November 13, 2014

The winds have shifted. I’ve been here two weeks now, and the prevailing winds are now from the northeast, off the Atlantic. When I first arrived, the winds were from the south, from the Caribbean. The difference is palpable.

The northeast winds are of course the trade winds. If you think about these winds from a meteorological perspective, they are the prevailing pattern of easterly surface winds found in the tropics. They strengthen or weaken based of the status of the North Atlantic Oscillation, the fluctuations in air pressure over the Azores (relatively high) and Iceland (relatively low).

The trade winds typically weaken and move north in the summer. Sunsets in Florida are redder in the summer, less so in the winter. The opposite is true here –dust from North Africa carried by the trades cause this. Drought conditions in North Africa create more dust and more dramatic sunsets. My maternal grandmother, a New Englander born and bred, used to say, when she saw a red sunset:

                                                Red sky in the morning

                                                Sailors take warning.

                                                Red sky at night

                                                Sailors delight.

Perhaps she was thinking of trade winds and North African dust when she taught me this.

The trades of course carried the Europeans across the Atlantic in the Age of Discovery. The sailing ships – Portuguese caravels, Spanish galleons, French nefs, Dutch fregats, British men-o-war – sailed south along the Iberian Peninsula and along the North African coast. This was often the most difficult part of the journey – ships could be becalmed for days or weeks before moving far enough south to get to the zone of consistent winds. Once there, they scooted across the Atlantic, carrying their cargoes of guns and germs, and, later, slaves. They returned by means of the Gulf Stream and the anti-trades. Perhaps I’ll visit those topics in another letter.

Caribbean air is warmer, more humid, almost steamy, hazier than North Atlantic air. Convective storms are common, with sometimes spectacular lightning displays. Historically, the average daytime high temperatures in San Juan drop almost five degrees during November, as the trade winds take hold, and rise by about that much in April, when the trades move north, allowing the Caribbean air to once again dominate. I wonder if and how plant life here adapts to these, to us, subtle changes in humidity, temperature and day length. I do know there is one tree that flowers, a bright red showy flower, only in February. I’ll have to do some research on that, starting by trying to learn the name of the tree.

But wait, you say: The trades blow over an ocean – doesn’t the air mass get saturated with water vapor? There are, after all, rain forests in Puerto Rico, aren’t there? Why are there no convective storms, thunder, lightning, associated with the trades?

Great question. It turns out the trades have an inversion layer, at about 10,000 feet, that prevents clouds from rising to create convective storms. You can actually observe this. Note the cloud structure the next time you fly into San Juan. Observe the clouds over the sea. You’ll be at about 35,000 feet, and the clouds will look like popcorn, all capped at the height of the inversion layer. Scattered showers for sure, but no thunderstorms.

So, it seems the weather pattern here has shifted to winter mode, and at just about the same time Croghan, Syracuse, Utica, Rochester got their first snowfall.

Neat, eh?

The images show the effects of the nice clear air. They were taken from Punta Escambron, looking east, along Condado and beyond. The first starts right at the end of Condado and you can see, on the horizon to the left, all the way to Punta Cangrejos and, to the extreme left, Isla La Cancora,  beyond Isla Verde, almost to Pinones.

The second, at a different scale, shows the Condado Vanderbilt on the right. Last year, the two buildings were a salmon color –they’ve been repainted to the off white you see here. The Marriot, in central Condado, is a bit to the left, the square building with red letters on top. The salmon colored building to the left, in the last group of high rise buildings, is Condado del Mar, where we stayed on our second trip to Puerto Rico. Ocean Park is the area with no high rises; Isla Verde begins beyond that.

See the Wikipedia entry for Trade Winds for more information.