Sunday, March 15, 2009

Spring Begins (Obviously and Yet Unofficially)

Last weekend, it was about 70 degrees today in the Philadelphia area, so the snows from the previous week were melting, snowdrops and crocuses were blooming, and dogwoods were budding. Unmistakable signs of spring.

The weather since then has been cooler, but daffodils and tulips are sprouting, and spring has undeniably arrived.

And yet somewhere in TV-land, some weatherman solemnly pronounced that spring does not “officially” begin for another week or two. And everyone will nod and agree, because everyone “knows” that spring begins with the vernal equinox on March 21.

These kinds of pronouncements about the “official” start of spring (or summer, or fall, or winter) always annoy me. Who declared that March 21 is the “official” start of spring? What legislature, executive or judicial officer, or authority made that decision?

If we go to an actual official source for weather and climate information, the National Weather Service of the National Oceanic and Atmospheric Administration (which is part of the United States Department of Commerce), we find that “spring” is defined as:

“The season of the year comprising the transition period from winter to summer occurring when the sun is approaching the summer solstice. In the Northern Hemisphere, spring customarily includes the months of March, April and May.”

There are similar definitions of summer (June, July, and August), autumn (September, October, and November), and winter (December, January, and February).

The National Weather Service applies these definitions in its operations, because it keeps seasonal statistics based on a spring that begins on March 1 and ends on May 31. When you read about the coldest (or warmest or wettest or driest) “spring on record,” you are reading about a “spring” that begins on March 1 and ends on May 31.

So in the most “official” source of weather information in the United States, we find that spring began on March 1.

But let’s forget for the moment about what is or is not “official” and look at what would be the most sensible or logical way to define the start of spring.

Let’s start with the assumption that each of the four seasons is of equal length, so each season will be about three months, or about 91 days. Spring is a transition from winter to summer, so let’s look at when winter ends and when summer begins. There are two or three different ways we might define winter (and summer):

1. Winter might be defined as the three months (or 91 days) with the least sunlight. Summer would be the 91 days with the most sunlight.

2. Winter might be defined as the coldest three months (or 91 days). Summer would be the 91 warmest days.

3. Seasons might also be defined by agriculture. Spring is the planting season, summer is the growing season, autumn is the harvest season, and winter is the season when you try not to freeze or starve until spring arrives.

As for the first possible definition, the shortest day of the year is the winter solstice on December 21, so that would be the middle of winter (and not the beginning of winter). Similarly, the summer solstice (June 21) would be the middle of summer. And indeed, until the invention of TV weathermen, December 21 was known as “midwinter,” and June 21 was known as “midsummer.” Going forwards 45 days from December 21, and backwards 45 days from June 21, we find that spring begins on February 4 and ends on May 7. This is also consistent with the celebration of May Day (on the first of May) as the beginning of summer.

Another of the possible definitions of “winter” and “summer” relate to 91 day period with the coldest (or warmest) weather. By that definition, winter would begin in early December and end in early March, while summer would begin in early June and end in early September. For example, a chart of a 30-year average of high and low daily temperatures for Philadelphia shows that the coldest 90 days of the year (with average daily high temperatures of no more than 47 degrees) begin on December 5 and end on March 5, while the 92 warmest days (with average daily high temperatures of 79 or above) begin on June 7 and end on September 7. So, looking at the transition from winter temperatures to summer temperatures, spring would begin on March 5 and end on June 7.

As far as the agricultural calendar is concerned, most crops are planted in March and April, and the harvesting of most crops begins in August. (In fact, the word "harvest" is from the old English word hærfest, which meant "autumn.") This suggests (once again), that summer begins in May and ends in August. This is also consistent with the celebration of May Day, on the first of May, as the beginning of summer, which puts the beginning of spring in early February.

All of these different approaches to defining spring would put the beginning of spring somewhere between the beginning of February and the first week of March, which makes the choice of March 1 by the National Weather Service eminently sensible and the choice of March 21 by TV weathermen and the publishers of calendars as decidely strange.

So how did we come to believe that spring “officially” began on March 21? Blame the astronomers.

The stars and constellations that are visible at night change during the year, and so astronomers like to refer to the appearance of the night sky during each season. Being astronomers (and not meteorologists), they want an astronomic event to divide the seasons, and so they somewhat naturally chose the equinoxes and solstices. And they wanted those events to mark the boundaries of the seasons and not the middles, so they made the vernal equinox the beginning of astronomic spring instead of the middle. (I have to admit that exactly how and when astronomers decided to move the summer solstice from the middle of summer to the beginning of summer is not yet clear to me.)

The National Weather Service explicitly recognizes the difference between meteorological seasons and astronomic seasons in its definition of autumn:

“Autumn: The season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice. Meteorological autumn (different from standard/astronomical autumn) begins September 1 and ends November 30.”

I referred to declarations of the “official” start of seasons as “annoying,” and I find them so for several reasons:

As I’ve explained above, it is factually incorrect, because no official or other authority has ever declared that spring begins on March 21.

It also results in TV weathermen producing commentary that is borderline gibberish, such as describing a warm day in early June as something unusual because “summer has not yet officially arrived.” News flash: It gets warm in June. For a weatherman to insist on a meaning for “summer” that does not include the warmest days of the year is idiotic and an insult to the intelligence of the listener (me).

Finally, defining an “official” start of seasons in a way that is out of touch with reality is disturbingly unnatural and in a way inhuman. It signifies to me that I am living in a society so out of touch with the natural world, and nature itself, that it would fix artificial dates and times to the change of the seasons and ignore the gradual and beautiful changes actually going on around us.