“What’s the weather going to be like this weekend?”
“How long until spring weather arrives?”
“Should I take my raincoat?”
While none of us can do anything about the weather, we all want to be able to predict it.
February 2 is a special day in such prognostication in the US: Groundhog’s Day. At 7:25 a.m. ET on that day each year in Gobbler’s Knob, Pennsylvania, a groundhog named Punxsutawney Phil is pulled out of his mansion of a den to forecast the end of winter by seeing — or not seeing — his shadow.
Punxsutawney Phil is a marmot, also called a “woodchuck” or “whistlepig.” More specifically, he's a member of the species Marmota monax. And he is male, since female marmots tend to hibernate longer, and would generally sleep through this day. (While there are a variety of gophers and ground squirrels in western Oregon, the only marmot native to our state, the Yellow-Bellied Marmot [Marmota flaviventris], lives east of the Cascades.)
The key to the prognostication is in the shadow, not the mammal. In ancient Europe, a similar forecast was made on the actions of hedgehogs and bears arising from hibernation. The idea was that a clear Candlemas Day — mid-point between Winter Solstice and Spring Equinox — meant more cold, wintery weather ahead; a cloudy Candlemas Day meant a milder entry into spring.
One would expect such predictions to be right at least some of the time by mere chance for the tradition to survive. But there may be another explanation for this well-traveled folklore.
Though not entirely reliable, weather sometimes cycles in a two-week pattern. Low-pressure systems generally bring precipitation and other "bad" weather; high-pressure systems generally bring clear skies and other "good" weather. In general, these pressure systems alternate, normally moving west to east in the northern hemisphere.
Putting all that together gives a plausible explanation in support of Groundhog Weather Forecasting. If the first week of February is sunny (high-pressure system dominating), the next low-pressure system will likely hit in mid-February, when the timing makes a cold winter storm more probable.
If the first week of February is cloudy (low-pressure system dominating), the next visiting weather system will be a high-pressure one. A cloudy Groundhog Day would mean that it would likely be later in the winter when the next low-pressure system came around. That two or three weeks would likely make that low-pressure system more spring-like; i.e., less snow and ice and more rain.
However, many factors can cause the "two-weeks" of the cycle to be unreliable, such as the relative size and the positions of the high- and low-pressure systems and the speed they're traveling, as well as various fronts and other interactions between the systems.
In fact, according to Stormfax’s analysis of 120 years’ worth of data, Punxsutawney Phil (and his many predecessors) has been correct only 39 percent of the time. <Sigh!>
However, based on the same principle of alternating pressure systems is the more reliable ditty: "Red sky at night, sailors delight; Red sky at morning, sailors take warning."
High atmospheric pressure means the atmosphere is denser in that region. More particles of air, and more particles of dust, increase the refraction of the incoming sunlight. When the sun sets with a very red sky, that's an indication of more dust (or pollution) or of a high-pressure system to the west. The prevailing westerly winds would drive a high-pressure system in the west our direction, bringing clear weather.
"Red sky at morning" would mean a high-pressure system is to the east at the rising sun. With the high-pressure system past, a rain-bearing low-pressure system is probably bearing down on us from the west: "sailors take warning."
The sky color at dawn/dusk is a more accurate prophesy because it predicts only into the very near future – the next couple of days—while foretelling weather a month or two away invites too much variety and chaos to be accurate.
While we cheer Punxsutawney Phil, it’s best to leave him to the business of being a marmot, rather than a weather forecaster.