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MARINE AND OCEAN WEATHER GLOSSARY

Cold front -The leading edge of a relatively colder airmass 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 anticyclonic wind shift.

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

Trough - [Trof], 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 KEY 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

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 airmass 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

Freezing spray - Spray in which supercooled water droplets freeze upon contact with exposed objects below the freezing point of water. It usually develops in areas with winds of at least 25 knots.

CONVENTIONS USED WITH WARNINGS FOR EXTRATROPICAL SYSTEMS

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 indicates gale force winds within the next 36 hours. When the term developing gale 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) or greater are "expected" by a certain time period. On surface analysis charts a developing storm indicates storm force winds forecast within the next 36 hours. When the term developing storm 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.

Gale - Refers to an extratropical low or an area of sustained surface winds (one minute) of 34 knots (39 mph) to 47 knots (54 mph).

Storm - Refers to a extratropical low or a area of sustained winds (one minute) in excess of 48 knots (55 mph).

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

CONVENTIONS USED WITH WARNINGS FOR TROPICAL SYSTEMS

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.

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.

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 frequentl report the highest 1/3rd of the waves observed.

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

High Seas - That portion of the Atlantic and Pacific oceans which extends from 20 to 40 nm 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.