Glossary

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.

 

Air

The mixture of gases and particles that make up our atmosphere.

 

Analogue

A past situation, which resembles the current situation, is called an analogue, which can then be used as a guide for what is to come. For example, by finding similar SOI patterns to the current year, we can then determine tropical cyclone events from that time to use as a guide as to what will happen this year.

 

Anemometer

A device used to measure wind speed.

 

Anomaly

The departure of an element from its long-period average value for the location concerned.  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 and clockwise in the northern hemisphere. Anticyclones are areas of higher pressure and are generally associated with lighter winds and fine and settled conditions.

 

Atmosphere

The mixture of gases surrounding a planet.

 

Autumn

One of the four seasons of the year. The transition months March, April and May in the southern hemisphere, and September, October and November in the northern hemisphere.

 

AWS

Automatic Weather Station.

Barometer

An Instrument that measures air pressure.

 

Bathymetry

The measurement of the depth of the ocean floor from the water surface and is the oceanic equivalent of topography.

 

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.

 

Slight chance, Medium chance, High chance, Very high chance

  • Slight chance is used when the likelihood is between 15% and 34%.
  • Medium chance is used when the likelihood is between 35% and 64%.
  • High chance is used when the likelihood is between 65% and 84%
  • Very high chance is used when the likelihood is 85% or higher.

 

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

Mass of water droplets or ice crystals caused by water vapour in the atmosphere condensing or freezing.

 

As air ascends through the atmosphere, it cools as it rises. The water vapour in the rising air condenses into droplets and forms clouds. As the droplets rises they form tiny ice crystals. When the droplets/ice crystals are large enough they may fall as rain, hail or snow.

 

Cloud cover

Forecasting terms:

Clear: Free from cloud, fog, mist or dust haze.

Sunny: Little chance of the sun being obscured by cloud. High level cirrus clouds are often thin and wispy, allowing a considerable amount of sunlight to penetrate them, sufficient to produce shadows. In this case the day could be termed 'sunny' even though more than half the sky may be covered in cirrus cloud.

Cloudy: Predominantly more cloud than clear sky for example, during the day the sun would be obscured by cloud for substantial periods of time.

Overcast: Sky completely covered with cloud.

 

Cold Fronts

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.

 

Condense

Change from a gas to a liquid.

 

Convection

The process generally associated with warm rising air and the formation of cloud. Local breezes, wind and thunderstorms are a result of convection in the atmosphere.

 

Cumulonimbus cloud

Heavy, puffy, heaped, dark cloud of great vertical depth, often bringing rain. Some have a distinctive anvil shaped head.

 

Cumulus

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

 

Cyclogenesis

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

 

Cyclone

Atmospheric circulations that rotate clockwise in the southern hemisphere, and anti-clockwise in the northern hemisphere. Cyclones are areas of lower pressure and generally associated with stronger winds, unsettled conditions, cloudiness and rainfall.  Sometimes referred to as Tropical Revolving Storms.

 

Cyclonic Storm

See Tropical Cyclone Terms

DALR

The Dry Adiabatic Lapse Rate (DALR) is the rate at which the temperature of unsaturated air changes as a parcel ascends or descends through the atmosphere.

The DALR is approximately 9.8 degree Celsius per 1 km.

 

Deep-layer mean wind

Average wind over a deep layer of the atmosphere, giving a representative steering wind.  In this case, deep is defined as spanning from the lower atmosphere all the way to the upper air (5-10 kilometres high or up to a pressure level of at least 500hPa)

 

Dew

Droplets of water deposited when air cools and the water vapour 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%.

 

Diurnal variation

The changes of value of meteorological elements i.e. temperature and pressure within the course of a (solar) day. More especially, it denotes the systematic changes that occur during the average day.

 

Downburst

Violent and damaging downdraught striking the surface of the Earth violently. Associated with a severe thunderstorm.

 

Drizzle

Fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.

 

Drought

Prolonged absence or marked deficiency of precipitation (rain).

 

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 storm which carries large amounts of dust into the atmosphere. Ensemble of particles of dust or sand energetically lifted to great heights by a strong and turbulent wind.

East Coast Lows

East Coast Lows are intense low-pressure systems which occur on average several times each year off the eastern coast of Australia, in particular southern Queensland, NSW and eastern Victoria.

 

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.

 

Evapourate

Change from a liquid to a gas.

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

As used by forecasters, no rain or other precipitation (hail, snow etc.). The use of fine is generally avoided in excessively cloudy, windy, foggy or dusty conditions. In particular note that fine means the absence of rain or other precipitation such as hail or snow - not 'good' or 'pleasant' weather.

 

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

Estimation of height, discharge, time of occurrence, and duration of a flood, especially of peak discharge, at a specified point on a stream, resulting from precipitation and/or snowmelt.

 

Flood Warning

Advance notice that a flood may occur in the near future at a certain location or in a certain river basin.

 

Fog

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

 

Freeze

Change from a liquid to a solid.

 

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.

 

Front (Occluded)

When the cold front moves faster than the warm front, and as it overtakes the warm front, the warm sector is closed and a combine front forms. This process is called occlusion.

 

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 winds averaging from 34 knots and up to 47 knots in coastal waters and high seas areas.

 

Geopotential height

Geopotential height approximates the actual (gravity-weighted) height of a pressure level above mean sea-level.

 

Geostrophic wind

When the wind is steady, horizontal, and flowing parallel to straight isobars it is called the geostophic wind. Where the Pressure Gradient force is exactly balanced by the Coriolis force.

 

Gradient wind

A steady, horizontal wind flowing along curved isobars is called gradient wind. Where there is imbalance between the pressure gradient and Coriolis forces.

 

When the pressure gradient force is greater than the Coriolis force, the flow takes on a curved path around low pressure.

 

When Coriolis is the larger force, the curved flow is around high pressure.

 

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 vapour 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 any sudden increase of wind of short duration, usually a few seconds.

Hail

Precipitation (falling) of particles of ice (hailstones). Usually spheroid, conical or irregular in form and with a diameter varying generally between 5 and 50millimetres.

Hail falls from clouds either separately or collected into irregular lumps.

 

Haze

State of atmospheric obscurity due to the suspension in the air of extremely small dry particles invisible to the naked eye.  Haze resembles a uniform veil over the landscape that subdues its colours. When viewed against a dark background (e.g. a mountain) it has a bluish tinge but it has a dirty yellow or orange tinge against a bright background (e.g. sun, clouds). Haze is distinguished from mist when the humidity is less than 90% at the time.

 

Heat wave

A period of abnormally hot weather lasting several days.

 

High pressure

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

 

Humid

Relatively high water vapour content in the air, often associated with warmer temperatures. Relative Humidity (RH) and the Dewpoint Temperature can be used as indicators of humidity.

 

Humidity

A measure of water vapour in the air.

Intertropical Convergence zone

An area of unstable atmospheric conditions dividing the northern hemisphere northeast trade winds and the southern hemisphere southeast trade winds. The ITCZ migrates north / south, expands and contracts following the seasonal movements of the sun and the relative strength of the prevailing trade winds.

 

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 joining places which have the same air pressure.

Jet stream

A flat, tubular current of air located in the tropopause, the area in the Earth's atmosphere located between the troposphere and the stratosphere. These powerful winds are generated by strong pressure gradients which reflect the great temperature differences at high altitudes.

Kelvin

Unit of thermodynamic temperature. Symbol: K

K = C + 273.2

Zero Kelvin is referred to as "Absolute Zero".

 

Knot

Unit of speed equal to one nautical mile per hour.

Land Breeze

A local offshore wind. At night, when the land cools more quickly 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 accompanying a sudden electrical discharge which takes place from or inside a cloud, or less often from high structures or the ground or from mountains. A large electrical spark.

 

Caused when the negative charge in the lower part of the cloud and the positive charge in the upper part of the cloud become so great that they can overcome the natural resistance of the air and discharge between negative and positive takes place.

 

Low pressure

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

 

LT/LST – Local Time / Local Standard Time

The globe is divided into Standard time zones and Local Standard Time is the time specified for each zone. This is usually done in terms of hours ahead or behind UTC.

Example of LT/LST zones are as follows:

(+10 UTC) Australian Eastern Standard Time - Qld, NSW, Vic, Tas

(+9.5 UTC) Australian Central Standard Time - NT, SA

(+8 UTC) Australian Western Standard Time - WA

(+8 UTC) China, Malaysia, Philippines, Singapore

(+7 UTC) Thailand, Vietnam

(+6.5 UTC) Myanmar

(+7 UTC) Western Indonesia

(+8 UTC) Central Indonesia

(+9 UTC) Eastern Indonesia

Mean

See Verification Statistical Terms

 

Median

See Verification Statistical Terms

 

Meteograms

Weather outlooks of wind, temperature, precipitation/rainfall, visibility, convective inhibition, CAPE, Total-total Index, and PABL height for 36 or 72-hours.  Presented as a graph, these outlooks are extracted directly from the WRF NMM and ARW mesoscale models run by OWS in-house.

 

Mid latitudes

The areas between about 30 degrees and 55 degrees latitude in the northern and southern hemispheres.

 

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.

 

Mirage

Optical refraction in the atmosphere consisting of images displaced from their true positions.

 

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 shift in the prevailing wind direction that brings with it different kind of weather.

 

The Asian monsoon covering India to Southeast Asia is associated with prevailing wind blowing from the southwest between May and September during the northern hemisphere summer-autumn season.  Wind during this monsoon season is warm and moist, bringing rain (SW Monsoon).

The wind shifts and blows from the northeast between October and April during the winter-spring season.  Wind during this monsoon season is cold and dry (NE Monsoon).

 

The northern Australian monsoon season generally lasts from December to March. It is associated with the inflow of moist west to northwesterly winds into the monsoon trough, producing convective cloud and heavy rainfall over northern Australia. These moisture-laden winds originate from the Indian Ocean and southern Asian waters. The north Australian wet season encompasses the monsoon months but can extend several months on either side.

 

MSL (Mean Sea Level)

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.

NE Monsoon (Northeast Monsoon)

The prevailing wind blowing from the northeast over Asia covering India to Southeast Asia during the months of December to March (northern hemisphere summer-autumn season).  Wind during this monsoon season is cold and dry (dry monsoon).

 

Nitrogen

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

 

Northern Monsoon Trough

The northern extent of the Inter Tropical Convergence Zone (ITCZ). Plays an important part in tropical cyclone development over the NW Pacific Ocean.

Offshore

The coastal waters zone between 12 and 60 nautical miles 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.

Percentile

The term for denoting thresholds or boundary values in frequency distributions. Thus the 5th percentile is that value which marks off the lowest 5 per cent of the observations from the rest, the 50th percentile is the same as the median, and the 95th percentile exceeds all but 5 per cent of the values.

When percentiles are estimated by ranking the items of a finite sample, the percentile generally falls between two of the observed values, and the midway value is often taken.

 

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.

Chance of precipitation

  • Slight chance is used when the likelihood is between 15% and 34%.
  • Medium chance is used when the likelihood is between 35% and 64%.
  • High chance is used when the likelihood is between 65% and 84%.
  • Very high chance is used when the likelihood is 85% or higher.

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 visibility 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. Visibility 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.

 

Probabilities, or Probabilistic Forecasts

An attempt to convey the uncertainty in a forecast by expressing its likelihood of occurrence as a probability or percentage.  High probabilities do not guarantee an outcome - they merely indicate that that outcome is highly likely. Probabilities are usually based on the frequency of occurrence in the historical record. For instance, if the chance of receiving above-median rainfall in a particular climate scenario is 60%, then 60% of past years when that scenario occurred had above median rainfall, and 40% had below-median rainfall.

Quartile

See Verification Statistical Terms

 

QFE (Field Elevation)

QFE is calculated by adjusting the station level pressure for the difference between the barometer level and the aerodrome reference level, assuming International Standard Atmosphere (ISA) conditions.

An Altimeter set to QFE will read zero when the aircraft is on the runway.

 

QNH

QNH is the pressure measured at station then reduced down to mean sea level pressure.

Rain

Precipitation of liquid water drops greater than 0.5 mm in diameter. In contrast to showers, it is steadier and normally falls from stratiform (layer) cloud.

 

Rainfall

The total liquid product of precipitation or condensation from the atmosphere, as received and measured in a rain gauge.

 

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.

SALR

The Saturated Adiabatic Lapse Rate (SALR) is the rate at which the temperature of a parcel of air saturated with water vapour changes as the parcel ascends or descends.

 

The SALR is often taken as 1.5 degree Celsius per 1000ft, although the actual figure varies according to the amount of water vapour present.

 

Sea

See Wave Terms

 

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 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.

 

Seasons

These definitions reflect the lag in heating and cooling as the sun appears to move southward and northward across the equator. They are also useful for compiling and presenting climate-based statistics on time scales such as months and seasons.

 

In the northern hemisphere, the seasons are define by grouping the calendar months in the following way:

  • Autumn - the three transition months September, October, November.
  • Winter - the three coldest months December, January and February.
  • Spring - the transition months March, April and May.
  • Summer - the three warmest months June, July and August.

 

In Australia (southern hemisphere), the groupings are the following:

  • Autumn - the transition months March, April and May.
  • Winter - the three coldest months June, July and August.
  • Spring - the three transition months September, October, November.
  • Summer - the three hottest months December, January and February.

Severe Cyclonic Storm

See Tropical Cyclone Terms

 

Showers

Precipitation, often short-lived but may last half an hour and heavy, falling from convective clouds. 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.

 

Southern Monsoon Trough

The southern extent of the Inter Tropical Convergence Zone (ITCZ)

Plays an important part in Tropical cyclone development over the Indian Ocean, Timor Sea and South Pacific Ocean.

 

Spring

The three transition months from winter to summer

Southern hemisphere - September, October and November

Northern hemisphere - March, April and May

 

Squall

A squall is a sudden increase in wind speed at least 15 knots with a top speed of at least 21 knots lasting at least one minute in duration. Squalls are produced when an unstable air is heated producing deep convection.  Downbursts from the mature cells lead to the development of a squall line or squall front.

 

After a squall event, the mean wind returns to near its previous value. A squall may  include many gusts.

 

Squall Warning

A Squall Warning is a statement which warns of a risk of squalls categorised as low, moderate and high.  Squall Warnings include the current location of squall line including direction of movement and speed, expected time of arrival at forecast location, and the expected intensity and duration of the squall event.

 

SST(s)

Stands for Sea Surface Temperature(s)

 

SSTA(s) - Sea Surface Temperature Anomalies

Anomalies in the surface temperature are associated with changes in the heat exchange between atmosphere and ocean, or changes in ocean currents or upwelling, and these changes can drive large changes in rainfall and atmospheric circulation patterns. SSTAs are therefore often strongly related to the development and maintenance of unusual climate patterns, such as ENSO.

 

Statistical Terms

See Verification Statistical Terms

 

Stratosphere

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

 

Stratus cloud

Low cloud forming a uniform layer.

 

Summer

The three hottest months December, January and February for the southern hemisphere, and June, July and August for the northern hemisphere.

 

Sunshine

Direct radiation from the sun, as opposed to the shading of a location by clouds or by other obstructions.

 

Supercell

A persistent, single, intense updraught and downdraught coexisting in a thunderstorm.

 

Super Typhoon

See Tropical Cyclone Terms

 

SW Monsoon (Southwest Monsoon)

The Asian monsoon covering India to Southeast Asia is associated with prevailing wind blowing from the southwest between May and September during the northern hemisphere summer-autumn season.  Wind during this monsoon season is warm and moist, bringing rain (wet monsoon).

 

Swell

See Wave Terms

 

Synoptic chart

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

 

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).

Temperature

A physical quantity characterising the mean random motion of molecules in a physical body.

 

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

Sudden electrical discharges manifested by a flash of light (lightning) and a sharp rumbling sound. Thunderstorms are associated with convective clouds (Cumulonimbus) and are more often accompanied by precipitation. They are usually short lived and hit on only a small area.

 

Time descriptions used in forecasts and warnings

Timings refer to Local Time:

  • Early in the morning: Expected to occur before 6am.
  • In the morning: Expected to occur between 6am and 10am.
  • Middle of the day: Expected to occur between 11am and 1pm.
  • During early afternoon: Expected to occur after 12 noon and 3pm.
  • In the afternoon: Expected to occur between 3pm and 6pm.
  • During the evening: Expected to occur between 6pm and 8pm.
  • Later in the evening: Expected to occur after 9pm.

 

Tornado

A whirlwind or mass of violently rotating winds having the appearance of a funnel-shaped cloud and advancing beneath a large storm system.

 

Total-totals

Measure or Index of atmospheric instability used to assess the strength of storms and for severe weather forecasts. A combination of calculations relating to the air temperature and dew point temperature at different levels of the atmosphere.

 

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.

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

Tropical cyclones are intense low pressure systems which form over warm ocean waters at low latitudes. Tropical cyclones are associated with strong winds, torrential rain and storm surges in coastal areas. Tropical cyclones can cause extensive damage as a result of the strong wind, flooding (caused by either heavy rainfall or ocean storm surges) and landslides in mountainous areas as a result of heavy rainfall and saturated soil.

When tropical cyclones attain maximum mean winds above 117 km/h (63 knots) they are called severe tropical cyclones in Australia, typhoons in the northwestern Pacific and hurricanes in the northeast Pacific and Atlantic/Caribbean.

 

Tropical Cyclone Terms:

Cyclonic Storm

The term used in the Bay of Bengal to describe a tropical cyclone with an intensity (as measured by 1 min average surface winds at 10m) greater than 33kt.

 

Depression or Severe Depression

The term used in the Bay of Bengal to describe a Tropical Cyclone with an intensity (as measured by 1 min average surface winds at 10m) less than 34kt.

Equivalent to NW Pacific Tropical Depression.

 

Intense Tropical Cyclone

Term used in SW Indian Ocean for Tropical Cyclone with an intensity of 90-114kt.

 

Moderate Tropical Storm

The term used in the SW Indian Ocean for Tropical  with an intensity between 34-47kt. Similar to Tropical Storm in the NW Pacific.

-

Severe Cyclonic Storm

The term used in the Bay of Bengal to describe a Tropical Cyclone with an intensity (as measured by 1 min average surface winds at 10m) greater than 64kts

 

Severe Tropical Cyclone

The term used in the Australia to describe a Tropical Cyclone with an intensity (as measured by 1 min average surface winds at 10m) greater than 64kt.  Severe Tropical Cyclone have categories 3, 4, and 5.

 

Severe Tropical Storm

An area of low pressure over tropical waters, with a well established surface wind circulation and a graded wind field increasing in strength towards the centre, and winds near the centre between 48-63kt.  The convective clouds have distinctive ‘spiral bands’.

Terms are used in the NW Pacific and SW Indian Ocean.

 

Super Typhoon

The most intense of typhoons with central surface wind speeds in excess of 130kts, and central atmospheric pressures lower than 904mb in the NW Pacific.

 

Super Cyclonic Storm

The term used in the Bay of Bengal to describe a tropical cyclone with an intensity (as measured by 1 min average surface winds at 10m) in excess of 120kt.

 

Tropical Cyclone

Term used in SW Indian Ocean for Tropical Cyclone with central surface winds speed of 64-89kt.

Term used in the Australia to describe a Tropical Cyclone with an intensity 34-63kt.  Tropical Cyclone have categories 1 and 2.

 

Tropical Depression

An area of low pressure over tropical waters, with a well established surface wind circulation and convective activity closely associated with the centre of the system. The wind field has graded winds increasing in strength towards the centre of the system, with wind speeds near the centre generally less than 33kts. The central atmospheric pressure is between 1002-1006mb.

 

Tropical Disturbance

A disorganised cluster of convective (rain bearing) clouds, close to or surrounding a weak surface circulation with a central pressure lower than the surrounding region. Also known in Australian waters as a tropical low.

 

Tropical Low

An area of low pressure in tropical regions that may or may not have associated convective activity or an associated surface circulation. A disorganised cluster of convective clouds, close to or surrounding a weak surface circulation with a central pressure lower than the surrounding region.

 

In Australia, Tropical Low is equivalent in intensity to the Tropical Depression category in the NW Pacific.

 

Tropical storm

An area of low pressure over tropical waters, with a well established surface wind circulation and a graded wind field increasing in strength towards the centre, and winds near the centre between 34-47kts. The convective clouds begin forming distinctive ‘spiral bands’.

 

Typhoon

Term used in the northwestern Pacific for a tropical cyclone with maximum winds above 117 km/h (63 knots).  A very intense and well developed tropical cyclone with a distinctive ‘eye’ usually visible at the centre of the system. The strongest surface wind speeds are in the ‘eye wall’ immediately surrounding the ‘eye’.  Typhoon with surface winds over 130 knots is called Super Typhoon.

 

Very Intense Tropical Cyclone

Term used in the SW Indian Ocean for Tropical Cyclone with central surface winds speed of 115kt and more.

 

Very Severe Cyclonic Storm

The term used in the Bay of Bengal to describe a tropical cyclone with an intensity (as measured by 1 min average surface winds at 10m) greater than 63 knots.  Similar to Typhoon category in the NW Pacific.

 

Global Tropical Cyclone Terminology
Australian name/Category NW Pacific Arabian Sea /Bay of Bengal SW Indian Ocean South Pacific
(East of 160E)
Tropical low Tropical depression Depression or severe depression Tropical depression Tropical depression
Tropical cyclone (1) Tropical storm Cyclonic storm Moderate tropical storm Tropical cyclone (Gale)
Tropical cyclone (2) Severe tropical storm Severe cyclonic storm Severe tropical storm Tropical cyclone (Storm)
Severe tropical Cyclone (3) Typhoon Very severe cyclonic storm Tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone (4) Typhoon Very severe cyclonic storm Intense tropical cyclone Tropical cyclone (Hurricane)
Severe tropical cyclone (5) Typhoon Super cyclonic storm Very intense tropical cyclone Tropical cyclone (Hurricane)

 

Tropical Revolving Storms

See 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.  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.

 

Typhoon

See Tropical Cyclone Terms

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.

 

Umbra

That part of the shadow cast by the Earth or the Moon within which the Sun is completely hidden.

 

Unsettled

Changeable, variable (Oxford Dictionary).

 

Unstable air

Air in which static instability exists. This condition is determined by the vertical gradients of air temperature and humidity.

 

Updraught

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

 

Upwelling

The rise to the surface of cold, deep ocean waters.

Vapour

Gaseous form of a substance.

 

Verification Statistical Terms

Average

Vague term that usually denotes the arithmetic mean, but it can also denote the median, the mode, the geometric mean, and the weighted means.

 

Bias

A measurement procedure or estimator is said to be biased if, on the average, it gives an answer that differs from the truth. The bias is the average (expected) difference between the measurement and the truth.

 

Correlation

A measure of linear association between two (ordered) lists. Two variables can be strongly correlated without having any causal relationship, and two variables can have a causal relationship and yet be uncorrelated.

 

Correlation coefficient

The correlation coefficient r is a measure of how nearly a scatterplot falls on a straight line. The correlation coefficient is always between −1 and +1. To compute the correlation coefficient of a list of pairs of measurements, first transform the data individually into standard units. Multiply corresponding elements of the transformed pairs to get a single list of numbers. The correlation coefficient is the mean of that list of products.

 

Deviation

A deviation is the difference between a datum and some reference value, typically the mean of the data. In computing the standard deviation, one finds the root mean square of the deviations from the mean, the differences between the individual data and the mean of the data.

 

Inter-quartile Range (IQR)

The inter-quartile range of a list of numbers is the upper quartile minus the lower quartile.

 

Mean, Arithmetic mean

A mean or average value over a stated period is the arithmetic mean. This is obtained by taking the sum of the individual values and then dividing the sum total by the number of values.  The mean of an element for a particular month is obtained by totalling all the values of that element for that month over the period of record and dividing by the number of values.

 

Mean absolute error (MAE)

MAE is a quantity used to measure how close forecasts or predictions are to the eventual outcomes. The mean absolute is an average of the absolute errors.

The mean absolute error is a common measure of forecast error in time series analysis.  The MAE, however, disregards the direction of over- or under- prediction.

 

Mean Squared Error (MSE)

The MSE is the mean of the square of the errors.  The MSE differs according to whether one is describing an estimator or a predictor.

As a predictor, if XF is the forecast value, XO is the observed/actual value and n is the number of samples, then

MSE(X) = (å (XF−XO)2) / n.

 

As an estimator, the MSE of an estimator of a parameter is the expected value of the square of the difference between the estimator and the parameter. In symbols, if X is an estimator of the parameter t, then

MSE(X) = E( (X−t)2 ).

 

* Where the expected value (E) is the predicted value calculated as the sum of all possible values each multiplied by the probability of its occurrence.

 

Median

"Middle value" of a list. The smallest number such that at least half the numbers in the list are no greater than it.

 

The median for the month is the value of an element which exceeds half the occurrences for that element for that month over the period on record. That is, there is a 50% probability of the element being below the median value. If the values for a month are ranked or arranged in increasing order of magnitude and there are an odd number of values, then the median is the middle value. If the number of observations is even, then the median is calculated by taking the mean of the two middle values.

 

With many meteorological quantities the mean and median values are close and the use of either values is acceptable. Although this may often be the case with annual rainfall, for shorter periods the mean can differ significantly from the median, as the mean can be influenced by an extremely heavy or light value, while the median is not. Hence the median is usually taken as giving a better description of the characteristics of rainfall.

 

Probability

The probability of an event is a number between zero and 100%.

 

Probability Distribution

The probability distribution of a random variable specifies the chance that the variable takes a value in any subset of the real numbers. The probability distribution of a discreet random variable can be characterized by the chance that the random variable takes each of its possible values.

 

Quantile

Quantiles are points in a distribution that relate to the rank order of values in that distribution. The qth quantile of a list (0 < q ≤ 1) is the smallest number such that the fraction q or more of the elements of the list are less than or equal to it. As such, if the list contains n numbers, the qth quantile, is the smallest number Q such that at least n×q elements of the list are less than or equal to Q.

 

Quartiles

A quartile is a type of quantiles. There are three quartiles.  By sorting the sample in increasing order, the middle value of the sorted sample (middle quantile, 50th percentile) is known as the median.

First quartile (Q1) is defined as the middle number between the smallest number and the median of the data set.

Second quartile (Q2) is the median of the data.

Third quartile (Q3) is the middle value between the median and the highest value of the data set.

 

Random Error

All measurements are subject to error, which can often be broken down into two components: a bias or systematic error, which affects all measurements the same way; and a random error, which is in general different each time a measurement is made.

 

Regression, Linear Regression

Linear regression fits a line to a scatterplot in such a way as to minimize the sum of the squares of the residuals.  The resulting regression line, together with the standard deviations of the two variables or their correlation coefficient can be a reasonable summary of a scatterplot if the scatterplot is roughly football-shaped.

 

Residual.

The difference between a datum and the value predicted for it by a model. In linear regression of a variable plotted on the vertical axis onto a variable plotted on the horizontal axis, a residual is the "vertical" distance from a datum to the line. Residuals can be positive (if the datum is above the line) or negative (if the datum is below the line). Plot residuals can reveal computational errors in linear regression, as well as conditions under which linear regression is inappropriate, such as nonlinearity.

 

Root-mean-square (RMS)

Also known a quadratic mean.  The RMS is the square-root of the mean of the squares of a set of numbers.  It is a measure of the average "size" of the numbers. To compute the RMS of a list, you square all the entries, average the numbers you get, and take the square-root of that average.

 

Root-mean-square error (RMSE)

The RMSE is a frequently used measure of the differences between values (sample and population values) predicted and the values actually observed.  In wind forecast verification, it indicates the differences between the forecast values and actual winds observed.

 

Scatterplot

A scatter plot (also called a scatter graph, scatter chart, scattergram, or scatter diagram) is a type of plot or mathematical diagram using Cartesian coordinates to display values for typically two variables for a set of data.

Scatter plots show how much one variable is affected by another. The relationship between two variables is called their correlation.

 

SD line

For a scatterplot, a line that goes through the point of averages, with slope equal to the ratio of the standard deviations of the two plotted variables.

 

Standard Deviation (SD)

A quantity expressing by how much the members of a group differ from the mean value for the group.  The standard deviation is the root mean square of the set of deviations between each element and the mean of the set.

 

Systematic error

An error that affects all the measurements similarly.  Systematic errors do not tend to average out.

 

Virga

Precipitation that evapourates before it reaches the ground.

 

Vorticity

Measure of the amount of local rotation or spin in the atmosphere at a specified level. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e. rotation about a vertical axis).

 

Vortex

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

Wave terms:

Extreme Wave Height

The greatest vertical distance between a wave crest and the succeeding or proceeding wave trough in any given observation. This is usually taken as 2.00 times the significant wave height.

Maximum Wave height

The average height of the highest one tenth of the waves in any given observation. This is usually taken as 1.67 times the significant wave height.

Peak Wave Period

Period in seconds between the swell of the primary swell component. The larger the time difference, the greater the amount of energy associated with the swell.

Primary Swell

Height and direction of the swell with the highest energy component. This is sometimes referred to as the dominant swell.

Sea waves

A water surface wave that is generated and sustained locally by the frictional drag of the wind over the water surface. These waves mainly move in the direction of the prevailing wind.

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.

Significant wave height (Sea and Swell Combined)

Also known as total wave height.  Traditionally defined as the mean height of the highest third of the waves.  The combined sea and swell describes the combined height of the sea and the swell that mariners experience on open waters.

Formula on how the combined sea and swell is calculated:

Total Wave Height = [(Wind Wave Height)2 + (Swell Wave Height)2]1/2

Swell

A water surface wave that has been generated by the wind at a distance from the observer and that travels through the area of the observer and is no longer sustained by the generating wind force. Typically swell is thought of as being generated by wind at least 30-40nm distant from observer.

Swell Period

Period in seconds between the swell of the particular swell train (primary, secondary).  The larger the time difference, the greater the amount of energy associated with the swell.

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 time interval between successive wave crests or troughs. However, in a random sea, a commonly used definition is the so-called zero-up crossing period, which is the time interval between two successive up crossings of the mean water level.

Wave Height

The vertical distance between a wave crest and the preceding or following wave trough.  However, in a random sea a commonly used definition is the so-called zero-up crossing wave height, which is the range of elevations (difference between highest crest and lowest trough) between two successive up crossings of the mean water level.

Wave Length

The mean horizontal distance between successive crests (or troughs) of a wave pattern.

Sea Scales in open sea

Sea scales (in open sea)
Description Height (metres) Effect WMO Sea State code
Calm (glassy) 0 No waves breaking on beach 0
Calm (rippled) 0 - 0.1 No waves breaking on beach 1
Smooth 0.1 - 0.5 Slight waves breaking on beach 2
Slight 0.5 - 1.25 Waves rock buoys and small craft 3
Moderate 1.25 - 2.5 Sea becoming furrowed 4
Rough 2.5 - 4 Sea deeply furrowed 5
Very rough 4-6 Sea much disturbed with rollers having steep fronts 6
High 6-9 Sea much disturbed with rollers having steep fronts (damage to foreshore) 7
Very high 9-14 Towering seas 8
Phenomenal over 14 Precipitous seas (experienced only in cyclones) 9

Swell description

SWELL
Description Wave Length Period Wave Height
Low swell of short or average length 0 - 200 m <=11 sec 0-2 m
Long, low swell over 200 m >11 sec 0-2 m
Short swell of moderate height 0-100 m <8 sec 2-4 m
Average swell of moderate height 100-200 m  8-11 sec 2-4 m
Long swell of moderate height over 200 m >11 sec 2-4 m
Short heavy swell 0-100 m <8 sec over 4 m
Average length heavy swell 100-200 m 8-11 sec over 4 m
Long heavy swell over 200 m >11 sec over 4 m

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 evapouration 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 evapouration from the wet-bulb thermometer depends on the humidity of the air - evapouration 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 Terms:

Mean wind speed

The mean wind speed over a period of time is the mean of many gusts and lulls. Usually only the mean wind is forecast, unless the gusts are expected to be a significant feature.  In all weather forecasts issued by OWS, the forecast wind value is a 10 minute mean wind, except for Tropical Cyclone intensity when a 1 minute mean is used.

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. Increase in wind speed is 16 knots or more to least 22 knots and lasting for a period of several minutes.  Squalls typically last at least 10 minutes but usually no longer than 40 minutes.  A squall may include many gusts

Surface Wind: wind speed and direction measured at 10 metres above the earth's surface. The surface wind drives wave generation locally, and is responsible for large swells generated by strong winds associated with intense storms.

Wind

Moving air parcel

Wind Gust: a gust is any sudden increase of wind of short duration, usually a few seconds.  In all weather forecasts issued by OWS, the forecast gust value is a 3 second mean wind speed value

Wind speed

The ratio of the distance covered by air parcel to the time to cover that distance.  The wind is a continuous succession of gusts and lulls (quiet intervals) associated with equally rapid changes of direction over a range which may exceed 30°.

 

Wind Warning

A Wind Warning is a statement which warns of winds ranging from strong, near gale, gale, strong gale, storm, violent storm and hurricane force coastal waters and high seas areas.

 

Winter

The three coldest months June, July and August in the southern hemisphere, and December, January and February in the northern hemisphere.

 

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 evapouration 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.

Zonal flow

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

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