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Posted at 10:45 AM ET, 11/20/2009

Sea & sky while cruising the Atlantic

By Steve Tracton

* Sunny days return: Full Forecast | Under pressure by air pressure? *

atlanticcruise1.jpg
Looking out at a shower over the open Atlantic. By Steve Tracton.

I recently returned from a 16-day cruise which included seven days at sea crossing the Atlantic from Lisbon, Portugal to Fort Lauderdale, Fl. Two of the highlights of my first ocean cruise were experiencing the vastness of the Atlantic Ocean and how otherwise dull weather for a weather geek like me can be quite noteworthy.

To cruise across the Atlantic is a totally different and indescribably exhilarating experience. Beyond the physical confines and many amenities (another story) of ship-board life there is nothing but sea, wind, waves, and dolphin sightings day after day.

Nowhere else have I been able to see the true horizon, the merging of sky and sea (assuming clear skies) in every direction with no visual obstructions from trees, buildings, hills, etc. Moreover, without the ever present light pollution surrounding urban areas, never was I so profoundly aware of the gradual, but dramatic transition from civil twilight following sunset to astronomical twilight (entire sky becoming fully dark). The comparable effect in reverse occurs with sunrise, but suffice to say, I slept through all opportunities for that experience.

Certainly Columbus and other early explorers were far less enamored with transatlantic voyages. Their journeys spanned at least a month in small, poorly constructed, and inadequately provisioned ships. They lacked reliable navigation equipment and nothing but sailor's folklore for anticipating weather and sea conditions. It's truly eye-opening to ponder the hardships endured by individuals on such voyages while looking over the railing of a modern cruise liner at the seemingly endless ocean - and doing so with no concern of sailing off the end of the Earth.

atlanticcruise2.jpg
Cumulus clouds extend to the horizon. By Steve Tracton.

Most of the cruise route across the Atlantic closely paralleled that of Columbus's first voyage westward from the Cannary Islands. Weatherwise this translates to conditions dominated by the prevailing trade winds blowing from the northeast across the Atlantic basin in the tropics. In August and September the trades can be the driver of tropical storm and/or hurricane development and motion. The same was possible but unlikely, especially during an El Nino year, while crossing the Atlantic the last half of October. And so, the weather during the voyage was characterized day after day by beautiful trade wind cumulus. Occasionally the clouds developed sufficiently in the vertical to interrupt otherwise ideal weather with a brief period of moderate rain.

Ideal weather? Sure, no doubt great for just about every other passenger. But not necessarily for weather obsessives like me. Without mentioning it to anyone, of course, I selfishly wished for an unexpected tropical storm to stir some excitement as the ships Captain maneuvered to avoid the worst. Even a nice thunderstorm complex would have sufficed. That was not to be.

Nevertheless there were two weather related consolation prizes. First, monitoring the lowly trade cumuli and shower activity proved interesting and even fascinating in some respects. Generally speaking vertical development of cumuli is limited by the subsidence of clear, dry, and warmer air associated with the circulation of the southern and eastern extremities of subtropical anticyclones. More often than not the "capped" cumuli spread horizontally creating a wide expanse of stratocumulus cloud cover.

With patience, however, I occasionally observed some cumuli break through the cap and grow vertically enough to generate rain showers. None as far as I could tell developed into a thunderstorm. However, I did see what looked like a wall cloud often observed with thunderstorms. As with these more familiar occurrences, the wall cloud I observed appeared to descend from the cloud base of a rain free area. The question I cannot answer yet is whether appearance here equates observationally and mechanistically in some respect to thunderstorm wall clouds.

The second consolation prize concerned the waves and swells experienced shipboard which, surprisingly so at first, were generated by an intensifying storm almost a 1500 miles away over the north Atlantic. I had not thought much about the ship's rolling as moderate 5-10 foot waves were strike the ship broadside - seemingly par for the course and fairly easy to become accustomed to.

My interest and curiosity awakened when a single larger wave rolled the ship enough to throw everything not tied down, including some passengers, to the floor. Not to worry the Captain assured us, it was unlikely to happen again. It did not, but there were a few occasions of some serious "rock and roll" that, for example, required holding fast to overflowing dinner plates and created surfable waves in the pool.

It dawned on me that the characteristics of the waves (here) about the ship (height, period, wave length) were inconsistent, with steady and relatively light trade winds. Though constrained by limited, very slow, and extremely expensive internet service (a major handicap for someone obsessed with being "connected"), I ultimately established (see figures) that the waves responsible for the ship rolling were actually swells emanating from the storm far to the north. Swell heights at the ship's latitude averaged between 5 and 7 feet (see video). I could only imagine the discomfort and difficulties experienced by passengers and crew aboard any ship in the reddish zones of waves more 20 feet or more in height.

Anyone out there with similar or complementary experiences? Let's hear about them.

Disclaimer: The cruise was aboard Holland America Line's Oosterdam with all expenses paid for myself and wife. In return I provided a set of lectures and hosted informal discussion periods as a participant in Holland America Line's Exploration Speaker Series.

By Steve Tracton  | November 20, 2009; 10:45 AM ET
Categories:  Education, Features, International Weather, Tracton  
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Comments

Really interesting trip. I always dreamed of doing something like that.

Posted by: Tom8 | November 20, 2009 1:18 PM | Report abuse

I have heard that thunderstorms are rare over the open ocean, except over such favored areas as our East Coast offshore, where temperature contrasts between the passing Gulf Stream and cold continental air coming off the landmass in winter serve to drive instability. The Outer Banks area has been known since Colonial times as an extremely stormy area by seafarers, one reason for its reputation as the "graveyard of ships".

The wall cloud you observed could have related to an oceanic waterspout. Generally waterspouts are less severe than tornadoes, and there are even "fair-weather waterspouts" similar to desert-area dust devils. Other waterspouts don't need thunderstorms but can arise from trade-wind and other cumulus congestus.

Posted by: Bombo47jea | November 20, 2009 1:58 PM | Report abuse

Perhaps you can take a cruise on a swordfish or king crab boat? I bet the weather would be a bit more interesting in those parts of the ocean.

Nice write-up Steve, I assume the temperature and humidity levels were at comfortable for your cruise.

Posted by: Kevin-CapitalWeatherGang | November 20, 2009 10:27 PM | Report abuse

You experienced at least one "dead man's roll", so called not because it can kill you, but because if it keeps up, you will wish you were dead. I'm surprised that the cruise ship's stabilizers didn't prevent it better than that.

A dead man's roll occurs when a boat takes swells or waves nearly broadside which are capable of tipping the boat near to it's calculated righting moment. The righting moment is different for each boat, based on size and weight. A boat heeled to its righting moment will feel as if it hangs there, hangs there, hangs there forever...and then, WHAM! The other side slams back down onto the water - and the cycle starts again, heeling up to the other side.

Not that I'd know how that feels. ;-)

Posted by: --sg | November 20, 2009 11:10 PM | Report abuse

There's one other MAJOR exception to the general rule that thunderstorms are rare over the open ocean, besides the Gulf Stream/cold air interaction I mentioned.

This is the Intertropical Convergence Zone or ITCZ, the "meteorological Equator" where the Northern Hemisphere and Southern Hemisphere trade winds meet. Thunderstorms forming along the ITCZ often rise to heights of 55,000 feet and higher since the convergence of the hemispheric tradewind belts results in rising air, which generally contains huge amounts of moisture.

In late summer of each hemisphere the ITCZ may be as far as ten or more degrees of latitude away from the Equator where Coriolis Force is weak. However the large westward moving mesoscale convective systems [MCS] which often form along the ITCZ produce winds which can be cyclonically affected by the Coriolis Force. These systems pass over open ocean which is at its warmest, and, when weak upper-air currents promote outflow at the top of the systems, they develop into the tropical cyclones we track during the hurricane season. Though not all tropical cyclones are produced by the ITCZ, it is the biggest source of such systems

Posted by: Bombo47jea | November 21, 2009 1:13 PM | Report abuse

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