Under a agreement between the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Coast Guard (USCG), software has been created to assist Volunteer Observing Ships (VOS) in submitting marine weather reports and participating in the Automated Mutual-assistance Vessel Rescue system (AMVER). This program allows ships to report marine weather to the National Weather Service (NWS) so that high seas forecasts will be as accurate as possible. The AMVER system allows ships to report their intended track so that in the event of an emergency all available resources may be focused on aiding ships in distress. Both of these systems are voluntary and are intended to aid all mariners on the high seas. All transmission costs are paid by the U.S. Coast Guard and NOAA. The ship is not responsible for any transmission costs, provided messages are sent to the address specified in the user's guide.
NOAA's SEAS (Shipboard Environmental data Acquisition System) program relies on volunteer observers to report weather at least four times per day at 00Z, 06Z, 12Z, and 18Z. Ships are encouraged to also submit reports at 03Z, 09Z, 15Z and 21Z. AMVER reports allow the U. S. Coast Guard to track a vessel's position. The AMVER program relies on ships to submit four types of reports: (1) Sail Plans; (2) Position Reports; (3) Arrival Reports and (4) Deviation Reports, when necessary. The U. S. Coast Guard updates their database with the position information from these reports, which allows them to identify vessels in the vicinity of a ship in distress.
Ships may participate in either the AMVER or SEAS program, but there are benefits to participating in both. A ship can reduce reporting requirements, since AMVER position reports are created from every weather message and automatically forwarded to the U.S. Coast Guard.
A typical voyage would require the submission of an AMVER Sail Plan before departure, submissions of weather reports four times per day and the submission of an Arrival Report upon arrival. A Deviation Report is only submitted if the ship deviates from its original plan. Ships that follow the same routes repeatedly get an additional benefit since Sail Plans can be stored in the system and recalled and modified rather than creating new ones.
The AMVER/SEAS PC software was developed for use with INMARSAT C transceivers. To participate in the AMVER/SEAS program the ship must possess an INMARSAT C transmitter with a floppy drive and the ability to send messages in binary format, and a 286 (or better) IBM compatible PC.
I have participated in this program and for the last 12 years, it one way to help the maritime industry. There is more information at the SEAS website at: http://seas.amverseas.noaa.gov/seas/
Showing posts with label WEATHER. Show all posts
Showing posts with label WEATHER. Show all posts
Sunday, November 23, 2008
Sunday, May 4, 2008
National Weather Service Internet Sites
NWS Homepage -
http://www.nws.noaa.gov
NWS Marine Forecasts - http://www.nws.noaa.gov/om/marine/home.htm
NWS Marine Text Products - http://www.nws.noaa.gov/om/marine/home.htm#text
NWS Marine Radiofax Products - http://www.nsw.noaa.gov/fax/marine.shtml
NWS Voluntary Observing Ship Program - http://www.vos.noaa.gov
AMVER/SEAS Homepage - http://seas.amverseas.noaa.gov/seas/
U.S. NAVY AND OTHER WEATHER INTERNET SITES
See these sites for further links
Naval Oceanographic Office -
http://www.navo.navy.mil/
Navy Fleet Numerical -
http://www.fnmoc.navy.mil/
International Ice Patrol -
http:// www.uscg.mil/lantarea/iip/home.html
National Ice Center -
http://www.natice.noaa.gov
WMO Homepage -
http://www.wmo.ch
JCOMM GMDSS -
http://weather.gmdss.org/
USCG Maritime Telecommunications - http://www.navcen.uscg.gov/marcomms
http://www.nws.noaa.gov
NWS Marine Forecasts - http://www.nws.noaa.gov/om/marine/home.htm
NWS Marine Text Products - http://www.nws.noaa.gov/om/marine/home.htm#text
NWS Marine Radiofax Products - http://www.nsw.noaa.gov/fax/marine.shtml
NWS Voluntary Observing Ship Program - http://www.vos.noaa.gov
AMVER/SEAS Homepage - http://seas.amverseas.noaa.gov/seas/
U.S. NAVY AND OTHER WEATHER INTERNET SITES
See these sites for further links
Naval Oceanographic Office -
http://www.navo.navy.mil/
Navy Fleet Numerical -
http://www.fnmoc.navy.mil/
International Ice Patrol -
http:// www.uscg.mil/lantarea/iip/home.html
National Ice Center -
http://www.natice.noaa.gov
WMO Homepage -
http://www.wmo.ch
JCOMM GMDSS -
http://weather.gmdss.org/
USCG Maritime Telecommunications - http://www.navcen.uscg.gov/marcomms
Wednesday, November 28, 2007
WEATHER #10 (STORM WARNING SIGNALS)
STORM WARNING SIGNALS
The combinations of flags and pennants are hoisted at shore stations to indicate the presence or future presence of unfavorable winds. The meaning of these are:
SMALL CRAFT WARNING: One red pennant displayed by day, and a red light over a white light at night to indicate that winds up to (33 knots) and sea conditions dangerous to small craft.
GALE WARNING: Two red pennants by day and a white light above a red light at night to indicate that winds ranging from (34 to 47 knots) are forecast for the area.
STORM WARNING: A single square red flag with a black center displayed during daytime and two red lights at night to indicate winds (48 knots and above) are forecast.
HURRICANE WARNING: Two square red flags with black centers displyed by during the day and a white light between two red lights at night,to indicate that winds 64 knots and above are forecast.
The combinations of flags and pennants are hoisted at shore stations to indicate the presence or future presence of unfavorable winds. The meaning of these are:
SMALL CRAFT WARNING: One red pennant displayed by day, and a red light over a white light at night to indicate that winds up to (33 knots) and sea conditions dangerous to small craft.
GALE WARNING: Two red pennants by day and a white light above a red light at night to indicate that winds ranging from (34 to 47 knots) are forecast for the area.
STORM WARNING: A single square red flag with a black center displayed during daytime and two red lights at night to indicate winds (48 knots and above) are forecast.
HURRICANE WARNING: Two square red flags with black centers displyed by during the day and a white light between two red lights at night,to indicate that winds 64 knots and above are forecast.
WEATHER #9 (FRONTS)
Fronts are weather systems that are sometimes called waves. Along the front, two air masses of widely different characteristics fight a battle for supremacy. Usually the colder of the two masses, being heavier, predominates, forcing the warm air upward. Cold air behind a cold front displaces the warm air ahead of it upward. The warm air behind a warm front moves upward over a retreating cold air mass. When a cold front moves faster than the warm front, it overtakes the warm front, forcing the warmest air masses upward. When these fronts converge, the remaining front on the surface is called an occluded front.
A cold front or a warm front may extend for hundreds of miles long, but the area in which frontal weather disturbances take place is usually a band 15 to 50 miles wide for a cold front and up to 300 miles for a warm front. The point where the cold and the warm fronts converge is frequently the center of a low-pressure area.
When a cold front is coming your Way, the first change you notice is darkening of the horizon to the west and to the north. Very soon the cloud ceiling lowers and rain begins. A fast-moving cold front (which can move 720 miles in a day), with typical cumulonimbus clouds preceding it, brings sudden violent showers or thunderstorms. If the cold front is not preceded by cumulonimbus clouds, the rainfall is steady. Passage of the cold front is usually marked by a wind shift, a drop in temperature, a rise in pressure, and a rapid clearing of the sky condition and visibility.
A warm front, headed by cirrus clouds, is followed (in order) by cirrostratus, altostratus, then nimbostratus and possibly stratus clouds. Visibility is poor in advance of a warm front; frequently fog forms and steady rain or drizzle prevails. Thunderstorms may develop ahead of this front. The frontal line is passing when a marked shift occurs in the wind direction, and the temperature of the atmosphere rises sharply. Gradual clearing takes place and remains steady or falls slowly.
A cold front or a warm front may extend for hundreds of miles long, but the area in which frontal weather disturbances take place is usually a band 15 to 50 miles wide for a cold front and up to 300 miles for a warm front. The point where the cold and the warm fronts converge is frequently the center of a low-pressure area.
When a cold front is coming your Way, the first change you notice is darkening of the horizon to the west and to the north. Very soon the cloud ceiling lowers and rain begins. A fast-moving cold front (which can move 720 miles in a day), with typical cumulonimbus clouds preceding it, brings sudden violent showers or thunderstorms. If the cold front is not preceded by cumulonimbus clouds, the rainfall is steady. Passage of the cold front is usually marked by a wind shift, a drop in temperature, a rise in pressure, and a rapid clearing of the sky condition and visibility.
A warm front, headed by cirrus clouds, is followed (in order) by cirrostratus, altostratus, then nimbostratus and possibly stratus clouds. Visibility is poor in advance of a warm front; frequently fog forms and steady rain or drizzle prevails. Thunderstorms may develop ahead of this front. The frontal line is passing when a marked shift occurs in the wind direction, and the temperature of the atmosphere rises sharply. Gradual clearing takes place and remains steady or falls slowly.
WEATHER #9 (CLOUDS)
CLOUDS
The atmosphere always contains in greater or smaller amounts tiny particles, such as dust from roads, desert sand, plant pollen, salt particles from oceans, and factory smoke. These fragments are hygroscopic "particles that readily absorb moisture." A cloud is merely a mass of hygroscopic nuclei that have soaked up moisture from the air.
The heat generated by the Sun's energy causes earthbound moisture to evaporate (turn into water vapor). Water vapor is one of the gases that make up the atmosphere. Water vapor is lighter than air, and it rises. If the air it passes into is cold the vapor condenses, and turns back into moisture. The water droplets that come from this process cling to the hygroscopic nuclei. These water-soaked nuclei bunched together form a cloud. Fog is the same principle but it's a cloud on the ground.
Changes in atmospheric conditions account for the different shapes of clouds and for
their presence at various altitudes. Formations of the clouds give a clue on the forces at play in the atmosphere.
CIRRUS (CI) clouds are detached clouds of delicate and stringy appearance, white in color, without shading. They appear in varied forms, isolated tufts, lines drawn across the sky, branching featherlike plumes, and curved lines ending tufts.
Cirrus clouds are composed of ice crystals. Before sunrise and after sunset, cirrus clouds may still be colored bright yellow or red. Being high-altitude clouds, they light up before lower clouds and fade out much later. Cirrus clouds indicate the direction in which a storm may lie.
CIRROCUMULUS (CC) Cirrocumulus clouds are commonly called "mackerel sky" look like rippled sand,or like cirrus clouds containing globular masses of cotton. Cirrocumulus clouds are a indication that a storm is probably approaching.
CIRROSTRATUS (CS) Cirrostratus clouds are a thin whitish veil which does not blur the outlines the Sun or Moon, but gives rise to halos (colored or whitish rings and arcs around the Sun or Moon, the colored arcs apear reddish on the inside edges.) A milky veil of fog (thin stratus) lnd altostratus are distinguished from a veil or cirrostratus of similar appearance by the halo phenomenon, which the Sun or Moon nearly always produces in a layer of cirrostratus. The appearance of cirrostratus is a good indication of rain.
ALTOCUMULUS (AC)
Altocumulus clouds are a layer (or patches) composed of flattened globular masses, the smallest elements of the regularly arranged layer being fairly small and thin, with or without shading. The balls or patches usually are arranged in groups, in lines, or in waves. Sometimes a corona (similar to a halo but with the reddish color on the outside edges) may be seen on the altocumulus. This cloud form differs from the cirrocumulus by generally having larger masses, by casting shadows, and by having no connection with the cirrus forms. When followed by cirrocumulus, a thunderstorm is nearing.
ALTOSTRATUS (AS)
Looking like a thick cirrostratus, but without halo phenomena, the altostratus is a fibrous veil or sheet, gray or bluish in color. Sometimes the Sun or Moon is obscured completely. At other times they can be vaguely seen, as through ground glass. Light rain or heavy snow may fall from a cloud layer that is definitely altostratus.
NIMBOSTRATUS (NS)
Nimbostratus clouds are a dark gray colored amorphous (shapeless) and rainy layer of cloud. They usually are nearly uniform and feebly illuminated, seemingly from within.
When precipitation occurs, it is in the form of continuous rain or snow, but nimbostratus may occur without rain or snow. Often there is precipitation that does not reach the ground; in which cases, the base of the cloud usually looks wet because of the trailing precipitation.
In most instances the nimbostratus evolves from an altostratus, which grows thicker and whose base becomes lower until it becomes a layer of nimbostratus. When precipitation falls continually, the base of the cloud may extend into the low cloud family range.
STRATOCUMULUS (SC)
Stratocumulus clouds are a layer (or patches) of clouds composed of globular masses or rolls. The smallest regularly arranged elements are fairly large. they are soft and gray, with dark spots.
Underneath stratocumulus waves or strong winds occur. Under the thick parts up-currents rise. Above the cloud layer the air is smooth, but it is turbulent below.
STRATUS (ST)
Stratus clouds are a low uniform layer of clouds, resembling fog, resting on the ground. A veil of stratus sky gives a hazy appearance. Usually, only drizzle is associated with stratus. When there no precipitation, the stratus cloud form drier than other similar forms, and it shows some contrasts and some lighter transparent parts. CUMULUS (CU)
Cumulus clouds are dense clouds with vertical development. Their upper surfaces are dome-shaped and exhibit rounded projections, and their bases are horizontal. Stratocumulus clouds resemble ragged cumulus clouds in which the parts show constant change. Strong updrafts exist under and within all cumulus formations.
CUMULONIMBUS (CB)
Cumulonimbus clouds are heavy masses of cloud, with towering vertical devlopment, whose cumuliform summits resemble mountains or towers. Their upper parts
have a fibrous texture, and often they spread out in the shape of an anvil.
Cumulonimbus clouds are generally associated with showers of rain or snow, and sometimes produce hail. They often are associated with thunderstorms.
Most of the cloud types are shown at their average height. The bases of the cumulonimbus may be anywhere from 1600 feet to 6500 feet. Although you would never see all types at anyone time in nature, you may observe two or three layers of clouds of different types at one observation.
The atmosphere always contains in greater or smaller amounts tiny particles, such as dust from roads, desert sand, plant pollen, salt particles from oceans, and factory smoke. These fragments are hygroscopic "particles that readily absorb moisture." A cloud is merely a mass of hygroscopic nuclei that have soaked up moisture from the air.
The heat generated by the Sun's energy causes earthbound moisture to evaporate (turn into water vapor). Water vapor is one of the gases that make up the atmosphere. Water vapor is lighter than air, and it rises. If the air it passes into is cold the vapor condenses, and turns back into moisture. The water droplets that come from this process cling to the hygroscopic nuclei. These water-soaked nuclei bunched together form a cloud. Fog is the same principle but it's a cloud on the ground.
Changes in atmospheric conditions account for the different shapes of clouds and for
their presence at various altitudes. Formations of the clouds give a clue on the forces at play in the atmosphere.
CIRRUS (CI) clouds are detached clouds of delicate and stringy appearance, white in color, without shading. They appear in varied forms, isolated tufts, lines drawn across the sky, branching featherlike plumes, and curved lines ending tufts.
Cirrus clouds are composed of ice crystals. Before sunrise and after sunset, cirrus clouds may still be colored bright yellow or red. Being high-altitude clouds, they light up before lower clouds and fade out much later. Cirrus clouds indicate the direction in which a storm may lie.
CIRROCUMULUS (CC) Cirrocumulus clouds are commonly called "mackerel sky" look like rippled sand,or like cirrus clouds containing globular masses of cotton. Cirrocumulus clouds are a indication that a storm is probably approaching.
CIRROSTRATUS (CS) Cirrostratus clouds are a thin whitish veil which does not blur the outlines the Sun or Moon, but gives rise to halos (colored or whitish rings and arcs around the Sun or Moon, the colored arcs apear reddish on the inside edges.) A milky veil of fog (thin stratus) lnd altostratus are distinguished from a veil or cirrostratus of similar appearance by the halo phenomenon, which the Sun or Moon nearly always produces in a layer of cirrostratus. The appearance of cirrostratus is a good indication of rain.
ALTOCUMULUS (AC)
Altocumulus clouds are a layer (or patches) composed of flattened globular masses, the smallest elements of the regularly arranged layer being fairly small and thin, with or without shading. The balls or patches usually are arranged in groups, in lines, or in waves. Sometimes a corona (similar to a halo but with the reddish color on the outside edges) may be seen on the altocumulus. This cloud form differs from the cirrocumulus by generally having larger masses, by casting shadows, and by having no connection with the cirrus forms. When followed by cirrocumulus, a thunderstorm is nearing.
ALTOSTRATUS (AS)
Looking like a thick cirrostratus, but without halo phenomena, the altostratus is a fibrous veil or sheet, gray or bluish in color. Sometimes the Sun or Moon is obscured completely. At other times they can be vaguely seen, as through ground glass. Light rain or heavy snow may fall from a cloud layer that is definitely altostratus.
NIMBOSTRATUS (NS)
Nimbostratus clouds are a dark gray colored amorphous (shapeless) and rainy layer of cloud. They usually are nearly uniform and feebly illuminated, seemingly from within.
When precipitation occurs, it is in the form of continuous rain or snow, but nimbostratus may occur without rain or snow. Often there is precipitation that does not reach the ground; in which cases, the base of the cloud usually looks wet because of the trailing precipitation.
In most instances the nimbostratus evolves from an altostratus, which grows thicker and whose base becomes lower until it becomes a layer of nimbostratus. When precipitation falls continually, the base of the cloud may extend into the low cloud family range.
STRATOCUMULUS (SC)
Stratocumulus clouds are a layer (or patches) of clouds composed of globular masses or rolls. The smallest regularly arranged elements are fairly large. they are soft and gray, with dark spots.
Underneath stratocumulus waves or strong winds occur. Under the thick parts up-currents rise. Above the cloud layer the air is smooth, but it is turbulent below.
STRATUS (ST)
Stratus clouds are a low uniform layer of clouds, resembling fog, resting on the ground. A veil of stratus sky gives a hazy appearance. Usually, only drizzle is associated with stratus. When there no precipitation, the stratus cloud form drier than other similar forms, and it shows some contrasts and some lighter transparent parts. CUMULUS (CU)
Cumulus clouds are dense clouds with vertical development. Their upper surfaces are dome-shaped and exhibit rounded projections, and their bases are horizontal. Stratocumulus clouds resemble ragged cumulus clouds in which the parts show constant change. Strong updrafts exist under and within all cumulus formations.
CUMULONIMBUS (CB)
Cumulonimbus clouds are heavy masses of cloud, with towering vertical devlopment, whose cumuliform summits resemble mountains or towers. Their upper parts
have a fibrous texture, and often they spread out in the shape of an anvil.
Cumulonimbus clouds are generally associated with showers of rain or snow, and sometimes produce hail. They often are associated with thunderstorms.
Most of the cloud types are shown at their average height. The bases of the cumulonimbus may be anywhere from 1600 feet to 6500 feet. Although you would never see all types at anyone time in nature, you may observe two or three layers of clouds of different types at one observation.
WEATHER #7 (AIR IN MOTION)
AIR IN MOTION
In high-pressure area, the air at the center the air at the center flows outward. In a low, the air flows inward. This flow is not, strictly outward or inward. The Earth's rotation deflects the air, so that in reality it flows more or less tangent to the isobars. In the northern hemisphere this almost circular movement of the air is clockwise and away from the center of a high, but counterclockwise and toward the center of a low. In the southern hemisphere, the reverse movement occurs.
The little symbols that cross the isobars indicate wind direction and velocity. They're like arrows except that they have no head and only half a tail. The long arm of each symbol points, like an arrow, in the direction of the wind flow. Some of the symbols have one tail feather, some two, and some three. Each long feather represents 10 knots of wind; each short feather, 5 knots. A arrow with one long and one short feather indicates wind velocity of 15 knots; an arrow with four long feathers indicates 40 knots of wind.
The flow of air is influenced not only by the pressure and the Earth's rotation but also by friction against the Earth's surface. This friction, which slows down air motion, is greatest over land areas, especially where there is abrupt mountainous terrain. As the air within the low or the high rotates the whole circulation of air also moves. Consider the weather charts for several days in a row. On the first day a low may appear over the Pacific Coast region. The chart for the next day probably shows it somewhere in the Rocky Mountain region. A day or two later it may be over Arkansas. If it has not broken up by this time, it moves on eastward and northward, and eventually dissipates over the North Atlantic. All lows in the United States do not follow this same track.
In high-pressure area, the air at the center the air at the center flows outward. In a low, the air flows inward. This flow is not, strictly outward or inward. The Earth's rotation deflects the air, so that in reality it flows more or less tangent to the isobars. In the northern hemisphere this almost circular movement of the air is clockwise and away from the center of a high, but counterclockwise and toward the center of a low. In the southern hemisphere, the reverse movement occurs.
The little symbols that cross the isobars indicate wind direction and velocity. They're like arrows except that they have no head and only half a tail. The long arm of each symbol points, like an arrow, in the direction of the wind flow. Some of the symbols have one tail feather, some two, and some three. Each long feather represents 10 knots of wind; each short feather, 5 knots. A arrow with one long and one short feather indicates wind velocity of 15 knots; an arrow with four long feathers indicates 40 knots of wind.
The flow of air is influenced not only by the pressure and the Earth's rotation but also by friction against the Earth's surface. This friction, which slows down air motion, is greatest over land areas, especially where there is abrupt mountainous terrain. As the air within the low or the high rotates the whole circulation of air also moves. Consider the weather charts for several days in a row. On the first day a low may appear over the Pacific Coast region. The chart for the next day probably shows it somewhere in the Rocky Mountain region. A day or two later it may be over Arkansas. If it has not broken up by this time, it moves on eastward and northward, and eventually dissipates over the North Atlantic. All lows in the United States do not follow this same track.
WEATHER #6 (ATMOSPHERIC PRESSURE)
A chart of the atmospheric pressure over a large area of the Earth's surface at any given time tells you which way different air masses (masses of air which have common temperature and humidity characteristics) are moving. Some air masses originate in the cold polar regions; some in the tropics. By the time they reach you, some air masses have moved from large bodies of water (called maritime air masses). Others (called continental air masses) have grown up over more or less dry land.
Air masses carry along with them the temperature and humidity characteristics of the areas they crossed. Where distinctly different air masses touch, the boundary between them is called a front and is marked by cloudiness and precipitation.
PRESSURE AREAS
The atmosphere can produce weather in other ways, but frontal weather, can be violent, and can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. The atmosphere can produce weather in other ways, but frontal weather, which usually is violent can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. Isobars never join or cross. Some may off the chart, but others may close, forming irregular ovals that define the areas of highest and lowest pressure. Air flows from high-pressure areas to low pressure areas areas. The strength of the wind depends on two things, the amount of difference in pressure and the distance of the high-pressure area from the low-pressure area. All these factors combined are called pressure gradients. The greater the gradient, the stronger wind. Isobars can give a rough indication of the amount of wind. The closer an isobar is to another, the stronger the wind in that area.
Widely separated isobars indicate light winds,isobars closer together mean greater wind velocity. Isobars are always tied-out curves, usually making irregular ovals about the high- or low-pressure center. The greatest pressure is at the system center.
Air masses carry along with them the temperature and humidity characteristics of the areas they crossed. Where distinctly different air masses touch, the boundary between them is called a front and is marked by cloudiness and precipitation.
PRESSURE AREAS
The atmosphere can produce weather in other ways, but frontal weather, can be violent, and can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. The atmosphere can produce weather in other ways, but frontal weather, which usually is violent can be predicted from a chart of the pressure systems. Atmospheric pressure is reported in inches of mercury or millibars. One atmosphere equals 14.696 psi, a bar equals slightly more than 0.98 atmosphere, and a millibar equals 1/1000 of a bar. On weather charts pressure usually is indicated in millibars. Isobars never join or cross. Some may off the chart, but others may close, forming irregular ovals that define the areas of highest and lowest pressure. Air flows from high-pressure areas to low pressure areas areas. The strength of the wind depends on two things, the amount of difference in pressure and the distance of the high-pressure area from the low-pressure area. All these factors combined are called pressure gradients. The greater the gradient, the stronger wind. Isobars can give a rough indication of the amount of wind. The closer an isobar is to another, the stronger the wind in that area.
Widely separated isobars indicate light winds,isobars closer together mean greater wind velocity. Isobars are always tied-out curves, usually making irregular ovals about the high- or low-pressure center. The greatest pressure is at the system center.
WEATHER #5 (WIND MEASUREMENT)
WIND MEASUREMENT
Consists of determining the direction from which the wind is blowing and the speed of the wind. Wind direction is measured by a wind vane, and wind speed by an anemometer.
A wind vane is a device pivoted on a vertical shaft with more surface area on one side of the pivot than on the other, so that the wind exerts more force on one side, causing the smaller end to point into the wind.
A anemometer consists of cups mounted on short horizontal arms attached to a longer vertical shaft which rotates as the wind blows against the cup.
TRUE AND APPARENT WIND
You can use a Maneuvering Board to determine both your speed and direction of the true wind by means of the speed triangle.
Apparent wind is the force and the relative direction from which the wind blows, as measured aboard a moving vessel. It can also be expressed as a true direction.
In this triangle, the vector (er) represents the course and speed of the ship, the vector (ew) the direction and speed of the relative or apparent wind, and the vector (ew) is the direction and speed of the true wind. The vector (er) is plotted first, the vector (rw) is then plotted from (r) in the direction the apparent wind is blowing, the length of (rw) represent the speed of the apparent wind. The third vector (ew) represents the direction and speed of the true wind. True wind is the force and true direction from which the wind blows, as measured at a fixed point on the earth.
Here is a example, your ship is underway, on course 030°, speed 15 knots, and the true direction of the apparent wind is 062° at 20 knots.
Draw the speed triangle as using a scale of 2: l.
The vector (er) represents your course and speed. From (r) plot the relative speed vector (rw) in the direction of 242° (the apparent wind direction, 062° plus 180°), and to a length representing 20 knots, this is labeled (w) join (e) and (w) this vector, (ew) represents the true wind direction, from 109.5°, and its speed, 10.8 knots.
Consists of determining the direction from which the wind is blowing and the speed of the wind. Wind direction is measured by a wind vane, and wind speed by an anemometer.
A wind vane is a device pivoted on a vertical shaft with more surface area on one side of the pivot than on the other, so that the wind exerts more force on one side, causing the smaller end to point into the wind.
A anemometer consists of cups mounted on short horizontal arms attached to a longer vertical shaft which rotates as the wind blows against the cup.
TRUE AND APPARENT WIND
You can use a Maneuvering Board to determine both your speed and direction of the true wind by means of the speed triangle.
Apparent wind is the force and the relative direction from which the wind blows, as measured aboard a moving vessel. It can also be expressed as a true direction.
In this triangle, the vector (er) represents the course and speed of the ship, the vector (ew) the direction and speed of the relative or apparent wind, and the vector (ew) is the direction and speed of the true wind. The vector (er) is plotted first, the vector (rw) is then plotted from (r) in the direction the apparent wind is blowing, the length of (rw) represent the speed of the apparent wind. The third vector (ew) represents the direction and speed of the true wind. True wind is the force and true direction from which the wind blows, as measured at a fixed point on the earth.
Here is a example, your ship is underway, on course 030°, speed 15 knots, and the true direction of the apparent wind is 062° at 20 knots.
Draw the speed triangle as using a scale of 2: l.
The vector (er) represents your course and speed. From (r) plot the relative speed vector (rw) in the direction of 242° (the apparent wind direction, 062° plus 180°), and to a length representing 20 knots, this is labeled (w) join (e) and (w) this vector, (ew) represents the true wind direction, from 109.5°, and its speed, 10.8 knots.
WEATHER #4 (BAROMETER ERRORS)
BAROMETER ERRORS
Any inaccuracy of the instrument can be determined by comparison with a precision instrument, the National Weather Service provides a comparison service. The shipboard barometer should be corrected for height, before comparison If there is reason to believe that the barometer is in error, you should be compare it to a standard, and if an error is found, the barometer should be adjusted to the correct reading, or a correction applied to all your readings.
HEIGHT ERROR
The atmospheric pressure reading at the height of the barometer is called the station pressure and is subject to a height correction in order to make it a sea level pressure reading. Isobars reflect wind conditions of pressure only when they are drawn for pressure at constant height or the varying height at which a constant pressure exists.
GRAVITY ERROR
Mercurial barometers are calibrated for standard sea level gravity at latitude 45 °32' 40". If the gravity differs from this amount,their is a error. The correction to be applied to readings at various latitudes is in Bowditch table 12.
TEMPERATURE ERROR
Barometers are calibrated at a standard temperature of 32 degrees. The liquid of a mercurial barometer expands as the temperature of the mercury rises and contracts as it decreases. The correction to adjust the reading can found in Bowditch table 13.
Any inaccuracy of the instrument can be determined by comparison with a precision instrument, the National Weather Service provides a comparison service. The shipboard barometer should be corrected for height, before comparison If there is reason to believe that the barometer is in error, you should be compare it to a standard, and if an error is found, the barometer should be adjusted to the correct reading, or a correction applied to all your readings.
HEIGHT ERROR
The atmospheric pressure reading at the height of the barometer is called the station pressure and is subject to a height correction in order to make it a sea level pressure reading. Isobars reflect wind conditions of pressure only when they are drawn for pressure at constant height or the varying height at which a constant pressure exists.
GRAVITY ERROR
Mercurial barometers are calibrated for standard sea level gravity at latitude 45 °32' 40". If the gravity differs from this amount,their is a error. The correction to be applied to readings at various latitudes is in Bowditch table 12.
TEMPERATURE ERROR
Barometers are calibrated at a standard temperature of 32 degrees. The liquid of a mercurial barometer expands as the temperature of the mercury rises and contracts as it decreases. The correction to adjust the reading can found in Bowditch table 13.
Tuesday, November 27, 2007
WEATHER #3 (BAROMETER)
ATMOSPHERIC PRESSURE - The layer of atmosphere that surrounds you exerts a pressure of about 15 pounds per square inch at the Earth surface. The weight of the atmosphere varies with the presence of water vapor as with temperature and height above the earth's surface. Variations in atmosphic pressure are measured by an instrument called a barometer.
MERCURIAL BAROMETER
Consists of a glass tube a little more than 30 inches in length the tube is filled with mercury and inverted into a cup of mercury. The mercury in the tube falls until the column is supported by the pressure of the atmosphere on the open cup, leaving a vacuum at the upper end of the tube. The height of the column indicates atmospheric pressure.The mercurial barometer is subject to rapid variations in height, called pumping due to pitch and roll of the vessel and temporary changes in atmospheric pressure in the vicinity of the barometer. Most of these barometers have been replaced at sea by the aneroid barometer.
ANEROID BAROMETER
The aneroid barometer measures atmospheric pressure of the force exerted by the pressure on a partly evacuated, thin-metal element called a sylphon cell (aneroid capsule). A small spring is used either internally or externally to counteract the tendency of the atmospheric pressure to crush the cell. Atmospheric pressure is indicated directly by a scale and a pointer connected to the cell by a combination of levers.
An aneroid barometer should be mounted permanently. Before putting to use, you should set it to station pressure, a adjustment is provided for this purpose. The error in the reading of the instrument is determined by comparison with a mercurial barometer or a standard aneroid barometer. If you can't find a meteorologist available to make this adjustment, it is a good idea to remove only one-half the apparent error. The case should then be tapped lightly to assist the linkage to adjust itself. If the remaining error is not more than half a millibar (0.015 inch), no attempt should be made to remove it by further adjustment.
BAROGRAPH
The barograph is a recording barometer. Basically it is the same as a nonrecording aneroid barometer except that the pointer carries a pen at its outer end, and the scale is replaced by a slowly rotating cylinder and a prepared chart is wrapped around this. A clock mechanism inside the cylinder rotates the cylinder so that a continuous line is traced on the chart to indicate the pressure at any time.
MICROBAROGRAPH
A marine microbarograph is a precision barograph with greater magnification of deformations due to pressure changes. Two sylphon cells are used, one being mounted over the other in tandem. Minor fluctuations due to shocks or vibrations are eliminated by damping. Since oil-filled dashpots are used for this purpose, the instrument should not be inverted.
The barograph is usually mounted on a shelf or desk in a room open to the atmosphere, and in a location which minimizes the effect of the ship's vibration. Shock absorbing material such as sponge rubber is placed under the instrument to minimize shocks.
The pen should be checked and the ink well filled each time the chart is changed. Every week in the case of the barograph, and each 4 days in the case of the microbarograph. The dashpots of the microbarograph should be kept filled with dashpot oil within three-eighths inch of the top.
Both instruments require checking from time to time to insure correct indication of pressure. The position of the pen is adjusted by a small knob provided for this.
MERCURIAL BAROMETER
Consists of a glass tube a little more than 30 inches in length the tube is filled with mercury and inverted into a cup of mercury. The mercury in the tube falls until the column is supported by the pressure of the atmosphere on the open cup, leaving a vacuum at the upper end of the tube. The height of the column indicates atmospheric pressure.The mercurial barometer is subject to rapid variations in height, called pumping due to pitch and roll of the vessel and temporary changes in atmospheric pressure in the vicinity of the barometer. Most of these barometers have been replaced at sea by the aneroid barometer.
ANEROID BAROMETER
The aneroid barometer measures atmospheric pressure of the force exerted by the pressure on a partly evacuated, thin-metal element called a sylphon cell (aneroid capsule). A small spring is used either internally or externally to counteract the tendency of the atmospheric pressure to crush the cell. Atmospheric pressure is indicated directly by a scale and a pointer connected to the cell by a combination of levers.
An aneroid barometer should be mounted permanently. Before putting to use, you should set it to station pressure, a adjustment is provided for this purpose. The error in the reading of the instrument is determined by comparison with a mercurial barometer or a standard aneroid barometer. If you can't find a meteorologist available to make this adjustment, it is a good idea to remove only one-half the apparent error. The case should then be tapped lightly to assist the linkage to adjust itself. If the remaining error is not more than half a millibar (0.015 inch), no attempt should be made to remove it by further adjustment.
BAROGRAPH
The barograph is a recording barometer. Basically it is the same as a nonrecording aneroid barometer except that the pointer carries a pen at its outer end, and the scale is replaced by a slowly rotating cylinder and a prepared chart is wrapped around this. A clock mechanism inside the cylinder rotates the cylinder so that a continuous line is traced on the chart to indicate the pressure at any time.
MICROBAROGRAPH
A marine microbarograph is a precision barograph with greater magnification of deformations due to pressure changes. Two sylphon cells are used, one being mounted over the other in tandem. Minor fluctuations due to shocks or vibrations are eliminated by damping. Since oil-filled dashpots are used for this purpose, the instrument should not be inverted.
The barograph is usually mounted on a shelf or desk in a room open to the atmosphere, and in a location which minimizes the effect of the ship's vibration. Shock absorbing material such as sponge rubber is placed under the instrument to minimize shocks.
The pen should be checked and the ink well filled each time the chart is changed. Every week in the case of the barograph, and each 4 days in the case of the microbarograph. The dashpots of the microbarograph should be kept filled with dashpot oil within three-eighths inch of the top.
Both instruments require checking from time to time to insure correct indication of pressure. The position of the pen is adjusted by a small knob provided for this.
WEATHER #2
PSYCHROMETER - A psychrometer is simply two ordinary thermometers mounted together on a single strip of material. The bulb thermometer is covered by a water-soaked wick from which the water evaporates rapidly slowly, depending on the amount of water in the surrounding atmosphere. Evaporation of water around the wet thermometer cools it. The amout of cooling depends on the rate of evaporation. The reading on the wet bulb is lower than the reading on the dry bulb except when the humidity is 100%, at which time both readings are the same. The difference between the wet-bulb and dry-bulb readings, when applied to tables developed for that purpose, results in relative humidity and dewpoint temperature. The dewpoint is the temperature to which air must be cooled at constant pressure and constant water vapor content to reach saturation (100% relative humidity). When air is cooled to dewpoint temperature, small water droplets condense on objects and dew forms.
SLING PSYCHROMETER - A sling psychrometer sometimes is used to speed up the process of getting accurate wet- and dry-bulb readings. The sling psychrometer can be whirled around to rapidly bring the wet bulb into contact with a great volume of air. This contact with air accelerates the evaporation rate. The person using the sling psychrometer should face the wind and should shield the instrument as much
possible from the direct rays of the sun. Whirling should not be too rapid because it force might displace the mercury columns in the thermometers. The whirling should be repeated until no further change can be detected in the wet-bulb reading.
DEWPOINT
The dewpoint is computed by using the psychrometer table. For example, a dry-bulb temperature of 60 F and a wet-bulb temperature of 50.5 F. The difference between the two readings is 9.5 F. This difference is called the depression of the wet bulb.
To compute the dewpoint, enter the table with the wet-bulb reading (50.5 F). Read across the top of the table to the proper depression column (9.5 F). Read the dewpoint temperature (42 F) directly from the intersection of the temperature row and the depression column.
SLING PSYCHROMETER - A sling psychrometer sometimes is used to speed up the process of getting accurate wet- and dry-bulb readings. The sling psychrometer can be whirled around to rapidly bring the wet bulb into contact with a great volume of air. This contact with air accelerates the evaporation rate. The person using the sling psychrometer should face the wind and should shield the instrument as much
possible from the direct rays of the sun. Whirling should not be too rapid because it force might displace the mercury columns in the thermometers. The whirling should be repeated until no further change can be detected in the wet-bulb reading.
DEWPOINT
The dewpoint is computed by using the psychrometer table. For example, a dry-bulb temperature of 60 F and a wet-bulb temperature of 50.5 F. The difference between the two readings is 9.5 F. This difference is called the depression of the wet bulb.
To compute the dewpoint, enter the table with the wet-bulb reading (50.5 F). Read across the top of the table to the proper depression column (9.5 F). Read the dewpoint temperature (42 F) directly from the intersection of the temperature row and the depression column.
WEATHER #1
The men who "go down to the sea in ships" fight a continuous close action with the elements that make up the weather. To seafarers the state of the weather is more important than it is to most people ashore. Accurate weather forecasting may not be as vital now as it was in the days of sailing but situations still arise in which the safety of a ship and the lives of her crew depend on the action you take to avoid the full fury of a storm. Even when actual safety is not considered, possible damage to the ship, her gear, and the like, should be minimized by security measures taken well in advance of a approaching storm.
THE ATMOSPHERE
The atmosphere (air) is a mixture of independent gases. Near the surface of the Earth the percentages by volume are approximately 78% nitrogen, 21 % oxygen, 1 % argon, with traces of other gases such as carbon dioxide, hydrogen, neon, and helium. Water vapor, which is found in relatively small but widely varying amounts; 1% of the total atmosphere may be taken as the average. The quantity of water vapor present is much greater in equatorial regions than in polar regions, and greater over the ocean than over land. The atmosphere has definite weight, called atmospheric pressure, and it is measured by an instrument called a barometer.
Large-scale changes in temperature, pressure, and water vapor content of the air cause the changes in weather. Warm air is lighter in weight and can hold more water vapor than cold air. Moist air with a temperature of 50°F is lighter than drier air of the same temperature because water vapor is lighter than air. Cold or heavy air has a tendency to flow toward and supplant warm or lighter air, and as the air begins to move, other forces come into play, making the movement of air masses and weather complex. Temperature, humidity, and atmospheric pressure are all factors in considering the weather. You probably don't need to be told that a thermometer is an instrument for measuring temperature. It is a glass tube of small bore in which either alcohol or mercury expands and contracts with the rise and fall of the temperature of the surrounding medium.
Most thermometers are mercury-filled and practically all of them use the Fahrenheit (F) scale, in which the freezing point of water is 32° and its boiling point is 212°. Temperature in meteorology, sometimes is expressed according to the Celsius (C) (formerly Centigrade) scale, in which the freezing point of water is 0° and its boiling point is 100°.
You might be to convert a Fahrenheit reading to Celsius, or vice versa. If 32 F is equivalent to 0 C, to change a Fahrenheit reading to Celsius you first subtract 32° and then multiply the remainder by 5/9. Say you want to change 41°F to Celsius Subtracting 32° from 41 ° gives 9°. Multiply by 5/9, and you get 45/9, or 5°C. To change from Celsius to Fahrenheit just reverse the procedure. First multiply Celsius temperature by 9/5, then add 32.
A thermometer must be read properly to obtain an accurate result. First, if you handle it, be sure that you do not touch the lower part of the glass containing the alcohol or mercury, because the heat from your body can affect the height of the mercury or column. Make sure that the top column is level with your eyes; otherwise you will be reading a higher or lower graduation than the one actual one.
THE ATMOSPHERE
The atmosphere (air) is a mixture of independent gases. Near the surface of the Earth the percentages by volume are approximately 78% nitrogen, 21 % oxygen, 1 % argon, with traces of other gases such as carbon dioxide, hydrogen, neon, and helium. Water vapor, which is found in relatively small but widely varying amounts; 1% of the total atmosphere may be taken as the average. The quantity of water vapor present is much greater in equatorial regions than in polar regions, and greater over the ocean than over land. The atmosphere has definite weight, called atmospheric pressure, and it is measured by an instrument called a barometer.
Large-scale changes in temperature, pressure, and water vapor content of the air cause the changes in weather. Warm air is lighter in weight and can hold more water vapor than cold air. Moist air with a temperature of 50°F is lighter than drier air of the same temperature because water vapor is lighter than air. Cold or heavy air has a tendency to flow toward and supplant warm or lighter air, and as the air begins to move, other forces come into play, making the movement of air masses and weather complex. Temperature, humidity, and atmospheric pressure are all factors in considering the weather. You probably don't need to be told that a thermometer is an instrument for measuring temperature. It is a glass tube of small bore in which either alcohol or mercury expands and contracts with the rise and fall of the temperature of the surrounding medium.
Most thermometers are mercury-filled and practically all of them use the Fahrenheit (F) scale, in which the freezing point of water is 32° and its boiling point is 212°. Temperature in meteorology, sometimes is expressed according to the Celsius (C) (formerly Centigrade) scale, in which the freezing point of water is 0° and its boiling point is 100°.
You might be to convert a Fahrenheit reading to Celsius, or vice versa. If 32 F is equivalent to 0 C, to change a Fahrenheit reading to Celsius you first subtract 32° and then multiply the remainder by 5/9. Say you want to change 41°F to Celsius Subtracting 32° from 41 ° gives 9°. Multiply by 5/9, and you get 45/9, or 5°C. To change from Celsius to Fahrenheit just reverse the procedure. First multiply Celsius temperature by 9/5, then add 32.
A thermometer must be read properly to obtain an accurate result. First, if you handle it, be sure that you do not touch the lower part of the glass containing the alcohol or mercury, because the heat from your body can affect the height of the mercury or column. Make sure that the top column is level with your eyes; otherwise you will be reading a higher or lower graduation than the one actual one.
BEAUFORT WIND SCALE
BEAUFORT WIND SCALE AND EFFECTS OBSERVED AT SEA
0 - under 1 kt Calm Sea like a mirror.
Beaufort Scale - Wind Speed - Effects Observed At Sea
1 - 1-3 kts Light Air - Ripples with appearance of scales, no foam crests.
2 - 4-6 kts Light Breeze - Small wavelets, crests of glassy appearance, not breaking.
3 - 7-10 kts Gentle Breeze - Large wavelets, crests begin to break, scattered whitecaps.
4 - 11-16 kts Moderate Breeze - . 5 - 1.25 meters high, becoming longer, numerous whitecaps.
5 - 17-21 kts Fresh Breeze - Moderate waves of 1.25 - 2.5 meters taking longer form, many whitecaps, some spray.
6 - 22-27 kts Strong Breeze - Larger waves 2.5 - 4 meters, forming whitecaps everywhere, more spray.
7 - 28-33 kts Near Gale - Sea heaps up, waves 4-6 meters, white foam from breaking waves begins to be blown in streaks.
8 - 34-40 kts Gale -Moderately high (4-6 meter) waves of greater length, edges of crests begin to break into spindrift, foam is blown in well marked streaks.
9 - 41-47 kts Strong Gale -High waves (6 meters) sea begins to roll, dense streaks of foam, spray may reduce visibility.
10 - 48-55 kts Storm - Very high waves (6-9 meters) with overhanging crests, sea takes a white appearance as foam is blown in very dense streaks rolling is heavy and visibility is reduced.
11 - 56-63 kts Violent Storm - Exceptionally high (9-14 meters) waves, sea covered with white foam patches,visibility stilI more reduced.
12 - 64 and over - Hurricane Air filled with foam waves over 14 meters, sea completely white with driving spray, visibility greatly reduced.
0 - under 1 kt Calm Sea like a mirror.
Beaufort Scale - Wind Speed - Effects Observed At Sea
1 - 1-3 kts Light Air - Ripples with appearance of scales, no foam crests.
2 - 4-6 kts Light Breeze - Small wavelets, crests of glassy appearance, not breaking.
3 - 7-10 kts Gentle Breeze - Large wavelets, crests begin to break, scattered whitecaps.
4 - 11-16 kts Moderate Breeze - . 5 - 1.25 meters high, becoming longer, numerous whitecaps.
5 - 17-21 kts Fresh Breeze - Moderate waves of 1.25 - 2.5 meters taking longer form, many whitecaps, some spray.
6 - 22-27 kts Strong Breeze - Larger waves 2.5 - 4 meters, forming whitecaps everywhere, more spray.
7 - 28-33 kts Near Gale - Sea heaps up, waves 4-6 meters, white foam from breaking waves begins to be blown in streaks.
8 - 34-40 kts Gale -Moderately high (4-6 meter) waves of greater length, edges of crests begin to break into spindrift, foam is blown in well marked streaks.
9 - 41-47 kts Strong Gale -High waves (6 meters) sea begins to roll, dense streaks of foam, spray may reduce visibility.
10 - 48-55 kts Storm - Very high waves (6-9 meters) with overhanging crests, sea takes a white appearance as foam is blown in very dense streaks rolling is heavy and visibility is reduced.
11 - 56-63 kts Violent Storm - Exceptionally high (9-14 meters) waves, sea covered with white foam patches,visibility stilI more reduced.
12 - 64 and over - Hurricane Air filled with foam waves over 14 meters, sea completely white with driving spray, visibility greatly reduced.
SEA STATE
ESTIMATING THE WIND AT SEA - The master and mates on board ships at sea can determine the speed of the wind by estimating its Beaufort Force. Through experience ships officers have various methods of estimating this force. The effect of the wind on the observer himself, the ship's rigging, flags, etc, is used as a criterion. Estimates on these give the relative wind which must be corrected for the motion of the ship before an estimate of the true wind speed can be found.
The most common method is the appearance of the sea surface. The state of the sea disturbance, the height of the waves, noticeing of white caps, foam or spray, depends on three factors.
1. The wind speed - The higher the speed of the wind, the greater is the sea desturbance.
2. The duration of the wind - At any point on the sea, the disturbance will increase the longer the wind blows at a given speed, until a maximum state of disturbance is reached.
3. The Fetch - This is the length of the stretch of water over which the wind acts on the sea surface from the same direction. For a given wind speed and duration, the longer the fetch, the greater is the sea disturbance. If the fetch is short, say a few miles a disturbance will be small no matter how great the wind speed is or how long it has been blowing.
There are other factors which can modify the appearance of the sea surface caused by wind alone. These are strong currents, shallow water, swell, precipitation, ice, and wind shifts.
A wind of a given Beaufort Force will produce a appearance of the sea surface provided that it has been blowing for a length of time, and over a long fetch. The effects of currents, shallow water, swell, precipitation, etc., should also be absent. The use of the sea criterion has the advantage that the speed of the ship need not be considered. The mariner observes the sea surface, noting the size of the waves, the white caps, etc., and then finds the criterion which describes the sea surface as he saw it. This criterion is associated with a Beaufort number, for a mean wind speed and range in knots are given. There are other factors besides the duration of the blow and the fetch that affect the appearance of the sea surface and these should be considered if they are present.
The most common method is the appearance of the sea surface. The state of the sea disturbance, the height of the waves, noticeing of white caps, foam or spray, depends on three factors.
1. The wind speed - The higher the speed of the wind, the greater is the sea desturbance.
2. The duration of the wind - At any point on the sea, the disturbance will increase the longer the wind blows at a given speed, until a maximum state of disturbance is reached.
3. The Fetch - This is the length of the stretch of water over which the wind acts on the sea surface from the same direction. For a given wind speed and duration, the longer the fetch, the greater is the sea disturbance. If the fetch is short, say a few miles a disturbance will be small no matter how great the wind speed is or how long it has been blowing.
There are other factors which can modify the appearance of the sea surface caused by wind alone. These are strong currents, shallow water, swell, precipitation, ice, and wind shifts.
A wind of a given Beaufort Force will produce a appearance of the sea surface provided that it has been blowing for a length of time, and over a long fetch. The effects of currents, shallow water, swell, precipitation, etc., should also be absent. The use of the sea criterion has the advantage that the speed of the ship need not be considered. The mariner observes the sea surface, noting the size of the waves, the white caps, etc., and then finds the criterion which describes the sea surface as he saw it. This criterion is associated with a Beaufort number, for a mean wind speed and range in knots are given. There are other factors besides the duration of the blow and the fetch that affect the appearance of the sea surface and these should be considered if they are present.
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