The Science of Groundhog Day

It’s Groundhog Day today in the world of meteorological superstitions. (In the world of multi-channel cable film networks, of course, it’s Groundhog Day every day.) But that’s just one example of the curious historical methods we use to guess what the weather will be, with mixed results.

The basis of Groundhog Day is that if, on February 2, an emerging groundhog quickly returns to his hibernation burrow, six weeks of poor weather will follow. If, instead, he remains out, then six weeks of good weather is on the books.

The closest thing to a logical explanation is that the groundhog looks for his shadow: on a clear (and thus cold) day he can see it and will likely retreat back into its burrow, while on a cloudy (and likely milder day) he can’t and will remain outside. According to old sayings which have been traced back in several countries, the clear or cloudy weather on the day (known as Candlemas Day, the midpoint of winter) gives a guide to the remaining winter climate.

The Groundhog element has now become an annual ceremony in several towns, most notably in Punxsutawney, Pennsylvania. How accurate the forecasts are is hotly disputed, partially because there’s plenty of room for interpretation in deciding whether an entire six-week period counts as good or bad weather. That’s particularly the case if the forecast is interpreted as being whether or not there will be “an early spring”, which can’t be objectively measured as spring is defined by date rather than weather.

In the United Kingdom, the closest equivalent is the superstition of St Swithun’s Day. This theory states that rain, or the lack of it, on July 15 will determine the rainfall for the following 40 days. The most commonly cited origin is that Swithun’s body was moved from an outside burial ground to an indoor grave on that day, during which a torrential downpour was blamed on his displeasure at being disturbed.

Given that several other saints (Medard in France, Godelieve in Flanders, among others) are cited in similar legends on different summer dates, it appears more likely a pattern emerged and then the saints received the credit by way of explanation. While consistent weather for 40 days is almost unheard of in Western Europe, what accuracy there is in the general patterns most likely comes from the location of jet streams during the summer, which affects whether high or low pressure systems dominate.

There are also several superstitions, which vary in wording from country to country, that hold that a red sky in the evening is a sign of good weather for the following day, but a red sky in the morning means bad weather is imminent. This has a solid meteorological basis: the red comes from the sun’s light passing through the atmosphere at a low angle. The red being visible requires a combination of a clear sky overhead and moisture-filled clouds either passing eastwards (evening) or approaching westwards (morning).

Some animal-based superstitions are accurate but not technically forecasts. Seagulls sitting on land tells you the water is choppy from excessive winds, though that’s more of a reaction to events. And a field-full of cows pointing in one direction can indeed indicate good or bad weather, though that’s mainly because they want to avoid having their backs to the wind, and a strong easterly wind is usually a sign of forthcoming unsettled weather. There’s also a theory that cows lying down is a sign of rain on the way (as they want to be sure to keep a patch of grass dry), though that’s not necessarily an eerie phenomenon and more of a sign that anyone or thing which spends all its time outdoors can spot the signs of impending rain.