Wayfinding / Non-Instrument Weather Forecasting
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Wayfinding / Non-Instrument Weather Forecasting Non-Instrument Weather Forecasting Dennis Kawaharada Photo below by S. Low: The appearance and movement of clouds are used to forecast weather Before the invention of modern weather instruments, seafarers relied on observations of nature to predict weather; today, while weather station and buoy reports, National Weather Service forecasts, and satellite photos are excellent sources of weather information, a mariner should also understand seasonal weather patterns and be able to read signs in the environment both before and during a trip: the wind, sea state, clouds, appearance of celestial bodies, smells, and animal behavior contain clues to current and approaching weather. The art of non-instrument weather forecasting can be used to confirm weather reports or to predict weather when instruments and reports are not available. Winds Basic to any observation of weather for sailing is knowing the direction and strength of the wind. Face the wind: when the pressure is even on both cheeks and ears, you are looking directly at the wind. You can tell direction also by watching in which direction the clouds are moving, the leaves of the trees are streaming, or the ripples on the ocean are running. Wind strength can be determined by feeling the wind, by watching the speed of clouds or the bend of trees and branches, and by observing the sea state (See "Wind Speed Tables" on what signs indicate what wind velocity.) But remember: the wind on land is affected by the land and may not be from the same direction or of the same strength as the wind at sea; you must be able to observe the sea or get a report from a vessel or buoy at sea to get an accurate reading of conditions at sea. Also, when you are on a moving vessel at sea, the true wind is different from the apparent wind: the apparent wind (the wind you feel) is a combination of the true wind, plus the wind your vessel is creating by moving. Knowing the direction of winds around high and low pressures systems can help a mariner understand wind patterns. The winds around high pressure systems move clockwise and outwards in the northern hemisphere and counterclockwise and outwards in the southern hemisphere.The winds around low pressure systems move counterclockwise and inwards in the northern hemisphere and clockwise and inwards in the southern hemisphere. In Hawai'i the prevailing ENE wind, called Moa'e or A'eloa, is generated by a high pressure system that is generally located to the N or NE of the islands. Hawai'i is situated toward the http://leahi.kcc.hawaii.edu/org/pvs/navigate/winds.html (1 of 11) [5/10/2004 1:36:24 PM] Wayfinding / Non-Instrument Weather Forecasting bottom edge of this system (called the North Pacific High), where the clockwise winds are blowing easterly. These so-called trade winds average about 14 knots, but may gust well over gale force, particularly when they are funneled through channels or mountain gaps. Wind strength is determined by (1) the strength of the high pressure; and (2) the steepness of the pressure gradient (the closer together the isobar lines are, the steeper the pressure gradient). The greater the pressure and the steeper the pressure gradient, the stronger the winds. The trade winds are the steadiest during the summer months, from May to September; during the winter months, the high pressure systems moves farther to the east, and the trade winds weaken, from October to April; however, migratory highs can bring strong and gusty easterlies during these months: "Winter months have the strongest trade wind episodes because of the passages north of the Islands by these highs but the average wind speeds do not reflect this-the average wind speed in summer is higher than that of winter in Hawaiian waters" (Haraguchi 12). When the high is centered directly over the islands, winds become light and variable. "The area under the center has light winds and low-height clouds with little precipitation outward to within about 300 miles of the center" (Haraguchi 12). Shifts in wind direction may indicate changes in weather. As low pressure systems travel from west to east, pushed along by the jet stream north of Hawai'i, or when low pressure systems form west of Hawai'i, the prevailing winds may shift southerly. Low pressure systems that remain stationary to the west of the islands bring warm, humid air from the south, and we experience a period of mugginess called Kona weather. As low pressure systems pass north of Hawai'i, cold fronts (cold air wedging under warm air) may sweep over the islands. At the leading edge of a cold front, the winds are southwesterly; rain and blustery winds often accompany these fronts. As the cold front passes, northerly winds fill in behind it, bringing dry cold air. As the cold front moves away from the islands to the east, the ENE trades may return. Ocean Surface-Swells, Seas, Currents Navigator Nainoa Thompson explains the difference between swells and seas, or locally-generated waves: "Swells are big waves generated by pressure systems far beyond the horizon, and they maintain their direction for long periods of time [and travel in the general direction of the winds generating them]. Seas are generated by local winds. Seas generally come downwind, but they may vary by as much as 30° on either side of the wind. When the wind changes, the seas become more of a mish-mash" (Kyselka 167-8). The height of swells depends on the strength of the wind generating them (velocity), the distance over which the wind is blowing (fetch), and the duration of the wind (time). Given a particular wind velocity, swells grow to a maximum size as the fetch and duration increase; they may persist for several days. "A 20-knot wind might make waves of some 6 to 8 feet ('significant wave height,' meaning the average height of the highest one-third of all the waves), if the wind blows for a day or so, over a distance of about 100 miles" (Burch 36). A diminishing swell indicates the wind generating it is diminishing. The farther away the wind generating the swell, the rounder the swells will appear, and the greater the distance between the swells. A 7-8 second interval between swells indicates a well-established wind pattern far off. Shorter intervals and steeper swells indicate that the wind system is closer. http://leahi.kcc.hawaii.edu/org/pvs/navigate/winds.html (2 of 11) [5/10/2004 1:36:24 PM] Wayfinding / Non-Instrument Weather Forecasting Major swells in Hawaiian waters include an ENE Swell generated by the trades; a North Swell, generated by winter storms in the North Pacific; and a South Swell, generated by southern hemisphere winter storms (bringing summer surf to the northern hemisphere). While not directly related to weather, currents affect the sea state: "A strong current flowing against the wind causes an enhanced chop and steepness to the seas, whereas a current flowing with the wind diminishes the seas just as dramatically. To recognize the effect, however, requires some experience at sea, since you must be able to conclude that the seas are not consistent with the wind" (Burch 132). Grimble notes that navigators in the Gilbert Island observe the ocean surface to determine the strength of currents before departure: "Before setting sail in the fair weather season, a Gilbertese mariner will sometimes spend several days in looking at the sea. If it is streaked in places with calm patches of an oily appearance he will refuse to start until these disappear, for they speak to him of strong currents" (238). Clouds David Seidman writes, "Clouds are the harbingers of weather. Their shape, height, color, and sequence foretell coming events." "High clouds are associated with the upper atmosphere and distant weather systems up to six hours away. If they are wispy and white, the weather will be fine. Lower clouds relate to the current weather or that which is soon to come. If they are dense and dark, change is imminent, usually for the worse. Notice if clouds are lowering or lifting, and if they are gathering or dispersing. Lowering or gathering usually brings wet weather. Lifting or dispersing means the weather will improve. A cloud's color seems obvious: the darker, the more dangerous. And a sharp-edged dark cloud is the most dangerous of all. In shape, flat clouds are characteristic of stable air, while lumpy, well-rounded clouds live in unstable air" (166). Satawal navigator Mau Piailug desribes how to read an approaching squall or squall line: "If the rain cloud is black, the wind isn't strong. If the cloud is brown, the wind is probably strong. If the cloud is high, there's not much wind, but maybe a lot of rain. If it's low, probably lots of wind. [As the clouds approach, if the ocean surface beneath the clouds] is black, you know it is a real strong wind. If it's the same color as the ocean near you, then it is not a strong wind. If the water is bumpy [beneath the cloud]you know there's a strong wind" (Kyselka 145). Dark, "bumpy" water (i.e., water full of ripples and small waves, which create shadows) is a sign of a strong local gust of wind; to anticipate such gusts, an experienced sailor watches for dark patches of water moving across the ocean surface. When the sun is relatively low on the horizon (e.g.