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Advection

The sideways movement of air in the lower atmosphere due to the differences in air pressure (commonly called wind). Process of transfer of air mass properties by the velocity field of the atmosphere.

Anomaly

The departure of an element from its long-period average value for the location concerned. For example, if the maximum temperature for June in Melbourne was 1 degree Celsius higher than the long-term average for this month, the anomaly would be +1 degrees Celsius. The current international standard is to use the 30 year average from 1961 to 1990 as the long-term average.

Anticyclones

Atmospheric circulations that rotate anti-clockwise in the southern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.

Australian Height Datum (AHD)

An average of the level of the sea around Australia in between high and low tide. It is used as the zero point when measuring or marking water height (measured straight up) e.g. on a map or flood marker. As an example, thirty metres above this 'sea level' would be written as '30m(AHD)'.

Barometer

A tool to measure air pressure which lets us predict change in weather.

Beaufort wind scale

A scale that uses observations of the effects of wind to estimate its speed.

Blizzard

Violent and very cold wind which is loaded with snow, some of which has been raised from snow covered ground.

Carbon dioxide

A gas (CO₂) present in the atmosphere which plays an important role in the greenhouse effect.

Celsius temperature scale

Thermodynamic scale of temperature. Temperature in degrees Celsius can be obtained from value in degrees Fahrenheit by the following formula: C = (F - 32) x 5/9

Chance of any rain

Chance of rain describes the likelihood of receiving a measurable amount of rain (>0.2mm) during the day at that location. For example, if the chance of rain for Mildura is 30%, it means that on 3 out of 10 days with similar weather conditions rainfall will be measured in the Mildura rain gauge. Where there may be a 30% chance of any rainfall, there is also a 70% chance of not receiving any rainfall at all.

Chlorofluorocarbons

Chemicals that release chlorine atoms that destroy ozone high in the atmosphere.

Cirrus cloud

High cloud, delicate, hair-like and feathery looking.

Climate

The atmospheric conditions for a long period of time, and generally refers to the normal or mean course of the weather. Includes the future expectation of long term weather, in the order of weeks, months or years ahead.

Cloud

A group of water drops or ice crystals that can be seen in the sky, caused by water vapour rising and cooling. Rain, hail or snow falls when the cloud can hold no more water or ice.

Cloud cover

Forecasting terms:

Clear: Free from cloud, fog, mist or dust haze.
Sunny: Little chance of the sun being hidden by cloud.
Cloudy: More cloud than clear sky. For example, during the day the sun would be hidden by cloud for a lot of time.
Overcast: Sky completely covered with cloud.

Combined Sea and Swell

Also known as total wave height, or significant wave height. Combined sea and swell describes the combined height of the sea and the swell that mariners experience on open waters. The height of the Combined sea and swell refers to the average wave height of the highest one third of the waves.

Convection

The movement of warm air, rising off the land, which helps to make cloud, local breezes, wind and thunderstorms.

Cumulonimbus cloud

Heavy, puffy, heaped, dark very tall clouds often bringing rain. Some have an anvil shaped head. Sometimes called a 'thunder head'.

Cumulus

Clouds with a woolly, heaped appearance that often produce rain.

Cyclogenesis

The rapid development of a low or intensification of a pre-existing one.

Cyclone

In Australia, a cyclone is a large clockwise loop of low pressure air that usually brings very strong or damaging winds, unsettled weather, cloudiness and very heavy rainfall.

(NOTE: Anticyclones - In Australia, an anticyclone is a large anti-clockwise loop of high pressure air, generally with light winds and fine and settled weather.)

Cirrus cloud

High cloud, delicate, hair-like and feathery looking, that may be seen on 'fine' days.

Desert

There are many different definitions which describe a desert. In general, arid areas (or deserts) are areas with low precipitation.

Dew

Droplets of water deposited when air cools and the water vapor in it condenses.

Dew-point temperature

This is a measure of the moisture content of the air and is the temperature to which air must be cooled in order for dew to form. The dew-point is generally derived theoretically from dry and wet-bulb temperatures, with a correction for the site's elevation. If the dry-bulb temperature is the same as the dew-point, the air is said to be saturated and the relative humidity is 100%.

Downburst

Violent and damaging downwards flow of air hitting the surface of the Earth violently, usually in a severe thunderstorm.

Drizzle

Steady(rain) in very small water droplets (less than 0.5 mm in diameter) very close to one another.

Drought

Drought is a long, unusually dry period when there is not enough water for normal needs.

Dry-bulb temperature

This is the shade temperature (degrees Celsius) registered by a mercury-in-glass thermometer exposed in a white louvered box or meteorological screen which is raised on legs one metre above the ground.

Dry

(As used by forecasters) Free from rain. Normally used when preceding weather has also been relatively dry, and dry weather is expected to continue for a day or so.

Dust storm

A wind storm which carries large amounts of dust or sand high into the air and may drop them far away e.g. dust from farm areas falling on a city.

East Coast Lows

East Coast Lows are intense low-pressure systems which happen on average several times each year off the eastern coast of Australia, in particular southern Queensland, NSW and eastern Victoria. They generally last only a few days, but can become worse over night. They can bring very strong winds, very heavy rainfall and sometimes cause coastal flooding.

El Niño

Nowadays, the term El Niño refers to the extensive warming of the central and eastern Pacific that leads to a major shift in weather patterns across the Pacific. In Australia (particularly eastern Australia), El Niño events are associated with an increased probability of drier conditions.

ENSO

Stands for El Niño-Southern Oscillation. 'El Niño' used here refers to the warming of the oceans in the equatorial eastern and central Pacific; Southern Oscillation is the changes in atmospheric pressure (and climate systems) associated with this warming (hence 'Southern Oscillation Index'' to measure these changes). 'ENSO' is used colloquially to describe the whole suite of changes associated with an 'El Niño' event - to rainfall, oceans, atmospheric pressure etc.

Equatorial trough

Zone of relatively low pressure which lies between the subtropical anticyclones of the two hemispheres.

Fahrenheit temperature scale

Thermodynamic scale of temperature. Temperature in degrees Fahrenheit can be obtained from value in degrees Celsius by the following formula: F = (9C/5) + 32

Fine

Dry weather with no rain of any kind, hail or snow.

Flash Flood

Flood of short duration with a relatively high peak discharge.

Flood

A flood occurs when water inundates (covers) land which is normally dry.

Flood Forecasting

Scientific opinion, based on past records and on weather and water conditions, about the likely height, amount of water, and time a flood will probably happen, and go on for, at a particular place on a river, stream or over land.

Flood Warning

Official advance notice that a flood may happen soon at a certain place or near a certain river.

Fog

A dense mass of small water droplets or particles in the lower atmosphere.

Front

The boundary between air masses having different characteristics.

Front (Cold)

In some regions along the polar front, cold dense air advances equatorwards, causing warm air to be forced aloft over its sloping surface. This portion of the polar front is known as a cold front.
Cold polar air is replacing warm tropical air.

Front (Warm)

In other regions along the front, warm air of lower density moves polewards, sliding over its sloping surface. This portion is called a warm front. Warm tropical air replaces cold polar air.

Frost

Deposit of soft white ice crystals or frozen dew drops on objects near the ground; formed when surface temperature falls below freezing point.

Gale Warning

A Gale Warning is a statement which warns of sea winds averaging from 34 knots and up to 47knots (equal to about 60 - 85 kph) in coastal waters and out at sea.

Greenhouse effect

A natural warming process of the earth. When the sun's energy reaches the earth some of it is reflected back to space and the rest is absorbed. The absorbed energy warms the earth's surface which then emits heat energy back toward space as longwave radiation. This outgoing longwave radiation is partially trapped by greenhouse gases such as carbon dioxide, methane and water vapor which then radiate the energy in all directions, warming the earth's surface and atmosphere. Without these greenhouse gases the earth's average surface temperature would be about 33 degrees Celsius cooler.

Global radiation

Global (short wave) radiation includes both that radiation energy reaching the ground directly from the sun, and that received indirectly from the sky, scattered downwards by clouds, dust particles etc.

Gust

A gust is a sudden burst of stronger wind that usually only lasts a few seconds.

Hail

Small roundish pieces of ice ('hailstones') (generally between 5 and 50 millimetres across,) which fall from clouds as separate bits or in clumps.

Harden Up

'Harden Up - Protecting Queensland' is a Green Cross Australia project. Harden Up is a statement of our ability as humans to endure natural disasters and be resilient to our changing climate. The term comes from a special report from the Australian Strategic Policy Institute. The report, called Hardening Australia: Climate change and national disaster resilience, encourages Australians to build resilience in infrastructure, business and the home and to be prepared for the unique risks that Australians will face as our climate changes.

Heat wave

A period of abnormally hot weather lasting several days.

High pressure

Atmospheric circulations that rotate anti-clockwise in the Southern Hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.

Humid

Moisture in warm air.

Humidity

Moisture in warm air.

Hurricane Force Wind Warning

A Hurricane Force Wind Warning is a statement which warns of winds averaging 64 knots or more in coastal waters and high seas areas.

Hydrology

An earth science concerned with the occurrence, distribution and circulation of waters on and under the earth's surface, both in time and space, their biological, chemical and physical properties, their reaction with the environment, including their relation to living beings.

Hydrometeorology

The study of the atmospheric processes that affect the water resources of the earth, including the study of the atmospheric and land phases of the hydrological cycle with emphasis on the interrelationships involved.

Indigenous Weather

Indigenous Australians have long held their own seasonal calendars based on the local sequence of natural events.

Inshore

The waters within 12 sea miles (about 22kms) of the coast.

Inversion, temperature

A temperature inversion occurs when the temperature of air increases with increasing height. Generally the temperature decreases with height in the lower atmosphere, called the troposphere. Low-level inversions generally form on clear calm nights due to cooling of the ground through loss of heat by radiation. The warm air on the ground is replaced by cooler air at the surface resulting in a temperature inversion. The inversion creates a boundary layer that restricts vertical motion and mixing of air between the two air masses either side. Low-level inversions act like a lid to trap pollutants resulting in smog over our cities.

Isobars

Lines on weather maps running through places which have the same air pressure.

Jet stream

A powerful current of air high above the Earth.

Knot

Measure of wind speed equal to about 1.8 km per hour.

Køppen's classification of climates

Classification of climate based on annual and monthly means of temperature and precipitation (rainfall) which also takes into account the vegetation limits. It is a tool for presenting the world pattern of climate and for identifying important deviations from this pattern.

King tide

On Australia's east coast, the highest tides happen during the winter months of June, July and August, and the summer months of December, January and February. The two highest tides, one in winter and one in summer, are known as the 'king tides'.

Land Breeze

A local offshore wind. At night, when the land cools more quickly, and to a greater extent, than the sea, the land breeze circulation is set up. Cooler air from the land flows offshore to replace the warm air rising over the sea. The air in contact with the sea warms and expands and the resulting changes in the pressure and temperature differences and distributions cause the land breeze circulation.

La Niña

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.

Lightning

The flash of light from a large electrical spark from or inside a cloud. Lightning can injure people and animals and damage trees and buildings if it reaches the ground.

Low latitudes

The southern hemisphere low-latitudes are considered to be the areas of the Earth north of about 30 degrees latitude. For Australia, this means the area north of a line from halfway between Perth and Geraldton (in Western Australia) to Bourke (in New South Wales). This part of Australia generally experiences a subtropical to tropical climate.

Low pressure

Atmospheric circulations that rotate clockwise in the southern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.

Mid latitudes

The areas between about 30 degrees and 55 degrees latitude. For Australia, this is the area south of a line from halfway between Geraldton and Perth (in Western Australia) to Bourke (in New South Wales). This part of Australia generally experiences a temperate climate.

Mist

Similar to fog, but visibility remains more than a kilometre.

MJO

Stands for Madden-Julian Oscillation, also known as the 30-50 day wave. This is a periodic enhancement of rainfall over the Australian tropics, which progresses across tropical latitudes roughly every 30-50 days. Satellite cloud loops and atmospheric pressure changes can signal passage of the wave over Australia, signalling a burst in monsoon (rainfall) activity during the tropical wet season.

Monsoon

A seasonal wind. The northern Australian monsoon season generally lasts from December to March. It brings cloud and heavy rainfall over northern Australia. The north Australian wet season, October to April, includes the monsoon months but can last for several months on either side. Parts of the North Queensland coast also have quite heavy rainfall throughout the cooler months.

MSL

MSL stands for Mean Sea Level. It is necessary to convert the pressure readings to equivalent mean sea level pressures, otherwise the important horizontal changes in pressure would be overwhelmed by vertical variations simply due to differences in height between observing stations. In this way, a Mean Sea Level Pressure (MSLP) map will then show pressures affected by changing weather conditions, not because of changing altitude.

Nitrogen

The most abundant gas in air, comprising 78% by volume. It is colourless and odourless.

Offshore

The coastal waters zone between 12 and 60 'nautical' miles (about 22 -110 km) from the coast.

Oxygen

The second most abundant gas in air, comprising 21% by volume. It is a colourless and odourless gas.

Ozone

One of the several gases that make up the Earth's atmosphere. It is the triatomic* form of oxygen and makes up approximately one part in three million of all of the gases in the atmosphere. If all the ozone contained in the atmosphere from the ground level up to a height of 60 km could be assembled at the earth's surface, it would comprise a layer of gas only about 3 millimeters thick, and weigh some 3000 million tonnes. Ozone is toxic at high concentrations because it reacts strongly with other molecules.

* Each ozone molecule is made up of three oxygen atoms.

Precipitation

Any or all of the forms of water particles, whether liquid (e.g. rain, drizzle) or solid (e.g. hail, snow), that fall from a cloud or group of clouds and reach the ground. (See Drizzle, Rain).

Duration of precipitation

Brief: Short duration.
Intermittent: Precipitation which ceases at times.
Occasional: Precipitation which while not frequent, is recurrent.
Frequent: Showers occurring regularly and often.
Continuous: Precipitation which does not cease, or ceases only briefly.
Periods of rain: Rain is expected to fall most of the time, but there will be breaks.

Intensity of precipitation

Slight or light:
- Rain: Individual drops easily identified, puddles form slowly, small streams may flow in gutters.
- Drizzle: Can be felt on the face but is not visible. Produces little runoff from roads or roofs. Generally visibility is reduced, but not less than 1000 m.
- Snow: Small sparse flakes. Generally visibility is reduced, but not less than 1000 m.
- Hail: Sparse hailstones of small size, often mixed with rain.
Moderate:
- Rain: Rapidly forming puddles, down pipes flowing freely, some spray visible over hard surface.
- Drizzle: Window and road surfaces streaming with moisture. Visibility generally between 400 and 1000 m.
- Snow: Large numerous flakes and visiblity generally between 400-1000 m.
- Hail: particles numerous enough to whiten the ground.
Heavy:
- Rain: falls in sheets, misty spray over hard surfaces, may cause roaring noise on roof.
- Drizzle: Visibility reduced to less than 400 m.
- Snow: Numerous flakes of all sizes. Visiblity generally reduced below 400 m.
- Hail: A proportion of the hailstones exceed 6 mm diameter.

Distribution of showers and precipitation

Few: Indicating timing not an area.
Isolated: Showers which are well separated in space during a given period.
Local: Restricted to relatively small areas.
Patchy: Occurring irregularly over an area.
Scattered: Irregularly distributed over an area. Showers which while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.
Sporadic: scattered or dispersed in respect of locality or local distribution. Charaterised by occasional or isolated occurrence.
Widespread: Occurring extensively throughout an area.

Probable Maximum Flood (PMF)

The most severe flood that scientists think is likely to happen at a particular place. The 'worst-case scenario' flood that would be caused if all the worst weather and water conditions happened at once.

Probable Maximum Precipitation (PMP)

The most rain that scientists think is likely to fall in a certain time over a certain size storm area at a particular place at a certain time of year.

Probabilities, or Probabilistic Forecasts

These forecasts use percentages, such as 60% (60 percent or 60 chances in every 100). This kind of forecast is usually based on how often something has happened in the past. For instance, if the chance of getting more than average rainfall if certain things happen is said to be 60%, then in 60 out of 100 of past years when things were similar it was higher than usual but in 40 out of 100 it was below.

High percentages do not mean something will happen, but that scientists think it is it is likely or probable.

Rain

Precipitation of liquid usually in drops bigger than 0.5 millimetres(mm) falling more steadily than showers.

Rain day

A rain day occurs when a daily rainfall of at least 0.2 mm is recorded.

Rainfall

The total which has fallen, as measured in a rain gauge.

Rainfall amount

Rainfall amount is the likely amount of rain in millimetres(mm) for the forecast period. Sometimes rain falls in a patchy pattern across an area with some places getting a heavy shower while an area nearby might miss out completely. On these days the rainfall range may be quite large, e.g. 5 to 30 mm. When steady rainfall is expected over a wide area, the range may be smaller, e.g. 10 to 15mm.

Relative humidity

Is a traditional indicator of the air's moisture content. It is the ratio of the amount of moisture actually in the air to the maximum amount of moisture which the air could hold at the same temperature. Relative humidity is normally expressed as a percentage and at saturation the relative humidity will be very close to 100%. The air can hold more moisture at higher temperatures, hence the relative humidity alone does not give an absolute measure of moisture content.

Ridge

A ridge is an elongated area of high pressure. It is indicated by rounded isobars extending outwards from an anticyclone and has associated with it a ridge line. The pressure at a point on the ridge is higher than at an adjacent point on either side of the line.

Sea and Swell, Combined

Combined sea and swell describes the total height of the sea and the rise in water of non-breaking waves ('swell') out at sea.

Sea and Swell

Peak Wave Period: Period in seconds between the swells of the primary swell component. The larger the time difference, the greater the amount of energy associated with the swells.
Primary Swell: Height and direction of the swell with the highest energy component. This is sometimes referred to as the dominant swell.
Sea waves: waves generated by the wind blowing at the time, and in the recent past, in the area of observation.
Secondary Swell: Height and direction of the swell with the second highest energy component.
Swell Period: See Peak Wave Period.
Swell waves: waves which have travelled into the area of observation after having been generated by previous winds in other areas. These waves may travel thousands of kilometres from their origin before dying away. There may be swell present even if the wind is calm and there are no 'sea' waves.
Wave period: the average time interval between passages of successive crests (or troughs) of waves.
Wave Height: Generally taken as the height difference between the wave crest and the preceding trough.
Wave Length: The mean horizontal distance between successive crests (or troughs) of a wave pattern.

Sea breeze

A local onshore wind. Cooler air from over the sea flows onto the shore to replace the warm air rising over the land. On sunny days the land heats up more quickly, and to a greater extent, than the sea. The air in contact with the land warms and expands and the resulting changes in the pressure and temperature differences and distributions cause the sea breeze circulation. At night, when the land cools more quickly, and to a greater extent, than the sea, the reverse land breeze circulation is set up.

Showers

Showers are often short lasting (but may last half an hour) but can be heavy. They usually begin and end suddenly.

Sleet

Generally refers to a mixture of rain and snow or falling snow that is melting into rain.

Smog

Smog (contraction for 'smoke fog') is a fog in which smoke or other forms of atmospheric pollutant have an important part in causing the fog to thicken, and have unpleasant and dangerous physiological effects.

Snow

Precipitation of ice crystals, most of which are branched (sometimes star shaped).

Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.

Squall

A squall comprises a rather sudden increase of the mean wind speed which lasts for several minutes at least before the mean wind returns to near its previous value. A squall may include many gusts.

Storm Force Wind Warning

A Storm Force Wind Warning is a statement which warns of winds averaging from 48 knots and up to 63 knots in coastal waters and high seas areas.

Stratosphere

Layer of the atmosphere between about 10 and 50 kilometres above the ground.

Stratus cloud

Low cloud forming a uniform layer.

A Strong Wind Warning

A Strong Wind Warning is a statement which warns of winds averaging from 26 knots and up to 33 knots in coastal waters.

Supercell

A long-lived intense thunderstorm containing a lot of wind movement. Supercell thunderstorms may produce very large hail, extraordinary wind gusts, powerful tornadoes and heavy rainfall.

Synoptic chart

Chart showing lines of equal pressure (isobars), corrected to mean sea level (MSL), over a broad area (eg Australia). Based on the synoptic observations taken simultaneously every 3 hours by weather observers and Automatic Weather Stations across Australia.

Synoptic scale

A horizontal length scale that corresponds to the size of the large-scale features of the lower atmosphere (ie the highs and lows over mid-latitude regions).

Stratus cloud

Low cloud in a layer or blanket. Stratus clouds may bring very light rain or snow. A cloudy day usually has a sky filled with stratus clouds hiding the sun.

Sea mile, nautical mile

A measure at sea equal to about 1.8 kilometres.

Severe weather warning

Severe Weather Warnings are issued for:

Winds of gale force (blowing at 63 km/h) or more
Wind gusts (short bursts of wind) of 90 km/h or more
Very heavy rain that may lead to flash flooding
Abnormally high tides (or storm tides) expected to be higher than usual high tide levels
Unusually large surf waves expected to cause dangerous conditions on the coast

Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is a measure of changes in the difference in measured air pressure between Tahiti and Darwin. Because of good records about how this has related to our rainfall in past seasons, we can use it to tell what Queensland's likely rainfall will be in the coming months.

Strong wind warning

A Strong Wind Warning is a statement which warns of winds averaging from 26 knots and up to 33 knots (or about 45 to 60 kph) in coastal waters. It is usually given by local or state government agencies.

Storm surge

A storm surge is a rise above the normal water level along a shore. In tropical areas, storm surges are caused by tropical cyclones as they come ashore. In a storm surge the water level rises above normal high tide levels, causing flooding on shore. The height of the water and the speed it rises depend on the strength of the cyclone and shape of the waterfront, among other things.

Temperature inversion

A temperature inversion occurs when the temperature of air increases with increasing height. Generally the temperature decreases with height in the lower atmosphere, called the troposphere. Low-level inversions generally form on clear calm nights due to cooling of the ground through loss of heat by radiation. The warm air on the ground is replaced by cooler air at the surface resulting in a temperature inversion. The inversion creates a boundary layer that restricts vertical motion and mixing of air between the two air masses either side. Low-level inversions act like a lid to trap pollutants resulting in smog over our cities.

Thunderstorm

A storm with sudden electrical discharges which make a flash of light (lightning) and a loud rumbling sound (thunder). Thunderstorms usually, but not always, bring rain. They usually only last a short time and hit a small area.

Tornado

A whirlwind or mass of circling air with high wind speeds at its centre.

Trade winds

East to southeasterly 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 northwesterly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters. As mentioned above, the trade winds in the southern hemisphere are east to southeasterly in direction. In the northern hemisphere however, the trade winds are east to northeasterly 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 cyclones

A tropical cyclone is an intense low pressure system which forms over warm ocean waters in Queensland, The Northern Territory and Western Australia.

A tropical cyclone usually brings strong winds and extremely heavy rain. It can also bring unusually high sea levels (storm surges) in areas near the coast.

A tropical cyclone can cause widespread damage as a result of the strong wind, flooding (caused by either heavy rainfall or ocean storm surges).It can cause landslides in hilly areas as a result of heavy rainfall and saturated soil.

If there are winds above 117 km/h (63 knots) it will be called a severe tropical cyclone.

(NOTE: A tropical cyclone may also be known (in other parts of the world) as a tropical storm. In the north western Pacific severe tropical cyclones are known as 'typhoons' and in the northeast Pacific and Atlantic/Caribbean they are called 'hurricanes'.)

Tropical storm

Term used in the northern hemisphere for a tropical cyclone.

Trough

A trough of low pressure is an elongated area where atmospheric pressure is low relative to its immediate surroundings. A trough of low pressure is sometimes indicated on the synoptic chart by a centre line or trough line denoted by a dashed line e.g. - - - - -. The trough line often extends outward from a low pressure centre, or an enclosed area of relatively low pressure. When moving across a trough from one side of a trough line to another, atmospheric pressure decreases as you approach the trough line. The atmospheric pressure increases again after you cross the trough line and move away. A change in wind direction will generally be observed as you cross from one side of the trough to the other. Compare with the definition for a Ridge and a Low.

Typhoon

Term used in the northwestern Pacific for a tropical cyclone with maximum winds above 117 km/h (63 knots).

Temperature

Celsius temperature scale

Temperature scale (sometimes called 'centigrade'). Water freezes at zero degrees Celsius (0 degrees C) and water boils at 100 degrees Celsius (100 degrees C).

Fahrenheit temperature scale. To work out a temperature in degrees Fahrenheit (degrees F), multiply the temperature in degrees C by 1.8 and add 32. E.g. 10 degrees C x 1.8 = 18 + 32 = 50. So 10 degrees C is the same as 50 degrees F.

Tsunami

The name Tsunami is used to describe a series of waves travelling across the ocean. These waves can measure up to hundreds of kilometres between wave crests in the deep ocean. They are very fast moving, and unlike normal waves, affect the movement of the water all the way to the sea floor. They tend to get higher as they get closer to land and, if high enough when they reach the land, can travel far inland and cause very serious damage.

Earthquakes are one of the things that cause tsunami. They can also be caused by underwater landslides, volcanoes, land slipping into the ocean, meteors, or even the weather when the atmospheric pressure changes very rapidly. The most common cause of tsunami is an under sea earthquake with a sudden change in a section of the earth's crust under or near the ocean. This rise or fall in sea level makes a tsunami wave.

Ultraviolet radiation

Electromagnetic radiation with wavelengths shorter than visible light, but longer than x-rays. Exposure to too much UV radiation can cause skin cancer.

Updraught

Upward moving current of air of small dimensions. A rapidly moving vertical wind as found in thunderstorms.

Virga

Precipitation that evaporates before it reaches the ground.

Vortex

Rotating mass of air or water, such as water going down a plug hole.

Water Resources

Water available, or capable of being made available, for use in sufficient quantity and quality at a location and over a period of time appropriate for an identifiable demand.

Water Resources Assessment

Determination of the sources, extent, dependability and quality of water resources for their utilisation and control.

Water vapour pressure

The atmospheric pressure which is exerted by water vapour (water in its gaseous state). It is one way of measuring the humidity of the air. At a given temperature, an increase of water vapour in the air corresponds to an increase in the humidity of the air. Water vapour is supplied to the atmosphere by evaporation of water from oceans, lakes, wet land surfaces or from vegetation (transpiration). Water vapour absorbs the Sun's radiation. As a result, the sunlight received at the Earth's surface will be more intense in a drier atmosphere.

Wet bulb temperature

Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air - evaporation is slower when the air is already full of water vapour For this reason, the difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity.

Wind

Usually only one wind speed is given in a weather forecast, unless it is expected to be very gusty. For instance, fresh, gusty southwest winds means that wind speed will generally be between 17 and 21 knots (or about 30 to 38 kph) and the mean wind direction will be from the southwest, but that there will also be much stronger gusts.

Gust: a gust is any sudden increase of wind usually for only a few seconds.
Squall: A sudden strong wind that rises up and may blow hard a number of times before it dies down.
Surface Wind: wind speed and direction measured at 10 metres above the earth's surface. The surface wind is what causes waves on the ocean. Large swells can be caused by strong winds in intense storms.

Zonal flow

1. Component of atmospheric circulation along a line of latitude, towards the east or west.

2. Atmospheric circulation along, or approximately along, parallels of latitude.

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