Glossary of weather terms

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

A prediction which informs us what the weather is going to be like within a specific location.

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 74–95 mph (64–82 kt or 119–153 km/hr). Storm surge generally 4–5 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 96–110 mph (83–95 kt or 154–177 km/hr). Storm surge generally 6–8 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 2–4 hours before arrival of the hurricane centre Small craft in unprotected anchorages break moorings.
• Category 3 — Winds 111–130 mph (96–113 kt or 178–209 km/hr). Storm surge generally 9–12 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 3–5 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 131–155 mph (114–135 kt or 210–249 km/hr). Storm surge generally 13–18 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 3–5 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 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.