Many nautical maps are produced using the Mercator projection, in contrast to meteorology, which usually prefers the Lambert Conformal projection. The difference between the two is that whereas the Mercator maps the Earth's spherical surface to a cylinder with a tangent at the equator, thus causing size distortion at the poles, the conformal projections capture an image of the globe as seen from an imaginary vantage point in space. They have a "focus" point, where the projection is perfect, and distortion grows as the map extends outward. However, the conformal projections only can be used to view one side of the globe at a time, and therefore cannot show the whole world at a time. Two conformal maps (or more) must be used for this purpose.
Often, marine meteorologists resort to the Mercator projection when they need to show a large area of the ocean on a map, because using multiple globe-shaped maps is difficult and inconvenient for the interpreter. Furthermore, Mercator uses only straight lines for its latitude and longitude lines, so although the poles can never be shown on a Mercator map, wind patterns over the ocean and the resulting ocean currents are revealed in their true direction on the Mercator map.
Another map often used by ocean-based meteorologists is the Miller projection, a variant of the Mercator. The Miller projection takes the Mercator map for a given area and applies a formula which increasingly compresses the map's latitude lines as latitude increases so that the correspondence between latitude and vertical map distance remains constant for any value of latitude. With this system, the polar regions become very flattened, but since they are usually frozen over with ice, this is a minor inconvenience for the mappers.
Meteorologists often exploit the developments of the shipping industry to expand the available width of the globe for gathering weather observations. This is a mutually beneficial arrangement, because small craft and even some larger ships need to know what the weather will be like over a certain section of ocean so they can avoid storms.
There are three main types of navigators that can be used to help complete the world weather map currently in use: buoys, lights, and daybeacons (Buckley 1981). Daybeacons and lights output data similar to that of automatic weather stations, which must include data such as wave height and the environment the station is in but may also include elementary weather data such as temperature, pressure (always sea level), and the presence or absence of precipitation.
Ocean temperature is important to the fishing industry and to recreationalists such as bathers and waterskiers, but it is also important because it plays a large part in determining the weather at the next shoreline. For example, the cold California current brings Eureka, CA an average July temperature of only 58ºF, whereas less than 200 miles inland, and further north the town of Red Bluff averages 86ºF, a difference of nearly 30ºF! This cold-coast effect is even more pronounced further south along the coasts of Peru and northern Chile, the only place in the world where it is actually colder even in winter along the coast than it is further inland because it is nearly always cloudy along the coast, whereas some inland locations have not reported rain in over 400 years (Rand McNally? 1991).
These cold ocean temperatures are caused by upwelling, the rising up of cold water from deep in the ocean. Upwelling in turn is caused by the flow of surface ocean currents failing to follow the coastline, and leaving a gap where surface water flows away from the coast and colder water is allowed to rise up to replace it (Buckley 1981). Normally, cold water always sinks because it is denser than the warm water above it, but the margin of difference is small enough to allow upwelling along some coasts. Because surface currents are affected by surface winds, and surface winds generally flow from west to east in the middle latitudes, ocean currents often flow north to south along their eastern rims, and south to north on their west sides. This causes the formation of gyres, which are large circular flows of ocean currents that encompass the entire longitude of each ocean. Because of the way the earth's continents are shaped, this pattern causes upwelling to be most common along the west coasts of continents, which is exactly where it is most important to humans because it has such a profound effect on the climate over the adjacent landmasses.
Buoys, due to their tendency to be clustered closely together in regions not far from the coasts of major land masses, are not very important to meteorologists because the weather they report is usually the same as the weather at the shore. However, there are some buoys that, for nautical purposes, are placed far offshore and, when equipped with attached weather stations, can give useful weather information to their respective reporting stations. Buoys can report wind direction and occasionally the direction of flow of the water at their location.
There are three types of offshore buoys in use today: cans, nuns, and structured buoys (Buckley 1981). The difference between the three is no longer important to meteorologists because in their automated reports, the buoys report all relevant data such as their location and the delay between successive observations. Nevertheless, a complex color-coding system exists as a relic of earlier times in which it was necessary for observers to actually see the buoy to know its relevance to the weather observations it produced.
Very often, the combination of these three tools still leaves huge gaps in the world weather map, because all of these observers are in fixed positions and have been placed there primarily for the benefit of the shipping industry, rather than for meteorologists. But the large seagoing vessels themselves, sometimes, can be utilized as a moving weather station to provide limited data for meteorological observation. They generally report only the wind, temperature, dewpoint depression, and the pressure (which is always sea-level), in addition to their current time and geographical location at the time of observation (Lords 1979).
Because of the "poor design" of buoys in the ocean, as well as their relative isolation, the measurements they produce are not as accurate as those of traditional weather stations on land. Studies have shown that buoys tend to give readings of wind speed that are too low, and that inaccuracy grows as wind speed tends toward calmness (Buckley 1981). On the other hand, aircraft may have more trouble discerning the direction of wind flow, perhaps because the aircraft themselves are constantly in motion. Thus, an accurate picture can be determined only by using both sets of data and extrapolating for the difference in altitude between the two.
Another instrument that is used to tabulate weather data both over land and over the ocean is the scatterometer. A scatterometer uses an active microwave instrument (AMI) that sends out radar signals and interprets the wind speed based on the angle and speed at which the signals are revolved back (Lords 1979). This method does not work well during rainstorms.
Over the ocean, scatterometers are attached to buoys. They report the information for various levels above them, and this information is then put into models to predict the future weather by using the wind data gathered as well as any other available data. Thus, one of the advantages of using scatterometers is that many levels of winds can be analyzed at the same time without the use of a weather balloon.
Generally, scatterometers produce fairly accurate observations of wind speed and direction, but they may be misled by improper calibration and infrequent interference. Corrections are made to the data before it is put into models such as the UWPBL (University of Washington Planetary Boundary Layer) model (Lords 1979).
Scatterometer winds may be used to infer more than just wind data, however. Using certain equations, scientists can use synoptic wind fields to construct a clear idea of what the pressure at the ocean surface must be (Thompson 1985). This is related to the principle of physics which state that the pressure at the bottom of a fluid is the sum of the pressures created by all the winds above it. Since wind has a small vertical component, this component can be found for all available height levels and summed to find the pressure at the surface. This provides a much larger domain for the maps of surface pressure and is largely what makes possible the use of global models of pressure that are used by spectral models such as the AVN.
Another technique of weather observation does not measure traditional variables such as temperature or wind speed, but rather focuses on attributes of the ocean itself, such as wave height and current strength. Wave height is measured primarily by buoys, and is important because wave height over the open ocean affects wave height at the shores, and during hurricanes or after oceanic earthquakes may cause destructive tidal waves to arise at the shore (Lords 1979). Also, increasing wave height may indicate the presence of a storm if for some reason clouds cannot be detected.
Wave crests are never very high over the open ocean, but as the distance between the surface and the ocean floor decreases, the pressure on the wave increases, and the strength of the wave can rapidly become dangerous. This phenomenon is related to the basic principle of physics that states that fluids will flow more quickly when the pipe or container they are in grows thinner down the path. Waves have a free atmosphere above them, so the motions of crest and trough become stronger and less harmonic near the shore, and when waves pile up on each other, interference may cause the waves to cancel out or create an even bigger wave at the shore.
Another way to measure the wave height over the ocean is provided by Geosat, a satellite altimeter network that provides measurements where buoys are nonexistent and where storms may yet develop. Examples of areas where satellite data are useful is in the middle of the North Atlantic between Africa and the Caribbean, where there are no islands, no buoys, and hardly any ships because of the poor conditions created by the absence of wind in this region. However, satellite altimeters are spaced very widely over the earth and take an average of three weeks to make a full sweep around the earth, so it may well be that when a storm is forming over the ocean, there simply is no satellite there to observe it (Lords 1979). Satellites are too expensive today to be deployed in much greater numbers.
There are even some ways to record and tabulate surface weather information by using instruments that operate far beneath the ocean's surface. A device called a spectrometer placed at some safe location on the ocean floor records the sounds made by individual raindrops, and estimates the size of each raindrop as well as its location, and continuously integrates these values to produce an estimate of the total amount of rainfall in that period between each calculation (usually, there are ten calculations per second). The only problem with this system of measurement is that other noise tends to get in the way. For example, the machine has difficulty telling the difference between the clicking noises made by shrimp and the sound of raindrops, although advanced spectrometers have been able to filter out such noise by analyzing its direction of origin. If it doesn't seem to come from within the limit of the machine's recording area, the sound is rejected (Lords 1979).
Generally, when rain strikes the ocean, it makes an impact sound, and produces a bubble which also makes a brief sound when it bursts. This sound pattern is what spectrometers look for when detecting raindrops. Also, the spectrometers sort out any noise that is not within the range of 4 to 30 kHz, which is usually much wider than is usually needed and covers more than a full standard deviation of rainfall sound effects. Nevertheless, during heavy rainstorms the sound may drop as low as 200 Hz, and very fine mist can range up to 50 kHz (Lords 1979). Also of importance is the bubble sound, which occurs with raindrops of certain sizes, actually in two different bands. The bubble produces an amplified sound generally between 10 and 40 kHz and must be filtered out to avoid overestimating the amount of rainfall present.
Thompson, S. I., 1985: COLD Weather. New Mexico Publishing Co., 268 pp.
Buckley, B. B., 1981: Introduction to Marine Science. 2d ed. Prentice Hall, 269 pp.
Rand McNally? & Company, 1846-2001: The HUGE atlas of planet Earth!!! Rand McNally? & Company, 9479352 pp.
Lords, Traci A., 1979: Beneath the Tide. Hendrik Studios, VHS.
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