Raymarine LIGHTHOUSE 4 Operation Instructions Manual page 246

Term Definition
away. All shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door
damage. Low lying escape routes are cut by rising water 3–5 hours before arrival of the centre of the hurricane. Major damage to
lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of
residential areas on low ground within 5–10 miles (8–16 km) of the shoreline may be required.
Isobar A line on a weather map linking areas with equal air pressure.
Lightning Discharge of static electricity in the atmosphere, usually between the ground and a storm cloud.
Low
Also called a 'depression' this region of low pressure can mean wet weather.
Low Pressure A mass of air that presses down only weakly on the surface of the Earth's surface as it is warmed and it therefore less dense.
Millibar A unit used to measure atmospheric pressure.
Occluded Front An area where warm air is pushed upwards as a cold front overtakes a warm front and pushes underneath it.
Precipitation Moisture that is released from the atmosphere as rain, drizzle, hail, sleet or snow, as well as dew and fog.
Pressure Centre A region of high or low pressure.
Squall line A non-frontal band, or line, of thunderstorms.
Super typhoon A typhoon that reaches maximum sustained 1 minute surface winds of at least 65 m/s (130 kt, 150 mph). This is the equivalent of a
strong category 4 or 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.
Tornado A funnel shaped whirlwind which extends to the ground from storm clouds.
Tropical cyclone
A low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern
Hemisphere, a counterclockwise circulation of winds near the earth's surface.
Tropical depression An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 mph
(33 kt) or less.
Tropical storm An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39–73 mph
(34–63 kt).
Tropics An area on the Earth's surface that lies between 30º north and 30º south of the equator.
Trough An elongated area of relatively low atmospheric pressure, usually extending from the centre of a low pressure region.
Typhoon The name for a tropical storm originating in the Pacific Ocean, usually the China Sea. They are basically the same as the hurricanes
of the Atlantic Ocean and the cyclones of the Bay of Bengal.
Wave cyclone A storm or low pressure centre that moves along a front.
Wave period The period is the time gap between successive waves and the longer the period the slower the waves travel.
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