Why does the quickest course to the Caribbean change with the time of year?

The winds: the U.S. Navy's pilot chart summaries count ships' weather reports for each one-degree square of ocean and each month, by the compass point the wind came from[1][2]. The dataset gives its general period as 1850 to 1970 and says the actual years vary by ocean; it does not say which years the North Atlantic covers. This page adds them up into five-degree squares, the size of the squares on the chart. January has 869,681 reports in the squares that get an arrow, and July has 892,089; together they are the 1,761,770 reports at the top of the page. A square with fewer than 300 reports gets no arrow, and so does a square whose middle is on land, though ships near its coast reported winds there; those reports are left out of these counts and of the speeds. Each arrow points the way the wind blew on average, and it is longer where the wind blew the same way more often.

The trade winds: they are "the prevailing easterly winds that circle the Earth near the equator"[3], and here they blow "from the northeast"[4]. This page shades a square when at least 2 of every 3 reports there, calms included, had the wind blowing toward the west, from the northeast, east or southeast. That line is the page's own choice. To find how far north they reach, it adds up the reports in mid-ocean, between 20 and 45°W, in each five-degree band of latitude, starts from the band where they blow most steadily, and goes north while the share stays at 2 in 3 or more. In January 79% of the reports between 20 and 25°N blow west and 55% between 25 and 30°N, so they reach 25°N. In July it is 82% and 56% one band higher, so 30°N. With a looser line of 1 report in 2, both months reach one band farther north, 30°N and 35°N, and July still turns 5 degrees north of January. Month by month, the trade winds reach 25°N from October to May and 30°N from June to September.

The turn: a degree of latitude is 60 minutes, and a nautical mile is one minute of latitude[8], so turning 5 degrees farther north is 5 × 60 = 300 nautical miles, or about 345 miles. The course is drawn green where it runs through a shaded square, in the trade winds, and orange elsewhere; January's course turns to a dashed gray line when the wheel turns to July. The course itself is the page's own: out of Saint-Malo around Brittany, past Cape Finisterre, south to the edge of the trade winds at 23°W, then west until it lies off Grenada, where the ship stops east of the pin so it never hides it. The dividers' points are set 5 degrees of latitude apart, so on the chart they reach from one turn to the other. A ship that rides the trade winds goes faster, which is why sailing ships from Europe used them and why "commercial ships use 'the trades'" still[3]. The page does not work out how many days a crossing takes, because that would depend on a ship's speed in each wind, which it does not have.

July also falls in the Atlantic hurricane season, which runs from June 1 to November 30, with its peak on September 10[9]; the page notes it in a chip and does not draw storms. The trade winds' average speed is the mean of every report's wind speed in the shaded squares on the chart, weighted by the number of reports: 13.4 knots in January and 12.7 in July. A knot is about 1.1508 miles an hour[8], so that is 15.4 and 14.6 miles an hour.

The ship's chip says which wind it is in, square by square along its course: "the trade winds" where the square is shaded, "the westerlies" where most reports in the square had the wind from the southwest, west or northwest, and "changeable winds" anywhere else, which is mostly around the high, where the wind blows from no one side most of the time. Where the course runs along the edge of a square or clips its corner, a stretch of only a few steps keeps the wind of the stretch before it, so the chip does not flicker, and the course's line keeps that stretch's color too.

The high: the brass weight sits on the five-degree square of open sea, between 20 and 45°N, with the highest average air pressure in the reports: 30–35°N, 10–15°W in January (1,021.5 millibars, 30.16 inches of mercury) and 35–40°N, 30–35°W in July (1,025.5 millibars, 30.28 inches of mercury). A millibar is 100 pascals and an inch of mercury is 3,386.389 pascals[10]. In January the square 30–35°N, 5–10°W reads a little higher (1,021.7 millibars), but its middle is on land. A 1997 study of the high puts its eastern center at 30°N, 20°W in early January, "just off the coast of Morocco and Mali", and its center at 30°N, 40°W in early July[6]. The National Weather Service says that in summer and fall the center is "near Bermuda"[7], farther west than either the reports or the study put it in July. All three agree that the high is farther west in summer than in winter. The ring of arrows around the weight is drawn to show which way the winds turn around a high, clockwise[5]; it is not measured.