hemisphere typically moves from west to east, due to the rotation of the Earth. Red clouds appear in the sky when the sun is setting or rising, illuminating clouds from below. The clouds appear red because sunlight travels through more of our atmosphere when it is near the horizon and all colors of the visible spectrum are filtered out except for red.

The red wavelengths of visible light are the longest, purple and blue the shortest. When sunlight travels through the atmosphere, it is scattered by small particles in the atmosphere. The shorter wavelengths, purple, blue and green, are scattered first, with longer wavelengths last. At midday, when the sun shines straight through a small amount of atmosphere, only the shortest wavelengths are scattered. We see the resulting combination of scattered light as a blue hue.

At dawn and dusk, however, sunlight travels through the atmosphere at a low angle encountering more small particle s that scatter light. By the time the sun is near the horizon, most everything except for red light is being scattered and we see red sunsets. This filtering of light is known as Rayleigh scattering.

If red clouds are seen in the morning, the sun rising in the east is illuminating clouds in the west. Since the clouds are moving from west to east, stormy weather is due. If, however, red clouds appear at night, the sun setting in the west is illuminating clouds in the east, clouds that have already passed through.

Here's another variation on the red sky saying: "Evening red and morning gray speed the traveler on his way. Evening gray and morning red bring down rain upon his head."

Next:
No weather is ill, if the wind is still.
High pressure systems contain cool, dry air that is denser than the warm, moist air found in a low pressure system. For that reason, the dense air in a high pressure area sinks. Clouds form when warm air rises, becomes full of moisture and congregates to form clouds and sometimes thunderstorms. As long as the air is sinking, clouds are prohibited from forming.

Additionally, the atmosphere is always trying to equalize itself to one pressure. (It can't because the sun is constantly warming parts of the Earth, making new warm air.) In its efforts, the cool, dense air flows from the area of high pressure to a neighboring area of low pressure. This airflow is felt as wind. So, if things are still, you're likely in an area of high pressure where no wind or storm clouds are blowing and the weather is nice.

Lastly, weather prediction based on sound:
The sharper the blast, the sooner it's past.
There's no special trick to this one. This just has to do with distance. Thunder is the sound made when a lightning strike occurs. The louder the thunder, the closer it is since sounds diminish over distances. If the thunder is very loud, then the cloud making lightning is very close. Since most storms are isolated, being at the heart of the storm - where the most lightning is forming - means the worst will soon pass and clear skies will follow.

For more about Punxsutawney Phil seeing his shadow, check out this Physics Central Groundhog Day Physics in Pictures.

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