Saturday, March 25, 2017

Understanding Coastal Breezes and Prevailing Winds

Since we have received some questions lately about the winds and sea currents in various areas that effected Nephi’s course to the Land of Promise, we are answering these in one article. Part of the problem seems to stem from the fact that while we assume reader questions or comments are based, at least in part, on more than their reading a single post, sometimes it turns out they were not and the reader’s knowledge, which we base on their written comment or question, is less than we had thought. This is often true about winds and currents, since at first blush, they appear to most people to be simple and understandable (which is far from correct).
Whether or not a person agrees with the South American location for the Land of Promise, they might want to better understand how winds and currents work since Lehi got from Irreantum, along the shore of Bountiful, to the Land of Promise on a ship the Lord showed Nephi in great detail how to build—and that ship was “driven forth by the wind,” not diesel powered or even sailed with much experience on the part of Lehi and Ishmael’s families and households.
    As some of our long-time readers have often stated to a few of the comments sent in that such readers should go back and read the entire blog and the books before entering into lengthy comments about already answered questions, but for the moment we will endeavor to make the winds and currents more understandable. In doing so, we mean no disrespect to anyone or any question or comment sent to us. We are just trying to make this information more clear, and why it is needed in the first place.
    It should be kept in mind that we have been talking here on this blog regarding wind and sea directions, paths, currents, etc., of well-established wind and sea patterns that are based upon unchanging factors, i.e., constant land mass configuration (continents and mountains etc., don’t change day to day or year to year, but remain constant), gravity, earth rotation, direction of spin (of the earth), etc. There are numerous factors involved in a sea current pattern as well as in “constant” air movement (winds).
Top: Deep Sea; Bottom: Coastal Waters, they appear very different but not always in the way they actually are

    However, that is not to say, nor have we ever suggested that sea and land breezes, i.e., winds and wind direction based on day to day climate conditions, land mass and sea temperatures, etc., do not change from day to day or even hour to hour, since they are not entirely determined by the factors that make up the constant wind and sea patterns. As an example, monsoons (like those in the Indian Ocean) are changing patterns, though their change is both well understood, predictable, and rare—they change only twice per year. On the other hand, constant or prevailing wind and sea current directions do not change throughout the year and time.
    In the area of changing wind directions (not sea directions) it should be noted, in coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds. That is to say, the topography and temperature of the earth effects temporary changes in the wind. In this, there are big differences between sea breezes and land breezes.
As an example, sea breeze blows from a large body of water toward or onto a landmass and develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes are more localized than prevailing winds (prevailing winds are constant, i.e., they prevail over extended periods of time, which means a ship could count on prevailing wind patterns, but not on fickle sea breezes, thus, from time to time, especially near land, winds change abruptly, which, by the way, is what allows small vessels with highly movable small sails (such as dhows) the capability to move along shores opposite to or in agreement with normal or prevailing winds).
    Thus, because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts and after sunrise. By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea. Thus sea breezes and land breezes are both important factors in coastal regions’ prevailing winds. The term offshore wind may refer to any wind over open water. Wind Speed fluctuates and the normal daily fluctuations of wind speed resulting from sea or land breezes.
    In addition, the sea has a greater heat capacity than land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the land rises, the land heats the air above it by conduction (directly transmitted). The warm air is less dense and so it expands, decreasing the pressure over the land near the coast. The air above the sea has a relatively higher pressure, causing air near the coast to flow towards the lower pressure over land. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea, and if a strong offshore wind is present, that is, a wind greater than nine miles per hour and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.
    Now, a sea breeze front is a weather front created by a sea breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, the front can sometimes trigger thunderstorms—all of which effects coastal sailing, but not ships at sea. If the flow aloft is aligned with the direction of the sea breeze, locations experiencing the sea breeze frontal passage will have benign, or fair, weather for the remainder of the day.
This is what allows a coastal vessel (the ancient traders) to basically move in any direction they wanted along the coast in most conditions—though not during strong monsoon weather, which overrides normal coastal breezes, and which caused the ancient India traders to  wait for the change of monsoon winds to go and return along India’s coasts.
    Now, having said that, it needs to be understood that once the ship leaves coastal waters, beyond the land breeze/sea breeze confrontations (which varies according to the exact topography of the land at any given point, i.e., flat plain, beaches, hills, mountains, cliffs, etc.), then the prevailing winds will always prevail, which is why they are called prevailing winds.
    Having studied the Indian Ocean and Sea of Arabia, Bay of Bengal, Indonesia Sea Through, etc., for many, many years, I can tell you that most people who write about or draw maps about the winds and currents, mix up coastal sailing with deep sea sailing. As an example, one of the interesting points in the Book of Mormon episode of Nephi’s sailing is when the storm came up and the ship was “driven back upon the waters for the pace of three days” (1 Nephi 18:13). Most Mesoamericanists, who believe that that deep sea vessels “driven forth before the wind” could have sailed eastward from Arabia to Indonesia, and through into the Pacific, miss the meaning of this story.
    First of all, Nephi tells us that they had been to sea “for the space of many days” (1 Nephi 18:9), and as such would have had to be many miles offshore, deep into the Sea of Arabia or more likely as far south as the Indian Ocean, since that is where the terrible storms occur (called Cyclones or Sea Typhoons), which whirl around and drive the sea into circles (turning a sailing vessel back in the direction from which it came). Now, three days of that, moving swiftly because of the power of the wind blowing it, if it had been near the coast in any way (like heading east toward Indonesia/Malay), the ship would have been driven into the coast and smashed against the rocky shore. But Nephi tells us where he went by telling us how his ship was configured, i.e., it was driven forth before the wind (1 Nephi 18:8,9).
Storms develop in the northern Indian Ocean and drive toward land

    Also, keep in mind that the Indian Ocean weather systems develop out to sea and moves toward land. If these storms last long enough, they move into land and that is where they do their damage. Three days and into the fourth day (1 Nephi 18:14) is a very long time for a tropical storm like the one Nephi describes as “a great and terrible tempest” that became “exceedingly sore.” Now note that Nephi says on the fourth day they were about to be “swallowed up in the depths of the sea” (1 Nephi 18:15 emphasis added), meaning they were still in deep ocean water, and at no time was anyone concerned about being driven into the shore, rocks, reefs, etc., near a shore. Thus, Nephi's ship could not have been heading eastward or it would have been driven into the shore long before the fourth day arrived.
    Now, with this understanding, a recent link sent to us by a reader, called with active maps, failed to mention that these maps were “current wind configurations,” i.e., how the wind was acting on that particular day at that particular time and are used so one can plan ahead as to where the wind might be blowing and how strong at any given time when planning trips, activities or short voyages. Those maps are not indicative of prevailing winds, but merely what is happening at any given moment. The point is, when we talk about winds and currents on this blog, we are talking about prevailing winds and constant currents that drove shipping across the deep oceans from the days of fixed sails before tacking to the Age of Sail that ended with the invention of paddlewheels, piston-engine steamships and finally diesel engines.
When Nephi tells us he “was driven forth before the wind,” he is telling us that his ship had basically a fixed sail and he was dependent upon the wind to move him in the direction of the Land of Promise, which was only in one direction away from the Land of Promise and eventually to a spot around the 30º South Latitude along the west coast of Chili in South America, as Frederick G. Williams once wrote.
    Another point to keep in mind is this is not coastal sailing like the early traders did on any continent at any time—even today coastal sailing is far different than deep ocean sailing and, in fact, coastal sailing requires far more expertise than simply sailing with the winds and currents in the deep ocean.

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