Terminology and Weather Symbols

Cold front -The leading edge of a relatively colder air mass which separates two air masses in which the gradients of temperature and moisture are maximized. In the northern hemisphere winds ahead of the front will be southwest and shift into the northwest with frontal passage.

Frontogenesis -The formation of a front occurs when two adjacent air masses with different densities and temperatures meet and strengthen the discontinuity between the air masses. It occurs most frequently over continental land areas such as over the Eastern US when the air mass moves out over the ocean. It is the opposite of frontolysis.

Frontolysis -The weakening or dissipation of a front occurs when two adjacent air masses lose contrasting properties such as the density and temperature. It is the opposite of frontogenesis.

Occluded front - The union of two fronts, formed as a cold front overtakes a warm front or quasi-stationary front refers to a cold front occlusion. When a warm front overtakes a cold front or quasi-stationary front the process is termed a warm front occlusion. These processes lead to the dissipation of the front in which there is no gradient in temperature and moisture.

Ridge - an elongated area of relatively high pressure that is typically associated with a anti-cyclonic wind shift.

Stationary front - A front that has not moved appreciably from its previous analyzed position.

Trough - An elongated area of relatively low pressure that is typically associated with a cyclonic wind shift.

Warm front - The leading edge of a relatively warmer surface air mass which separates two distinctly different air masses. The gradients of temperature and moisture are maximized in the frontal zone. Ahead of a typical warm front in the northern hemisphere, winds are from the southeast and behind the front winds will shift to the southwest.

LOW & HIGH PRESSURE SYSTEMS AND MISCELLANEOUS TERMS USED

Low pressure with a number such as 99 means 999 mb and with 03 means 1003 mb. High pressure with a number such as 25 means 1025 mb.

Extratropical low - A low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity.

Low pressure - An area of low pressure identified with counterclockwise circulation in the northern hemisphere and clockwise in the southern hemisphere. Also, defined as a cyclone.

High pressure - An area of higher pressure identified with a clockwise circulation in the northern hemisphere and a counterclockwise circulation in the southern hemisphere. Also, defined as an anticyclone.

New - The term "NEW" may be used in lieu of a forecast track position of a high or low pressure center when the center is expected to form by a specific time. For example, a surface analysis may depict a 24-hour position of a new low pressure center with an "X" at the 24-hour position followed by the term "NEW", the date and time in UTC which indicates the low is expected to form by 24 hours.

Rapidly intensifying - Indicates an expected rapid intensification of a cyclone with surface pressure expected to fall by at least 24 millibar (mb) within 24 hours.

Squall - A sudden wind increase characterized by a duration of minutes and followed by a sudden decrease in winds.

Fog -Over the marine environment the term fog refers to visibility greater than or equal to 1/2 NM and less than 3 NM. Fog is the visible aggregate of minute water droplets suspended in the atmosphere near the surface.

Dense fog -Over the marine environment the term dense fog refers to visibility less than 1/2 NM. Fog is the visible aggregate of minute water droplets suspended in the atmosphere near the surface. Usually dense fog occurs when air that is lying over a warmer surface such as the Gulf Stream is advected across a colder water surface and the lower layer of the air mass is cooled below its dew point.

Sea fog - Common advection fog caused by transport of moist air over a cold body of water.
FREEZING SPRAY

CONVENTIONS USED WITH WARNINGS FOR EXTRATROPICAL SYSTEMS

Complex gale/storm -An area in which gale/storm force winds are forecast or are occurring, but in which more than one center is the generating these winds.

Developing Gale -Refers to an extratropical low or an area in which gale force winds of 34 knots (39 mph) to 47 knots (54 mph) are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING GALE" label indicates gale force winds within the next 24 hours. When the label is used on the 48 hour surface forecast and 96 hour surface forecast charts, gale force winds are expected to develop by 72 hours and 120 hours, respectively.

Developing Storm -Refers to an extratropical low or an area in which storm force winds of 48 knots (55 mph) to 63 knots (73 mph) are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING STORM" label indicates storm force winds forecast within the next 24 hours. When the label is used on the 48 hour surface and 96 hour surface charts, storm force winds are expected to develop by 72 hours and 120 hours, respectively.

Developing Hurricane Force -Refers to an extratropical low or an area in which hurricane force winds of 64 knots (74 mph) or higher are "expected" by a certain time period. On surface analysis charts, a "DEVELOPING HURRICANE FORCE" label indicates hurricane force winds forecast within the next 24 hours. When the label is used on the 48 hour surface and 96 hour surface charts, hurricane force winds are expected to develop by 72 hours and 120 hours, respectively.

Gale - Refers to an extratropical low or an area of sustained surface winds (averaged over a ten minute period, momentary gusts may be higher) of 34 knots (39 mph) to 47 knots (54 mph).

Storm - Refers to a extratropical low or an area of sustained winds (averaged over a ten minute period, momentary gusts may be higher) of 48 knots (55 mph) to 63 knots (73 mph).

Hurricane Force - Refers to a extratropical low or an area of sustained winds (averaged over a ten minute period, momentary gusts may be higher) in excess of 64 knots or higher(74 mph).

Small Craft Advisory - Refers to areas within the coastal waters with sustained winds of 18 knots (21 mph) to 33 knots (38 mph).

Heavy Freezing Spray -Spray in which supercooled water droplets freeze upon contact with exposed objects below the freezing point of water at the rate of greater than 2 cm/hr. It usually develops in areas with winds of at least 25knots.

CONVENTIONS USED WITH WARNINGS FOR TROPICAL SYSTEMS

Hurricane - A tropical cyclone with closed contours, a strong and very pronounced circulation, and one minute maximum sustained surface winds 64 knots (74 mph) or greater. A system is called a hurricane over the North Atlantic, Gulf of Mexico, North Pacific E of the dateline, and the South Pacific E of 160E.

Intertropical Convergence Zone - (ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

Tropical cyclone - A non-frontal, warm-core, low pressure system of synoptic scale, developing over tropical or subtropical waters with definite organized convection (thunderstorms) and a well defined surface wind circulation.

Tropical depression - A tropical cyclone with one or more closed isobars and a one minute max sustained surface wind of less than 34 knots (39 mph).

Tropical storm - A tropical cyclone with closed isobars and a one minute max sustained surface wind of 34 knots (39 mph) to 63 knots (73 mph).

Typhoon - Same as a hurricane with exception of geographical area. A tropical cyclone with closed contours, a strong and very pronounced circulation, and one minute maximum sustained surface winds of 64 knots (74 mph) or greater. A system is defined as a typhoon over the North Pacific W of the dateline.

NOTE: It can be difficult to determine the central pressures of tropical depressions, tropical storms, and hurricanes/typhoons and at times no estimates or measurements is provided by a hurricane or typhoon specialist. An estimate of central pressure may be provided over the Atlantic. Otherwise an XXX is used in place of actual or estimated pressures associated with these systems and an XX is used for forecast central pressure.

SEAS
Combined seas -The combination of both wind waves and swell which is generally referred to as "seas".

Primary swell direction - Prevailing direction of swell propagation.

Significant wave height - The average height (trough to crest) of the 1/3rd highest waves. An experienced observer will most frequently report the highest 1/3rd of the waves observed. The generation of waves on water results not in a single wave height but in a spectrum of waves distributed from the smallest capillary waves to larger waves. Within this spectrum there is a finite possibility of each of the wave heights to occur with the largest waves being the least likely. The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The random nature of waves implies that individual waves can be substantially higher than the significant wave height. In fact, observations and theory show that the highest individual waves in a typical storm with typical duration to be approximately two times the significant wave height. Some reported rogue waves are well within this factor of two envelope. Waves higher than roughly twice the significant wave height fall into the category of extreme or rogue waves.
Swell - Wind waves that have moved out of their fetch or wind generation area. Waves generated by swell exhibit a regular and longer period than wind waves.

MISCELLANEOUS TERMINOLOGY
Coastal Waters - Includes the area from a line approximating the mean high water along the mainland or island as far out as sixty nautical miles including the bays, harbors and sounds.

High Seas - That portion of the Atlantic and Pacific oceans which extends off the Western and Eastern US coasts and extends to 35W in the Atlantic ocean and to 160E in the Pacific Ocean. The area includes both the coastal and offshore waters.

Offshore waters - That portion of oceans, gulfs, and seas beyond coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by a specific latitude and longitude points.

A Sailors Knife or a Pointless Story

Thought I'd give you a break from Celestial Navigation and Terrestrial Navigation and pass this interesting bit of information on the "Sailors Knife".

The lore involves the points on sailor's knives, or rather the lack there of. Sailor's rope knives traditionally have no points. As far as I can discover, there is no functional reason for this. After all, a knife with a point will cut line just as well as a knife without a point. The story I have seen and heard holds that, on most ships, sailor's were not allowed to carry pointed knives, to discourage them from stabbing each other when the strains of a long voyage under sail got to be too much for some of them to bear.

A friend of mine found this book "The Making of a Sailor", or Sea Life Aboard a Yankee Square-Rigger, by one Frederick Pease Harlow. The book was published in 1928, but recounts the author's experiences at sea as a boy about six decades earlier. On pages 90-93 Harlow describes the outset of a voyage to Melbourne, Australia. The first and second mates are choosing up the port and starboard watches for the voyage from the newly hired crew. The first mate, naturally, chooses first. I choose you, What's your name? Hans, sir, he replied. Let me have your knife, requested the mate, who stood on top of the main hatch, with a hammer in his hand, which he was all the while turning and twisting. Upon receiving the sheath-knife, which is as much a part of sailor's uniform as his overalls and is always carried in a sheath or scabbard, hanging from a strap about the waist and back of the hips, where it is handy for cutting line, for a sailor is not dressed without his knife, the mate put the point of the knife across the iron band on top of the combings of the hatch and struck a sharp blow with the hammer, breaking off the point.

You probably didn't have the mate, in your last ship, break the point off your knife. said Mr. Burris. But I always keep a ship sweet and clean by seeing that every knife aboard the ship has no point. This is for your own protection. If you get into a fight with a shipmate you know you can't stick him with your knife or he, you. Knowing this you both will fight like men and use your fists, the weapons God has given you to fight with. Returning the knife to Hans, he was told to stand over to the port rail. The second mate then chose Jim Dunn. His knife was broken in the same manner and he was told to stand over to the starboard rail. The selection and knife breaking continued until the 14 men and two boys were all divided into two watches.

Going forward, I followed the men into the forecastle. A big Irishman by the name of O'Rourke, was much put out by having his knife point broken and was saying as I entered, I don't know phat ye's fellers tink about it, an I haven't been to sea for ten years, but fifteen years ago I sailed in the (Clipper) ships Live Yankee (built 1853, wrecked 1861) and the Phantom and if ships of that caliber can make a vi age widout breakin our knives, why in the --- does an old tub like this wan want to do it?

The answer to seaman O'Rourke's question very likely lay in a Federal maritime law enacted in 1866, during the ten year interval when he had been ashore. First Mate Burris had in fact not gone nearly as far as the new law required. The Act of Congress of July 27, 1866, which is still the law (Title 46 u.s. Code, Section 710) reads:

Carrying sheath knives. No seaman in the merchant service shall wear any sheath knife on shipboard. It shall be the duty of the master of any vessel registered, enrolled, or licensed under the laws of the United States, and of the person entering into contract for the employment of a seaman upon any such vessel, to inform every person offering to ship himself of the provisions of this section, and to require his compliance there with, under a penalty of $50 for each omission.

Note that the penalty for a violation falls not on the sailor, but on the captain or the owner's agent. As far as I know, U.S. Navy sailors at this time, and up until World War II, were only allowed to carry folding knives, and those had to have square tipped blades.