McGHEST/"Monsoon Meteorology" states that the average onset date for the summer monsoon rains is June 1 at Kerala (8° N), June 10 at Bombay (19° N), and June 15 at Delhi (28.5° N), affecting the entire Indian subcontinent by mid-July. The monsoon rains withdraw southward beginning in September.
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More detailed data is available in books and magazines geared for sailors and pilots. Since West Virginia is quite a distance from the sea, I am not very sanguine about finding a treasure trove of relevant nautical literature. Still, there are people in Grantville with nautical or aeronautical experience, and some basic books on navigation and meteorology could be in their private libraries.
For example, there is a reasonable chance that one of the many editions of Bowditch's American Practical Navigator came through the RoF. Chapter 35 of the 1995 edition provides the "generalized pattern of actual surface winds" in January-February and July-August over the ocean, worldwide. It also notes that the North Atlantic westerlies average 25 knots in winter and 14 knots in summer, but are unsteady (blowing between south and northwest 74% of the time). The Southern Hemisphere westerlies are steadier, and 17-27 knots, with the greatest strength at 50° S. http://www.irbs.com/bowditch/pdf/chapt35.pdf
Bowditch isn't very informative about the trade winds. However, a book in my personal library-which has not been proven to be in Grantville-says that the NE trade winds have an average strength of Beaufort force 3-4 (7-16 knots), but can reach force 6-7 in Jan-Mar. Their direction and speed is very steady. The trade wind zone is 2-25° N in winter and 10-30° N in summer, but they are less reliable at the northern margin. See Cornell, World Cruising Routes 34.
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If there are books on "wind power" in Grantville, they may have a world map showing wind speed distribution. Continental maps prepared in the early 1980s are duplicated in Gipe, Wind Power 406ff (2004), and I believe also in his pre-RoF Wind Power for Home and Business (1993). Of course, these maps do not show wind direction, but the up-timers would figure that the prevailing zonal winds (polar easterlies, mid-latitude westerlies, tropical easterly trades) would extend, in modified form, over the continents.
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The Ring of Fire itself will perturb the post-RoF weather-the Great Colonial Hurricane of 1635 will not strike the same places on the same day that it did in the old timeline-but prevailing winds are an aspect of climate, not weather, and shouldn't change much.
The Hidden Treasure-Trove of Wind Knowledge: Ships' Logbooks
Even if we didn't have this info in available Grantville literature, it can be reconstructed to some degree by the down-timers, from ships' logbooks. "Even from the beginning of the great voyages in the 16th century, the positions and weather observations were taken every day. From the 17th century onwards, the observations were in tabular form." (Garcia-Herrera 3). "A Royal Order issued in 1575 required masters and pilots from the Spanish ships who navigated in the Carrera de Indias (the route from the mainland to the colonies in America) to keep a record of each trans-Atlantic journey, including a detailed description of the voyage and of any geographical discoveries, winds, currents, and hurricanes. The completed logbooks had to be delivered to the Professor of Cosmology in the Casa de Contratacion." (Prieto 38).
Naturally, the older the logbook, the less likely it is to have escaped loss or destruction, and I have relatively little information on the nature of logbooks compiled prior to 1750. However, sailors are a conservative lot, and there is reason to believe that the logbooks of the 1630s and before present similar information, albeit in more archaic terms.
The earliest logbook I have been able to read was from HMS Experiment (1697), it was tabular, and recorded the date, the day of the week, the wind direction (on the 32 point compass), the course, the distance sailed that day, the latitude and longitude, bearings to landmarks, and a description that begins with a remark about the weather, including the force of the wind (e.g., "small winds with much thunder, lightning and raine"). (Wheeler 135ff). The description might also mention what sails were set. Records were made at least daily.
Thus, there are three different indications of the wind force: the verbal descriptor, the amount of sail carried, and the distance traveled (the last being a function of the wind speed and direction relative to course). (Wheeler 68). This allows some degree of quantitative analysis of the logbooks right off the bat; wind force terms may be arranged by average distance sailed (Fig. 4). Once anemometers are placed on ships, it will be possible to correlate these traditional indications with wind speed.
At least in the British Navy, logbooks were kept by the captain, the lieutenants, and the master. Diligence was assured by the fact that the officer didn't get his pay until he handed in his logbook. In the East India Company, the journals were kept by the captain, the first mate, and occasionally the second mate. Enough British logbooks have survived to the present so that it was possible to reconstruct the probability of wind from each of the cardinal directions for each month of the year, in the English Channel, during 1685-1700. (Wheeler).
I can't speak about period Portuguese logbooks, because they were pretty much all destroyed by the Lisbon earthquake and fire. Spanish logbooks recorded all astronomical observations, and the distance traveled, course, and wind direction every two hours. (Garcia 16).
In Dutch logbooks, including information about wind force was routine by around 1600. (Koek 82). Dutch eighteenth-century logbooks anticipated the Beaufort scale descriptions by use of wind terms that specified the sail set, e.g., "double reefed topsail wind." (83). But bear in mind that the terminology in the early-seventeenth century must have been different, as reefing wasn't practiced then.
The French captains did keep logbooks, but in the seventeenth century it apparently was not commonplace for them to record meteorological observations in them. One exception was Georges Fournier (1593-1652), who in his 1643 treatise urged his fellows to record wind direction and quality. (Prieto 41).
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The utility of this wind data is of course dependent on the accuracy of the computation of the place of observation, expressed as latitude and longitude. At the time of the Ring of Fire, you had perhaps half a degree accuracy for shipboard observations of latitude (with occasional several degree flubs). As for longitude, this was determined by dead reckoning, and became less and less reliable the longer you were at sea (so, on a major voyage, you could be off by tens of degrees). This will change as a result of Grantville knowledge, but it will change slowly. The raw logbook data will have to be corrected for the differences between compass (magnetic) and true directions, and for changes in reference meridian. (Wheeler 97ff).
In the nineteenth century, Maury studied American and British logbooks. By then, of course, navigation was more accurate, and addition the wind speeds were quantified at least as Beaufort force and possibly by anemometer (haven't checked this). Maury created track charts which showed what winds were encountered where and when by particular ships. From these, he then prepared "pilot charts" that showed, for a particular "grid box," the frequency of winds from different 16 compass points, for each month. From his charts, he created sailing directions. His directions and charts reduced the average New York-San Francisco passage time from 180 to 133 days. (Lewis; Maury), and New York to Rio from 41 to 21 days. (De Villiers 68).
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Sailors, of course, aren't going to know anything about overland winds. In 1582-97, the landlubber Tycho Brahe kept a meteorological daybook on the Island of Ven, with an eleven level wind scale, but this was exceptional. (De Villiers 61; Huler 82). There are undoubtedly documents that make reference to winds so strong that they destroyed property, but it's doubtful that there are many systematic records of wind direction and force over land.