21 Oct - Island Spotting and a Lesson on Weather
RRS Ernest Shackleton Diary
Position at 1200 (UTC -2): 08°17'N 27°21'W
Next destination: Montevideo
ETA: November 03 2001, 1700 UTC.
Distance to go: 3134 NM
Total Distance Sailed this Season: 3210NM
Wind: E x S, Force 4. Vessel pitching to a long swell, overcast with rain.
Barometric pressure: 1012.7 mb
Sea state: Slight
Air temperature: 26.1°C.
Sea temperature: 28.5°C.
The past week has been a good one for all onboard the Ernest Shackleton, with the weather changing from good to excellent, and the temperature rising slowly day by day as we continue in a south-westerly direction, making for the Brazilian coast.
The voyage has been akin to a tour, with us passing a multitude of islands. On Monday we transited the Arquipélago Da Madeira, at about 32°N 17°W, passing first the Ilha de Porto Santo and then the Ihla da Madeira on the starboard side, with the Ilhas Desertas off on the port side. The islands were discovered in the fifteenth century by the Portuguese and it has been under Portuguese Sovereignty since, with the exception of between 1581 to 1640 (Spanish domination) and a short period in the early part of the nineteenth century when it was occupied by British troops.
Tuesday, and it was time to pass two of the Canary Islands, first Isla de la Palma and then Isla de Hierro. Isla de la Palma has two mountain ranges that rise in the north part of the island and extending SSW, the other traverses the island from N to S. Isla de Hierro is further to the SW.
Moving away from the Islas Canarias there was then a couple of days with just a few passing ships to be seen, until Friday lunchtime when we passed some of the islands of the Republic of Cape Verde. This group of islands lies some 385 miles to the west of the African continent between the parallels of 14° and 18°N, and the meridians of 22°40'W and 26°30'W. The Arquipélago de Cabo Verde consists of ten islands and five islets, and these can be divided into two groups. The island of Sâo Nicolau was the first to be passed, this belonging to the Barlavento (Windward) group. This island is some 388 sq km. Having passed this, we then continued for some six to eight hours before passing between the island of Fogo, 476 sq km, and the islet of Brava, 67 sq km, which form part of the Sotavento (Leeward) group. The islands were uninhabited when they were discovered in 1460 by Diogo Gomes, with the first settlers arriving in 1462. The islands became the independent Republic of Cape Verde on July 05 1975, having formerly been an autonomous overseas territory of Portugal.
The following definitions are taken from the Collins Compact
Island: A piece of land completely surrounded by water.
Islet: A small island.
Saturday night had us all eating out on the Helideck with the first BBQ of the season. This went on into the early hours of the morning and even survived a number of heavy showers of rain.
During this week there have been a few wildlife sightings, with the Captain spotting a shark in the vicinity of Madeira, and a number of turtles were also seen later in the week. This morning saw a single swallow flying along with the vessel and later on in the morning a large number of dolphins were seen moving away from us.
Sunday also saw us pass the yacht Minmens, bound for Brazil, being sailed single handed by a Frenchman. The weather at the time of passing was not too good and he called us up requesting a forecast for the next few days, which we happily supplied. Later in the day we passed a further two yachts, who we suspect, are travelling to the same destination as Minmens.
As you will have seen from the above information, the weather has changed in the past twenty four hours, with thunder storms, heavy rain showers and lightening becoming common. The reason for this is that we have entered what is known as the Intertropical Convergence Zone (ITCZ). The following extract from the book Meteorology for Mariners will hopefully explain what is happening to the weather in this area:
The doldrums is the name given to the zone of light and variable winds, often associated with heavy rain or thunderstorms, which form a narrow, roughly latitudinal, belt occupying a position between the equator and about lat 12° N, which varies with longitude and with season. In the days of sailing ships the zone was dreaded on account of the risk of being becalmed. This is the zone where the trade-wind systems of the northern and southern hemispheres converge together and where, after prolonged heating over the equatorial ocean, some of the air finally ascends vertically.
The term INTERTROPICAL CONVERGENCE ZONE (ITCZ) concentrates on the latter concept, i.e. on the meeting place of the converging trade-wind belts of the northern and southern hemispheres whereas the term DOLDRUMS refers particularly to the tropical region of light winds. Apart from this difference of emphasis, they refer to the same phenomenon.
A METEOSAT image taken at 1200UTC October 21 2001, clearly showing the ITCZ (boxed area). These images are available to us using special satellite receivers from Datrtcom.
The ITCZ was formerly known as the INTERTROPICAL FRONT by analogy with the fronts of higher latitudes but the name was changed because of the rather vague and variable character of the accompanying weather characteristics.
Sometimes the northern boundary of the zone will be marked by rain storms, squalls or thunderstorms, and sometimes it is the southern boundary which is so marked. On one occasion one boundary of the zone will be marked by bad weather with frontal characteristics, but a vessel two or three days later will report nothing untoward in the same region.
The position given for the ITCZ for today by the Brazilian Weather Bureau was: 06n020w 06n030w, 08n 040w and 12n 050w with 3/4 degrees wide with moderate/heavy showers and thunderstorms in the whole band.
Forthcoming events: Cross the Equator on Tuesday, with King Neptune and his entourage arriving on Wednesday afternoon.
The next update will be written on Sunday October 28.