The seasonal lag

Winter, which usually arrives on or about December 22, never seems to be appropriately on time, no matter where you live. In the southern realms of the Northern Hemisphere, the first day of winter can be quite mild, even tropical, while in the north, cold air has been infiltrating since late September and several inches of snow may already be on the ground. Depending on where you live, you are either surprised that winter has snuck up on you or incensed that someone would make a big deal about the season's entrance long after its arrival.

Those of us living in the Northern Hemisphere may well take heart in the first day of winter, for it means that the Sun has reached its most southerly extreme - it's summertime in the Southern Hemisphere - and will soon be heading back toward the north, bringing with it more direct light and longer days. In fact, by late January, and definitely by mid-February, you can tell that it doesn't get dark quite as early as it did in mid-November and December.

Even as the Sun halts its southerly advance and begins its slow return to the north, the coldest days still lie ahead for those in the Northern Hemisphere. Granted, with daylight saving time not in effect during the autumn and winter months, the days seem that much more abbreviated, but still by December 22 the Sun sets around 5 o'clock and it is dark by 6 o'clock. (The Sun sets even earlier the further north you are.) Arctic air whisks in from the north whipping up little whirlwinds of leaves or snow, while over-running warm air from the south covers the sky with an unbroken blanket of slate-gray stratus clouds. For northerners, the winter solstice is not cause for celebrating the Sun's turnaround; it just means that the weather will be getting colder before it starts getting warmer again.

Why is this the case? With each passing day, the Sun's rays shine more directly on the ground. Shouldn't it, therefore, be coldest at the winter solstice and warmer thereafter?

It would, if Earth's surface temperature were solely dependent on the changing angle of the Sun. But it's not. Day-to-day weather is more dependent on the balance of incoming solar heat and outgoing radiation reflected from both the surface and the atmosphere. As long as more heat is being reflected than absorbed, the temperature falls. Hence, in the Northern Hemisphere around December 21 or 22, even though the Sun's elevation at noon is lowest and the daily duration of sunshine is at its minimum, the coldest days occur in January and February, as the Sun advances northward. Not until the rate of heating overtakes the rate of radiative cooling do the daily temperatures begin to rise, as they do in the spring.

Conversely, after the first day of summer, with the Sun slipping ever southward, temperatures reach their warmest weeks later, in July and early August. They don't begin to abate until the amount of heat the Earth returns to space exceeds the amount it receives, which occurs during the autumn months. This delay between the official onset of winter and summer and their associated temperatures is called the lag of the seasons.

For some latitudinal extremes in the Northern Hemisphere, the lag of the seasons can be ludicrous. Very often, cooler weather doesn't become noticeable in the extreme south until well into October, and even then, the few cool days are adjoined by a string of warm, humid ones. For example, in south Florida and Texas (latitude 26° N to 30° N or so), mild Christmases are not unusual, and you can often sit in your shirtsleeves sipping iced tea while basking in the warm sun in January. Conversely, I've seen nearly ten inches of heavy, wet snow dumped on Milwaukee (latitude 42°N) in April (almost a month after the spring equinox), and near-freezing temperatures in early June, (a couple of weeks before the summer solstice).

In such cases there doesn't seem to be so much of a seasonal lag as a seasonal identity complex, which, as I said earlier, often leads to surprise and malediction. The weather hasn't caught up with the month it's supposed to occur in, and we end up hurling invectives at the calendar.

Imagine for a moment, though, what life would be like if the Sun never performed its annual shift across the sky, but was always directly overhead at local noon at the equator? Then, as seen from everywhere else, the Sun's path would transcribe the same daily arc across the sky, rising and setting at the same point on the horizon and at pretty much the same time every day. There would be no change in the seasons and little or no variation in weather. Those who lived in the northern and southern extremes of the world would forever experience cool or cold weather, while those living closer to the equator would be subjected to invarying heat.

Fortunately, Earth's axial tilt of 23.3° saves us from such celestial ennui.* Over weeks and months, the Sun appears to move gradually north, then south, then back again. Seasons change, stabilize, destabilize, then change again. Weather fluctuates, winds shift, sunlight and shadows vary. The sky transforms itself as the clouds and stars of the seasons come and go. And whether we acknowledge it or not - or give in to it - these rhythms are embedded in our culture in many ways. We may hunt, gather, or fall in love beneath a Harvest Moon; brew up deep discussions around cozy winter fireplaces; let our spirits soar like wellstruck baseballs in the clear skies of a temperate spring; and become like children at play again in the summer.

Even if humans did not exist on this planet, Earth itself would still be compelled to reflect the changes wrought throughout the year by the simple, inherent tilt of its axis. Since we are here to experience these changes, we can't help but embody them, too, personifying not only the seasons, but our very connection to the stars.

Also this week:

• The Andromeda Galaxy is overhead in the evening sky for observers in the Northern Hemisphere. (See November 1 - 7.)

• Bright star Achernar (magnitude 0.4) in Eridanus the River is near the meridian around 7 o'clock in the Southern Hemisphere and for observers in the more southerly latitudes of the Northern Hemisphere.

* More accurately, the tilt of Earth's axis amounts to 23°.26'. I've rounded up to 23.3° because, for general purposes, 4 arcminutes is not that significant.

Autumnal Equinox

Autumnal Equinox

Vernal Equinox

The seasons are caused by Earth's 23.3° axial tilt.

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