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Weather Glossary


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A


ABSOLUTE HUMIDITY

A type of humidity that considers the mass of water vapour present per unit volume of space. Also considered as the density of the water vapour. It is usually expressed in grams per cubic meter.
 

AIR MASSES 

Air masses are large parcels of air that have approximately uniform characteristics of temperature and moisture. Air masses acquire their characteristics by remaining over a land or ocean surface for a considerable period of time.
 

ALTOCUMULUS

Clouds characterised by white, grey or both patch, sheet or layers composed of rounded masses or rolls.
Related Term: Cloud
 

ALTOSTRATUS

Clouds characterised by greyish or bluish cloud sheets with parts thin enough to reveal the sun at least vaguely.
Related Term: Cloud
 

ANEMOMETER

An instrument that measures wind speed in kilometres per hour.
 

ATMOSPHERE

The envelope of air surrounding the earth and bound to it by gravitation. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.
 

ATMOSPHERIC PRESSURE

The pressure exerted by the atmosphere at a given point. It is measured in hectapascals.
Related Term: Barometric pressure
 

AUTUMN

The season which occurs as the sun approaches the winter solstice, and characterised by decreasing temperatures in the mid-latitudes. This refers to the months of September, October, and November in the Northern Hemisphere and the months of March, April, and May in the Southern Hemisphere.
Related term: Season, Spring, Summer, Winter


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B


BAROMETER

A barometer is an instrument for measuring the pressure of the air, due to the weight of the column of air above it. The measurement unit of atmospheric pressure is hectopascal (hPa)
Related Terms: hectapascals
 

BAROMETRIC PRESSURE

The pressure exerted by the atmosphere at a given point. It is measured in hectapascals.
Related Term: Atmospheric pressure
 

BEAUFORT SCALE

A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

The Bureau of Meteorology uses a similar scale for forecasts and warnings, although the terms 'near gale', 'strong gale' and 'violent storm' are not normally used;
Beaufort scale number Descriptive term Units in km/h Units in knots Description on Land Description at Sea
0 Calm 0 0 Smoke rises vertically Sea like a mirror.
1-3 Light winds 19 km/h or less 10 knots or less Wind felt on face; leaves rustle; ordinary vanes moved by wind. Small wavelets, ripples formed but do not break: A glassy appearance maintained. 
4 Moderate winds 20 - 29 km/h 11-16 knots Raises dust and loose paper; small branches are moved. Small waves - becoming longer; fairly frequent white horses.
5 Fresh winds 30 - 39 km/h 17-21 knots Small trees in leaf begin to sway; crested wavelets form on inland waters Moderate waves, taking a more pronounced long form; many white horses are formed - a chance of some spray
6 Strong winds 40 - 50 km/h 22-27 knots Large branches in motion; whistling heard in telephone wires; umbrellas used with difficulty.  Large waves begin to form; the white foam crests are more extensive with probably some spray
7 Near gale 51 - 62 km/h 28-33 knots Whole trees in motion; inconvenience felt when walking against wind. Sea heaps up and white foam from breaking waves begins to be blown in streaks along direction of wind.
8 Gale 63 - 75 km/h 34-40 knots Twigs break off trees; progress generally impeded. Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks along the direction of the wind.
9 Strong gale 76 - 87 km/h 41-47 knots Slight structural damage occurs -roofing dislodged; larger branches break off. High waves; dense streaks of foam; crests of waves begin to topple, tumble and roll over; spray may affect visibility.
10 Storm 88 - 102 km/h 48-55 knots Seldom experienced inland; trees uprooted; considerable structural damage. Very high waves with long overhanging crests; the resulting foam in great patches is blown in dense white streaks; the surface of the sea takes on a white appearance; the tumbling of the sea becomes heavy with visibility affected. 
11 Violent storm 103 -117 km/h 56-63 knots Very rarely experienced - widespread damage Exceptionally high waves; small and medium sized ships occasionally lost from view behind waves; the sea is completely covered with long white patches of foam; the edges of wave crests are blown into froth.
12+ Hurricane 118 km/h or more 64 knots or more The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected

 

Related Term: Anemometer
 

BIG BANG

A theory which explains the creation of the universe in terms of an explosion or "Big Bang".
 

BLACK DWARF

A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
Related terms: Star, Red Dwarf, Yellow Dwarf, Red Giant
 

BROADSCALE FLOODING

When widespread rain, causes water to cover a large area, creeks and rivers overflow causing widespread flooding.
Related terms: Floods, severe thunderstorms
 

BUSHFIRES

Bushfires are a natural hazard that places areas in risk of extreme fire damger. There are two types of bushfires – forest fires and grass fires.
Related Term: Wildfires


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C


COLD FRONT

The boundary, or 'leading edge' of where a cold airmass meets warmer air. Cold fronts are most common through the mid-latitudes, for areas such as Perth, Melbourne, Adelaide and Hobart. 

Cold fronts can generate strong winds and also trigger shower and thunderstorm activity. Usually a shift in wind direction is involved, bringing in a new, substantially cooler airmass to a region.

Temperature drops of 20 degrees can occur in summer for cities such as Melbourne as these cold fronts pass through.

Often a hot continental airmass will be drawn across the city by northerly winds ahead of a cold front but then a shift in wind direction to the southwest will deliver a new, cooler airmass originating over the Southern Ocean.

CONVERGENCE ZONE

A region of converging winds where rainfall can be enhanced. For example, a region of converging northeast and southeast winds along a trough line on the Queensland coast can amplify rainfall over one area, especially if the area of convergence is slow moving. Other factors such as atmopsheric instability and moisture levels are also important in such rainfall events. 

CHAPARRAL

Vegetation that contain highly flammable oils that explode and promote bushfires.
Related Terms: Eucalypt Forest, Maquis
 

CIRROCUMULUS

A cloud characterised by thin, white patch, sheet or layer of cloud without shading, composed of grains or ripples.
Related Term: Cloud
 

CIRROSTRATUS

A cloud characterised by transparent whitish cloud veil, of fibrous (hair-like) or smooth appearance totally or partially covering the sky and may cause a halo phenomenon.
Related Term: Cloud
 

CIRRUS

A cloud characterised by wispy high level cloud, usually above 6000metres, dominated by ice crystals. They are often a sign of approaching bad weather.
Related Term: Cloud
 

CLIMATE

The average daily and seasonal weather events over a long period of time. Related terms: Climatic Zone, Tropical, Temperate, Polar 
 

CLIMATE CHANGE

This refers to changes in global temperatures over time. It includes ice ages and the warmer inter glacial periods. Global warming is a key feature of climate change in the 20th Century.
 

CLIMATIC ZONE

An area of the globe which has a similar climate. For example, the tropical zone being hot and wet and found between the Tropics of Cancer and Capricorn (23.5 degrees N&S)
Related Terms: Tropical, Temperate, Polar
 

CLOUD

A visible collection of minute particles of matter, such as water droplets and/or ice crystals, in the free air. A cloud forms in the atmosphere as a result of condensation of water vapour. Condensation nuclei, such as in smoke or dust particles, form a surface on which water vapour can condense.
Related Terms: Altostratus, Altocumulus, Cirrus, Cirrostratus, Cumulus, Cumulonimbus, Stratocumulus, Stratus, Nimbostratus.
 

CLOUD SEEDING

Cloud seeding is a technique for increasing rainfall or precipitation using naturally occurring clouds. Cloud seeding is only effective if suitable clouds are present.
Related Term: Cloud
 

CONDENSATION

The process by which water vapour undergoes a change in state from a gas to a liquid. It is the opposite physical process of evaporation.
Related Terms: Evaporation, Precipitation, Transpiration
 

CONTINENTAL AIR MASS

An air mass that originates over land. These air masses are dry and may originate in polar or tropical areas.
Related Term: Air Mass
 

CONVECTION RAINFALL

Cloud seeding is a technique for increasing rainfall or precipitation using naturally occurring clouds. Cloud seeding is only effective if suitable clouds are present.
Related Term: Cloud
 

CONTINENTAL DRIFT

Scientists believe that until 180 million years ago, there was one large global continent that eventually split apart and drifted northward. This caused changes to the oceans that redistribute heat throughout the globe, while wind systems were also altered.
 

CORAL BLEACHING

Coral bleaching is the loss of colour from corals under stressful environmental conditions.
 

CUMULONIMBUS

Heavy and dense cloud, with considerable vertical extent in the form of a mountain or huge towers. At least part of its upper potion is smooth. Fibrous or striated and nearly always flattened and often spreads out in the shape of an anvil or large plume. Under the base of these clouds there are frequently low ragged clouds.
Related Term: Cloud
 

CUMULUS

A cloud characterised by fluffy clouds that have a low level base, but can extend to great heights. They are caused by rising air currents and may be associated with showers and rain.
Related Term: Cloud
 

CYCLONE 'EYE'

The dense cloud ring where winds are light and the sky is clear.
Related Terms: Tropical Cyclone


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D


DAY

The period of time defined by the earth's rotation on its axis. It represents the time needed for one complete revolution of the earth about its own axis. It is approximately equal to 23 hours, 56 minutes, and 4.09 seconds.
Related term: Night
 

DECIDUOUS

When plants lose their leaves and are dormant (do not grow) during the winter months.
Related Term: Hibernate/Hibernation, Seasons
 

DIURNAL

Patterns in weather that are part of a day-night cycle, such as minumimum temperatures usually occurring early in the morning, before maximum temperatures are reached in the afternoon. Sea and land breezes are an example of a diurnal pattern.
 

DRIFTING BUOYS

Automatic weather stations that record important information about ocean temperatures that assist in the prediction of El Nino events and atmospheric pressure.
 

DROUGHT

A period of below average rainfall that disrupts the normal way of life.
Related Term: rainfall deficiency, rainfall variability.


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E


EARTH

The earth is one of nine planets that orbit around the sun. The earth is the only planet which supports human life.
 

EARTH'S AXIS

An imaginary line joining the North Pole and the South Pole through the centre of the earth on which the earth spins once every 24 hours. The earth is tilted on its axis approximately 23.5 degrees from the vertical.
 

EDDY

A small disturbance of wind in a large wind flow, which can produce turbulent conditions.
 

EL NINO

El Niño was named by Peruvian fisherman who noticed warmer than usual waters around Christmas. There is usually cool and nutrient rich waters along this section of coast due to wind-driven upwelling but in El Niño years, the warmer waters would decrease the fish-stocks for the locals. Due to this Christmas time onset, it was named El Niño, meaning "the boy child or Christ child".

Although originally named for a local warming of the ocean near the coast of Peru in South America, "El Niño" now refers to a sustained warming over a large part of the central and eastern tropical Pacific Ocean.

Combined with this warming are changes in the atmosphere that affect weather patterns across much of the Pacific Basin, including Australia. These altered weather patterns often help promote further warming of the ocean because of the changes they cause in ocean currents.

There is a strong correlation between the El Niño pattern and below average rainfall in northern and eastern Australia.
 
Related Terms: La Nina, Southern Oscillation Index, Upwelling
 

EQUATORIAL AIR MASS

An air mass that originates in the equatorial area. It is hot and wet.
Related Terms: Air Mass
 

EQUINOX

The time of the year when the sun is vertically overhead at the equator. When this occurs when day and night are of equal length. This occurs on 22 March and 22 September (or one day either side).
Related Terms: Solstice, Seasons, Earth's Rotation/Orbit
 

EUCALYPT FOREST

Vegetation that requires fire to regenerate. The volatile eucalypt oils can assist the spread of fires in these forests.
Related Terms: Chaparral, Marquis
 

EVAPORATION

The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapour. It is the opposite physical process of condensation.
Related Terms: Condensation, Precipitation, Transpiration.
 

EXOSPHERE

This region is considered the very outer limits of the earth's atmosphere. It is estimated to be some 400 plus miles (640 kilometres) above the surface.
Related Terms: Troposphere, Stratosphere, Mesosphere, Thermosphere


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F


FLASH FLOODS

Flash flooding occurs when relatively short, intense bursts of rainfall, often from thunderstorms.
Related Term: Floods, Broadscale Floods
 

FLOODS

Floods occur when water covers land, which is normally dry. They may result from prolonged or very heavy rainfall, severe thunderstorms, monsoonal (wet season) rains in the tropics, or tropical cyclones.
Related Term: Flash Floods, Broadscale Floods
 

FLOOD WARNING

The rating of the severity of a flood.
Minor Flooding - minor flooding causes inconvenience such as closure of minor local roads and low bridges.
Moderate Flooding - low lying areas will be inundated requiring removal of stock, equipment and evacuation of isolated homes. Main traffic bridges may be covered.
Major Flooding - higher areas will be inundated with isolation of towns and properties, causing extensive damage.
Related Term: Floods
 

FOG

Condensation of water vapour in the air that occurs close to the ground surface.
 

FOSSIL FUELS

Non renewable resources used to generate energy, especially electricity. Fossil fuels add carbon dioxide to the atmosphere causing an acceleration of the Greenhouse Effect.
 

FRONTAL RAINFALL

When 2 air masses of different temperatures meet it is called a front. The surface between these air masses is called a frontal surface. When air masses move across a region the previous air mass will be displaced.
Related Term: Convectional Rainfall, Orographic Rainfall 


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G


GALES

A wind with average speeds from 51 to 87 km/hour.
Related Terms: Winds
 

GLOBAL WARMING

Gases within the atmosphere that reduce heat loss to the atmosphere. These gases include carbon dioxide and methane and when present in large mounts can cause global warming.
 

GOLDILOCKS PLANET

This refers to the temperature of the earth and its ability to support life relative to Venus and Mars. “Venus is too hot, Mars is too cold and Earth is just right”.
 

GREENHOUSE EFFECT

The presence of carbon dioxide and water vapour in the earth’s lower atmosphere which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space, keeping the Earth warm as in a blanket.
 

GREENHOUSE GASES

A mixture of gases in the atmosphere that are capable of retaining heat reflected back into the atmosphere. Carbon dioxide and methane are two of these gases.


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H


HALOES

When ice crystals form in the upper atmosphere they are associated with rings around the Sun and can come singly or in pairs.
 

HEAT BALANCE

The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).
 

HEAT CRAMP

Muscular pains and spasms caused by heavy exertion is and early indication of heat stress. Related Topics: Heat Exhaustion, Heat Stroke, Heat Wave
 

HEAT EXHAUSTION

Occurs when body fluids are lost through heavy sweating. Blood flow to the skin increases, causing a decrease of flow to the vital organs. This causes a mild shock with symptoms of cold, clammy and pale skin, together with fainting and vomiting. If not treated the victim may suffer heat stroke.
Related Topics: Heat Cramp, Heat Stroke, Heat Wave
 

HEAT STROKE

Is life threatening. The victim's temperature control system, which produces sweating to cool the body, stops working. The body temperature may exceed 40.6ºC potentially causing brain damage and death if the body is not cooled quickly.
Related Topics: Heat Cramp, Heat Exhaustion, Heat Wave
 

HEAT WAVE

These occur due to a period of prolonged excessive heat.
Related Topics: Heat Cramp, Heat Stroke, Heat Exhaustion
 

HECTAPASCALS

A measurement of air pressure recorded by a barometer.
Related Topics: Air Pressure, Barometer
 

HIBERNATE/ HIBERNATION

The seasonal impact of climate on some animals. Hibernation occurs in winter. For example, bears go to sleep through the winter.
Related Topics: Deciduous, Seasons
 

HUMIDITY

The amount of water vapour in the air measured in absolute terms or relative to the temperature. Absolute Humidity (corresponding to the quality of waterin weight) contained at a specific time in a certain volume of air). Relative Humidity ,(percentage corresponding to the ratio between the existing water weight and the maximum water weight.
 

HUMIDEX INDEX

This index is calculated from the combination of measured relative humidity and the effective temperature: the humidex or comfort index allows evaluating the comfort of your environment.
Related Topics: Humidity, Hygrometry
 

HYGROMETRY

The humidity measurement is hygrometry and its measurement unit is the percentage (%) of the relative humidity of the air.


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I


ICE AGES

Periods of glaciation where global temperatures have declined. The maximum period of glaciation occurred during the Pleistocene period that dates back 1 million years.

INDIAN OCEAN DIPOLE

The Indian Ocean Dipole (IOD) is an oscillation of sea surface temperatures through the western part of the Indian Ocean near Indonesia and northwest Australia.

A negative phase of the IOD sees warmer than usual sea surface temperatures in this western region, which in turn leads to wetter than usual conditions for adjacent land areas in the northwest of Australia, particularly around the Kimberley and Pilbara districts of Western Australia.

The reverse is true for a positive IOD, with cooler than usual waters off the northwest coast and reduced rainfall for adjacent land areas.
 

INSOLATION

Incoming solar radiation that reaches the earth as heat or light. It includes ultra violet light, visible light and infra red waves.


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L


LA NINA

Translates from Spanish as "the girl-child". The term La Niña refers to the extensive cooling of the central and eastern Pacific Ocean. In Australia (particularly eastern Australia), La Niña events are associated with increased probability of wetter conditions. A La Niña event is sometimes called an anti-ENSO (anti-El Niño-Southern Oscillation) event.

There is usually an enhancement of the southeast trade winds across the Pacific Ocean in a La Niña year, while an earlier onshore of the tropical cyclone season often occurs in Australian waters. There is also typically a 50% increase in tropical cyclone formation compared with a neutral year (neither La Niña or El Niño).

Related Terms: El Nino, Southern Oscillation Index
 

LIGHTNING

Is the flash associated with thunder clouds when the cloud becomes electrically charged. The charge may pass within the cloud, between clouds or between the earth and the cloud.


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M


MARITIME AIRMASS

An airmass originating over the ocean, bringing moist conditions. Maritime air masses can be tropical or polar in origin.
Related Term: Airmass
 

MARQUIS

Vegetation that requires fire to regenerate. The volatile oils in the herbs in thse areas can assist the spread of fires.
Related Topics: Eucalypt Forests, Chaparral
 

MESOSPHERE

The layer of the Earth’s atmosphere between the Stratosphere and Thermosphere. In this layer temperatures decrease rapidly with height.
 

METEOROLOGY

The science and the study of the atmosphere and atmospheric phenomena. A meteorologist studies the atmosphere and atmospheric phenomena.
 

MONSOON

The seasonal reversal of winds caused by changes in pressure systems due to the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season.


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N


NIGHT

The period when the earth’s is facing away from the sun due spinning on its axis. It occurs between sunset and sunrise.
Related term: Day
 

NIMBOSTRATUS

Grey cloud layer, often dark, the appearance of which is more or less diffuse by more or less continuously falling rain or snow. It is thick enough to blot out the sun.
Related Term: Cloud


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O


ONSHORE WINDS

Winds that move from the ocean to the land. They are often associated with cool breezes and rainfall.
 

ORBIT

The path of an object around another object. For example, the Earth orbits around the sun once every 365 ¼ days and travelling through space at about 30km/second.
Related term: Rotation
 

OROGRAPHIC RAINFALL

Orographic rainfall is caused when air is forced to rise over a mountain barrier.
Related Term: Convectional Rainfall
 

OZONE LAYER

An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, 15 to 20 kilometers above the earth's surface.


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P


PLANET

A spherical body which revolve around the sun in elliptical paths known as orbits. Planets shine only by reflected light, unlike the stars and sun. The earth is one of eight planets in our solar system. They are (in order closest from the sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Related term: Orbit, Earth
 

POLAR/ POLES

The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 66 1/2 and 90 degrees latitude, both North and South of the equator and experiences very cold conditions.
Related Terms: Climatic Zone, Tropical Zone, Polar Zone.
 

PRESSURE SYSTEM

Areas where there is low or high levels of barometric pressure. These measures of pressure are used to determine weather characteristics.
Related Terms: High Pressure, Low Pressure


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R


RAINBOW

Rainbows are caused by refraction and each colour is bent at a slightly different angle resulting in the curved bow shape. A rainbow can be described as sunlight spread out into its spectrum of colours directed to our eyes by water droplets.
 

RAINSHADOW

The dry leeward side of a mountain caused by warming, sinking air. This causes dry conditions while the windward side is wet due to the uplift and cooling causing orographic rainfall.
Related Term: Orographic Rainfall
 

RAINFALL DEFICIENCY

A rainfall deficiency exists when the amount of rainfall is below the annual average rainfall. A severe deficiency exists in a district when rainfall for three months or more is in the lowest 5 per cent of records. A serious deficiency lies in the next lowest 5 per cent i.e. lowest 5 per cent to 10 per cent of historical records for a three month or longer period.
 

RAINFALL VARIABILITY

The variation in rainfall from one year to the next (or month or season)
 

RELATIVE HUMIDITY

The amount of water vapour in the air is measured as a percentage. For example, when the RH is 100% the air is said to be saturated and will not absorb any more moisture.
 

ROTATION

The spinning of a body, such as the earth, on its own axis. One rotation of the earth occurs once in approximately 24 hours.
Related term: Orbit


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S


SEA BREEZE

In coastal areas, as the land heats faster than the ocean, it expands more quickly than the air over the ocean. This causes the air to rise and low pressure systems develop. The cooler high pressure air from the ocean moves in to fill the gap. This causes a local circulation of air between the land and sea that moderates temperatures in coastal areas.
 

SEASONS

The differences in temperature caused by the earth’s rotation around the sun and the tilt of the earth on its axis.
Related term: Spring, Summer, Autumn, Winter
 

SEVERE THUNDERSTORM

Severe thunderstorms are defined as those that produce hail stones or with a diameter of 2cm ( the size of a $2 coin) or more, wind gusts of 90 km/h or greater, flash flooding and or tornadoes.
 

SOLSTICE

A solstice occurs when the position of the earth reaches its maximum inclination towards the sun in any one hemisphere. On June 21-22, in the northern hemisphere, the sun’s rays strike the earth vertically at the Tropic of Cancer, while this occurs in the southern hemisphere on December 22-23.
 

SOUTHERN OSCILLATION INDEX (SOI)

An index of atmospheric pressure differences between Australia and Tahiti. This provides an indication of the likelihood of an El Nino and La Nina event. A sustained period of negative SOI readings correlates with a La Nina pattern, with higher than usual pressure readings over Tahiti and lower than usual readings over Darwin. The reverse is true under El Nino patterns. 
Related Terms: El Nino and La Nina
 

SPOT FIRES

Fires that break out ahead of a fire front due to wind gusts that transport airborne embers ahead of the front.
 

SPRING

The season of the year which occurs as the sun approaches summer solstice, and characterised by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the Southern Hemisphere and the months of March, April, and May in the Northern Hemisphere. 
 

STAR

A large ball of gas that creates and emits its own radiation.
Related terms: Red Dwarf, Yellow Dwarf, Black Dwarf, Red Giant
 

STORM SURGE

A rise above normal sea level caused by strong onshore winds and/or reduced atmospheric pressure. Storm surges often accompany tropical cyclones.
Related term: Tropical Cyclone
 

STRATOCUMULUS

Grey or whitish or both patch, sheet or layer clouds which almost always has dark parts of rounded masses.
Related Term: Cloud
 

STRATUS

Layer like clouds that can cover the whole sky. They generally occur below 2000metres and are produced by the uplift of stable air at a front or over a mountain. They are associated with light rain or drizzle.
Related Term: Cloud
 

SUMMER

This is the period between the summer solstice and the equinox. It is characterized as having the warmest temperatures of the year, when the sun is directly over the hemisphere. This refers to the months of December, January and February in the Southern Hemisphere and June, July and August in the Northern Hemisphere.
 

SYNOPTIC CHART

This map is also called a weather map. It is a summary of the weather conditions experienced at a particular place at a particular point in time. See The Weather Channel Synoptic charts here.
Related term: Weather map.


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T


TEMPERATURE

This is the degree of heat or cold. It is measured by a thermometer in degrees Celsius and may vary between daily or seasonally.
 

TEMPERATURE ZONES

Locations with climates with distinct winter and summer seasons, typical of regions found between the tropics and the poles. Australia is found mostly within the temperate zone.
Related terms: Climatic Zones, Tropical Zone, Polar Zone
 

THERMOSPHERE

This is the layer of the atmosphere located between the mesosphere and outer space. It is a region of steadily increasing temperature with altitude, and includes all of the exosphere.
Related terms: Troposphere, Stratosphere, Mesosphere, Exosphere.

THUNDERSTORMS
Large cumulonimbus or thunderstorm clouds form when there are sufficient levels of atmospheric moisture and instability. This thunderstorms often form due to the presence of high moisture levels in the atmosphere, along with a trigger such as a cold front or low pressure trough. Day time heating of the ground by the sun can initiate the formation of thunderstorms when the other atmospheric conditions are met. Lightning and thunder are the most obvious features with thunderstorms but a large anvil shaped cloud is usually present as well. Heavy rain, flash flooding, damaging winds, large hail and tornados can develop with thunderstorms, especially with a bigger and longer lived variety known as 'supercell thunderstorms'.
 

TILT

The inclination to the vertical. The earth is tilted on its axis approximately 23.5 degrees from the vertical. This influences weather patterns.
Related term: Earth’s Axis
 

TORNADOES

A violently rotating column of air in contact with and extending between a cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena.
 

TRADE WINDS

These are the east to south-easterly winds (in the Southern Hemisphere) which affect tropical and subtropical regions, including the northern areas of Australia. During the monsoon season in northern Australia, the easterly trade winds are replaced by moist north-westerly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters.
 
In the Northern Hemisphere however, the trade winds are east to north-easterly in direction. It means that in both hemispheres, they tend to blow from the east to the west and towards the equator. Sometimes the trade winds will just be called "easterly" to avoid having to specify the hemisphere.
 

TROPICAL CYCLONE


Click here for Tropical cyclone safety tips

A 'warm-cored' low pressure system that requires certain atmospheric conditions to be met.
1. A region of sustained and oranized convection (cumulous cloud formation) over a region of open ocean
2. Sea surface temperatures of over 27 degrees celsius (26.5 in some special cases)
3. Low amounts of wind shear. That is no strong changes in wind speed in direction with height
4. More than 5 degrees north of south of the equator so that the coriolis force can have an influence

Tropical cyclones are referred to by different names in different parts of the world. In North America, the common term is hurricane, in the region of Japan and the Phillipines, the term Typhoon is used.

There is a scale for determining the strength of a Tropical Cyclones based on surface wind speeds;

Category Strongest gust (km/h) Typical effects
1 Tropical Cyclone Less than 125 km/h
Gales		 Minimal house damage. Damage to some crops, trees and caravans.Boats may drag moorings.
2 Tropical Cyclone 125 - 164 km/h
Destructive winds		 Minor house damage. Significant damage to signs, trees and caravans. Heavy damage to some crops. Risk of power failure. Small boats may break moorings.
3 Severe Tropical Cyclone 165 - 224 km/h
Very destructive winds		 Some roof and structural damage. Some caravans destroyed. Power failure likely.
4 Severe Tropical Cyclone 225 - 279 km/h
Very destructive winds		 Significant roofing and structural damage. Many caravans destroyed and blown away. Dangerous airborne debris. Widespread power failures.
5 Severe Tropical Cyclone More than 280 km/h
Extremely destructive winds		 Extremely dangerous with widespread destruction.


TROPICAL CYCLONE SAFETY


If you live in a tropical region of Australia you need to be ready for a tropical cyclone to impact your area, here are some tips.
  • Keep up to date with warnings by watching The Weather Channel or observing the warnings on the weather channels website (check warnings here). Follow any advice provided in these warnings.
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  • Well before a cyclone hits tidy up your yard and tie down anything that may become a projectile.
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  • Ensure you have an emergency kit that is up to date, including necessities such as: Non perishable food, drinking water, medicines, torch with working batteries, a first aid kit, blankets and special requirements for infants, the elderly and pets. Pack these things in a watertight container.
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  • Keep a battery operated radio handy to listen to your local radio for advice as the cyclone draws closer. Make sure it works and keep spare batteries in case of extended power outages.
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  • Charge your mobile phone, make a list of emergency contacts and pack insurance papers and other important documents into a safe container.
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  • Follow advice from emergency authorities or the police. If asked to evacuate, follow the instructions given to you. Plan to leave well before the cyclone is due. Take the emergency kit and dress appropriately, with strong shoes.
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  • If you are staying at home, keep all family members inside. Turn off electricity, gas and water. Unplug appliances.
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  • Shelter in the strongest part of the house which is usually the bathroom, or the cellar if you have one. Wrap yourself in blankets and if the building starts to break up, shelter under a table or mattress.
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  • Keep away from windows.
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  • Don’t assume it’s safe to go outside when the winds get lighter – it may be the calm eye of the cyclone. If this is the case, the winds will strengthen again and cause you harm if you are outside. Do not go outside until you have been given the all clear.
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  • After a cyclone, there may be a lot of debris and damage around. Remember this can be very dangerous and it is never safe to go sightseeing.
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  • Check on your neighbours.
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  • Stay away from fallen powerlines and safety check all electrical appliances before use.
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  • Boil water before drinking.
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  • Adhere to safety advice from local authorities.


TROPICAL ZONE

The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It includes the equatorial region, an area of high temperatures and considerable precipitation.
 

TROPOSPHERE

The lowest layer of the atmosphere located between the earth's surface to approximately 17 kilometres into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.
Related Terms: Stratosphere, Mesosphere, Thermosphere, Exosphere.


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U


UNIVERSE

Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain.

UPWELLING
Upwelling is often a wind-driven phenomenon that allows, cold, nutrient rich water to rise to the surface along the coast. In the Southern Hemisphere, surface water is deflecting to the left of the prevailing wind direction.

Therefore, under strong and sustained north/northeast wind conditions along the New South Wales coastline, surface water is deflected to the left (or east), allowing colder water to reach the coastal fringe. Beach-goers in New South Wales may notice a temperature drop of 5-6 degrees celcius in summer after a 24-48 hours of strong north/northeast winds. This drop in temperature may be even greater for areas on the south coast of New South Wales around Jervis Bay. The closer proximity of this region to the continental shelf can allow water to rise from even greater depths, bringing even colder water to the surface.

Strong south to southwest winds moving up the Peruvian coastline help to provide regular upwelling of cold, nutrient waters. This upwelling tends to break down in El Nino years, leading to a decrease in fish and sea life (due to the lower nutrient levels). This is what originally lead to the naming of the Pacific-wide weather phenomenon of El Nino.
Related terms: El Nino and La Nina

URBAN FLOODING

Flooding in urban areas caused by changes to the movement of water by the replacement of natural surfaces with impermeable surfaces that increase runoff and divert excess water via drains. Urban flooding usually occurs as a flash flood that inundates city areas due to the inability of drains to contain excess water.
 

URBAN HEAT ISLAND

Urban areas tend to be warmer than surrounding areas due to the hard surfaces of concrete and brick that store heat during the day and release it into the atmosphere at night. Cities also generate their own heat through industrial processes and from commercial and household sources. The warmer conditions in urban areas are called heat islands.


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W


WATER CONSERVATION

Reducing water usage by adopting practises that minimise water use.
 

WATER CYCLE

The movement of water in all its states (solid, liquid and gas) between the earth, the atmosphere, and the oceans.
 

WATER STATE

The form in which water exists - solid (ice), liquid (water), and gaseous (water vapour). It is a vital, life-sustaining part of life on earth.
 

WATER VAPOUR

Water in gaseous form. It is one of the most import constituents of the atmosphere.
Related Terms: Water State
 

WEATHER

The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind.
Related term: Climate
 

WEATHER FORECASTING

These are weather predictions that have occurred throughout time. In the Divine era, weather was thought to be related to gods and spirits. Over time, ordinary people developed stories and rhymes related to weather forecasting. With scientific instruments and computers, weather forecasting has become more accurate.
 

WEATHER MAP

This map is also called a synoptic chart. It is a summary of the weather conditions experienced at a particular place at a particular point in time.
Related term: Synoptic Chart
 

WEATHER STATIONS

Places from where weather information is recorded. These include automatic weather stations and drifting buoys in the ocean that provide meteorologists with important information about ocean temperatures related to El Nino events and the recording of atmospheric pressure.
 

WEATHER VANES

Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions.
 

WET BULB THERMOMETER

A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity.
Related Terms: Relative Humidity
 

WHITE DWARF

A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!
Related terms: Star, Red Dwarf, Yellow Dwarf, Black Dwarf, Red Giant
 

WILD FIRES

In other countries bushfires are called wildfires indicating that they are out of control and causing a severe bushfire risk. There are two types of wildfires – forest fires and grass fires.
Related Term: Bushfire
 

WIND

Horizontal movements of air are called winds. They originate from differences in air pressure, which may be the result of temperature differences.

For a scale describing wind strengths and their impact on land and sea activities, please refer to the Beaufort Wind Scale.

Related Term: Beaufort Wind Scale

WIND DIRECTION

The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east . It is reported to true north, or 360 degrees on the compass, and expresses to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc).
 

WIND ROSE

These identify the wind speed and direction relative to the frequency of winds specified either as the number of degrees clockwise from the north, or as one of the 8 or 16 compass points. The branches of a wind rose are divided into segments, which represent wind speed ranges from that direction. The length of each segment is proportional to the frequency of the winds blowing within the corresponding range of speeds from that direction.
Related Terms : Wind Direction, Wind Vane
 

WIND TUNNELS

In urban areas wind may be funnelled along “wind corridors” at high speed created by the presence of tall buildings. These are called wind tunnels.
 

WIND VANE

An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.
Related Terms: Wind Direction, Wind Rose.
 

WINTER

It is characterized in Australia as the 3 coldest months of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of June, July, and August in the Southern Hemisphere and December, January February in the Northern Hemisphere. Winter in the United States of America is often defined as the period from the winter solstice (shortest day of the year) to the equinox (equal day and night).
Related terms: Seasons, Summer, Autumn, Spring.


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Y


YEAR

The time taken for the earth to complete one rotation around the sun which is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year.

Leap years do not occur in years divisble by 100, unless they are also divisble by 400. For example, 2100 will not be a leap year but 2000 was.
 
Related terms: Rotation, Orbit
 


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