Rain, Sleet, or Snow?

Storm clouds form over the Menai Straights, North Wales.

On my walk to work recently, I passed a woman broadcasting her thoughts on the weather to the world, and the person on the other end of her phone:  “We’ve had sleet, which is too warm for snow; and we’ve had hail, which is cold enough for snow; so we should have had snow!”.

I’d missed the sleet, but I wouldn’t forget the hail in a hurry; pea sized missiles had drawn a deafening clatter from the roofs of cars, and had driven a pavement-full of pedestrians to shelter in doorways from the prickly assault.  Snow had been the last thing on my mind, but for Mobile Telephone Lady, it was clearly a concern. 

So why the apparent contradiction of near-simultaneous sleet and hail?  The answer lies in the clouds that spawn them:

Time to Turn?

Leaf strewn streets are as inevitable as autumn itself.  The discarded detritus of another year’s growth always ends up rasping across pavements, clogging the gutter, or chasing the wakes of cars as we speed towards winter.  The annual transformation of our trees’ canopies, from green to a spectrum of burgundies and browns, is a global spectacle. It’s surprising then, that the trigger for this landscape altering phenomenon remains something of a mystery.

The most obvious trigger is day length:  For a deciduous tree, there’s no benefit to a canopy of energy-sapping leafs in the short days of the winter months.  Better to forgo photosynthesis, avoid frost damage, and discard your leaves.  But it’s not the shortening of autumn days that triggers the transition; if it were, a cascade of autumnal colour would flow from the north every year, turning the leaves of Maryland and Madrid simultaneously, alongside other trees sharing the same latitude.

Naming a Monster

Hurricane Tomas passes over Jamaica on 5th November 2010.

Hurricanes Igor and Tomas have been sent to the history books, relegated for the destruction they wrought in last year’s Atlantic hurricane season.

We’ve been naming large storms for over a century, to make their identification easier, but it’s a system so successful that names like Igor and Thomas sometimes have to be thrown away.

Until the middle of the last century hurricanes were often christened with the name of an associated event, such as the names of boats that they damaged, or the public holiday that they were spotted on.  Official naming crept in from the 1940s, but even then the practice took time to spread.

Most tropical regions name their storms alphabetically, and the Atlantic naming system, to which Igor and Tomas belonged, is no exception; here there are six lists of names, with one list used every year.  But with only six lists, names come back quickly, and that’s the problem.  Take hurricane Katrina, which struck New Orleans in 2005; six years on few of us will have forgotten the images of that stricken city.  Next year though the 2005 list will be in use again.  For those whose homes and lives were destroyed the prospect of another Hurricane Katrina is unimaginable.  So in 2006 the World Meteorological Organisation’s Hurricane Centre announced that Katrina, along with the names of four other storms, would be removed from the list.   Next year the first Hurricane Katia might appear with most of us unaware of its former incarnation.

Meanwhile, the people of Newfoundland are re-building after Hurricane Igor, and Jamaica, Haiti, the Turks and Caicos Islands, and their Caribbean neighbours will continue the cleanup after Hurricane Tomas.  Three weeks ago, those names too were struck from the list, but their legacy remains.   All will be hoping that their replacements, Ian and Tobias, go somewhere else in 2016.

Image: GOES satelite image of Hurricane Tomas - NASA storm images.

Text copyright: Christopher Lee, 2011 - First published 5^th April 2011)

The Shifting Shadows of the Solstice

What did you do on 21^st December?  Struggling to remember?  That’s not a surprise; the 21^st December often passes with little fanfare, in Britain at least.  Most of us are too pre-occupied with Christmas shopping, or planning the New Year’s Eve party, to consider its significance.  A shame, perhaps, because the events of the 21^st are far more important; in fact without them, we probably wouldn’t be celebrating New Year at all.

To understand why, we have to turn to the Earth’s annual journey around the Sun.  As we hurtle through space, our distance to our parent star changes little; just over one-and-a-half percent – a mere two million miles!  But a few million miles aren’t enough to create our seasons; to generate those familiar yearly transformations requires a twist, and that twist is literal:

The school-child’s illustration of the Solar System describes it best:

Rather than sit upright in its orbit, the Earth’s axis is tilted; inclined at an angle of 23.5 degrees.  That small twist in the solar jigsaw has profound effects back on Earth:  In winter, in the Northern Hemisphere, the North Pole points away from the Sun.  As the Earth spins on its axis (one whole each day), the polar region remains in shadow.  Here, night last for 24 hours a day, and at the pole itself, the night is six months long.  Things are little better further south, with short days, and long nights, that lengthen as one moves towards the Equator.

In so much shadow, little of the Sun’s heat reaches the Northern Hemisphere, and winter is in full swing.  Travel south of the Equator, though, and the sun brings summer, with the seasonal regularity we have come to expect.  Here, then, is the origin of our seasons; a constant see-saw of light and shadow as our planet ploughs along its orbit.  That see-saw governs the lengths of our days and nights, the growing seasons for our food, and ultimately, our weather.

And the 21^st December?  Well that date marks the Winter Solstice; the date when the North Pole is furthest from the Sun, and the Northern Hemisphere experiences its shortest day of the year.  From then onwards, the shadow begins to recede, slipping south in anticipation of the Northern Hemisphere’s summer, and shortening our nights by a few minutes every day.

So next year, those of you who enjoy a good New Year’s party, spend a few minutes contemplating the Earth’s shifting shadow; not only does it bring our winter snows, autumn storms, and summer monsoons, it’s also responsible for the growing season that gave you your glass of wine, or bottle of beer.  And for those who can’t stand the New Year’s parties, console yourself with this:  New Year is just a change of date; the real New Year was 11 days ago, on the Winter Solstice.

Text and image copyright: Christopher Lee, 2011 - First published 6^th January 2011.