Raymarine e7D, Installation And Operation Handbook, Page: 188 / 270

16.10 Glossary of weather terms
Term Definition
Cold front The boundary between two different air masses where cold air pushes warm air out of the way and brings colder weather.
Cyclone A large area of low atmospheric pressure, characterized by inward spiralling winds. A “low” also called a “depression”. Also the name
used for a hurricane in the Indian Ocean and Western Pacific.
Depression An area of low pressure. Also called a cyclone.
Dry line A region where there is a strong gradient in dew point temperatures. It is often found in a region where strong thunderstorms develop.
Forecast Something that tells us what the weather is probably going to be like.
Front
The boundary between two masses of air with different temperatures (i.e. a mass of cold air and a mass of warm air).
High Also known as an ’anticyclone’ an area of high atmospheric pressure with a system of winds rotating outwards. This usually means dry
weather. It is the opposite of a ’low’.
High Pressure A mass of air that presses down strongly on the surface of the Earth because it is being cooled and is therefore more dense.
Hurricane
A violent, spiralling storm that forms over the Atlantic Ocean, with winds over 120 kph. Such storms usually have a lifespan of several days.
Also known as a typhoon or tropical cyclone. There are 5 levels of hurricane:
• Category 1 — Winds 7495 mph (6482 kt or 119153 km/hr). Storm surge generally 45 ft above normal. No real damage to building
structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also,
some coastal road flooding and minor pier damage.
• Category 2 — Winds 96110 mph (8395 kt or 154177 km/hr). Storm surge generally 68 feet above normal. Some roofing material, door,
and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable dam age to
mobile homes, poorly constructed signs, and piers. Coastal and low lying escape routes flood 24 hours before arrival of the hurricane
centre Small craft in unprotected anchorages break moorings.
• Category 3 — Winds 111130 mph (96113 kt or 178209 km/hr). Storm surge generally 912 ft above normal. Some structural damage to
small residences and utility buildings with a minor amount of curtain wall failures. Damage to shrubbery and trees with foliage blown off
trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Low lying escape routes are cut by rising
water 35 hours before arrival of the centre of the hurricane. Flooding near the coast destroys smaller structures with larger structures
damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13
km) or more. Evacuation of low lying residences with several blocks of the shoreline may be required.
• Category 4 — Winds 131155 mph (114135 kt or 210249 km/hr). Storm surge generally 1318 ft above normal. More extensive curtain wall
failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction
of mobile homes. Extensive damage to doors and windows. Low lying escape routes may be cut by rising water 35 hours before arrival of
the centre of the hurricane. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be
flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km).
• Category 5 — Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18 ft above normal. Complete roof
failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or 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 35 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 510 miles (816 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 3973 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.
188 e7 / e7D / e95 / e97 / e125 / e127 / c95 / c97 / c125 / c127