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Ipanema beach in Rio de Janiero

Glossary Of Terms

Cold front

The transition zone where a colder denser air mass collides with and replaces a significantly warmer (less dense) air mass. Within the frontal transition zone (i.e. just ahead of the main front) marks the region where there's a quick, short-lived increase in temperature coupled with oscillating surface atmospheric pressure values. As the main frontal boundary crosses a region, the temperature drops while the surface atmospheric rapidly rises in response to the increase in air density.

Great Dividing Range

An extensive chain of mountains that spans 3,500 km (2175 mi) north to south along the eastern coast of Australia. The southern end of this mountain range is the location of Australia's highest mountain (i.e Mt Koscuiszko - 2,230m above sea-level). This mountain chain separates the drier inland parts of Australia from the more humid and wetter coastal plains.

High pressure systems

An area of raised atmospheric pressure which results in wind blowing in a anticlockwise direction in the Southern Hemisphere and in a clockwise direction in the Northern Hemisphere. Also known as an anticyclone.


Are powerful downdraughts (initially formed in thunderstorms) that cover a horizontal extent of less than 4 km. This type of downdraught pushes cold high altitude air (sometimes accompanied with precipitation) rapidly to the surface where it spreads out radially with often sufficient force so as to produce severe wind damage. Microbursts present a significant hazard to aircraft that are taking off or landing since the aircraft can flip if caught in one.

Mixing ratio

A measure of how much moisture is in the air. Technically speaking, it's the ratio between the mass of water vapor (expressed in grams) to the mass of dry air (measured in kilograms) in a given volume of air. In tropical coastal regions of the world, it's common to see mixing ratios of around 20-28g/kg. Generally, most people begin to complain that the air is too humid (thus causing discomfort) when the mixing ratio rises above 25g/kg on a windless day.

Mathematically, the mixing ratio (r) is:

r =   mv

where mv is known as the mass of water vapour (g) and md is the mass of dry air (kg).

North-west cloud band

Is a mid-to-high level cloud band that forms in the eastern Indian Ocean (near Indonesia) and stretches towards the south-east across Australia for several thousand kilometres. This type of cloud band frequently occurs from April to September/October and is an important contributor to the rainfall in south-eastern Australia throughout these months.

The north-west cloud bands presence indicates that tropical air is pushing southwards and is starting interact with cooler air masses.

Polycyclic aromatic hydrocarbons (PAH's)

Used to describe a specific type of persistent organic pollutant (POP). PAH's normally form due to incomplete combustion of domestic waste, biomass (wood), building debris, coal, natural gas, oil, vehicle emissions (diesel and gasoline) as well as from many others sources. The majority of these combustion sources produce ultrafine particulates and so easily penetrate deeper into our respiratory tract.

The main reason why there is widespread scientific interest in PAH's is due to their carcinogenic and mutagenic causing properties (particularly benzo[a]pyrene (B[a]P) and benz[a]anthracene (B[a]A).

Relative humidity

A measure of how close the air is to saturation. The term relative humidity is still used rather carelessly in the media and in meteorological broadcasts to the public and is not readily used by meteorologists for scientific atmospheric studies since it is temperature-dependent. For example, if the air temperature in London is 5°C and the relative humidity at the time is 95%, does it feel more humid than Singapore where the air temperature is 31°C and the relative humidity is 80%? Of course not. The relative humidity levels are higher in London but you would feel that the air is cold and damp. In comparison, Singapore air would cause thermal discomfort since there's more moisture present.

Relative humidity (RH) can be written as:

RH =   r

where r is known as the actual mixing ratio and rs is the mixing ratio at saturation at the same temperature.

Southerly buster (occur along south-east coast of Australia and New Zealand)

Southerly busters are a type of severe shallow cold front that produces strong gusty southerly winds in excess of 29 knots (54km/h). This occurs in conjunction with a dramatic drop in temperature of 10-15°C over 5-30 minutes.

Southerly busters occur during the warmer months from mid-September through to February and generally form over the eastern coastlines of Tasmania, New Zealand and quite frequently along Australia's south-eastern mainland along the coastal plain between Gabo Island (i.e far southern New South Wales) and Coffs Harbour (i.e. a town located on the mid-north coast of New South Wales). However, the classic southerly busters occur between Nowra and Newcastle (Australia). Depending upon how unstable the air is, the leading edge of the cold front may or may not be accompanied by an ominous looking roll cloud.

In terms of its formation; this particular type of cold front becomes trapped by mountain ranges in eastern Victoria as it tries to cross perpendicular to them. As a result, the western edge of the front stalls and lags behind the coastal edge of the cold front. Thisin turn helps the coastal edge of the front to accelerate and swing northwards once it passes Gabo Island. In all cases, a southerly buster occurs when the depth of the cold air propogating behind the cold front is lower than the height of the Great Dividing Range. Additionally, the coastal part of the cold front rushes northwards and remains perpendicular to the mountain ranges while strong heating is being maintained ahead of the leading edge of the front.

Temperature inversion

An increase in temperature with height. This is quite noticeable if you happen to climb a mountain during the winter months. Since cold air is heavier, cold air tends to drain into low lying regions near the base of the mountain. If you were to climb several hundred metres above the valley floor on such a night, you would notice that the temperature would be significantly warmer. That's why farmers prefer to plant their crops on hill slopes rather than near the base of a mountain so their crop will not be damaged by low temperatures during the winter.

Temperature inversions can form above any surface (flat or topography) when the air is stable due to the presence of a high pressure system (i.e. anticyclone). Inversions are most prominent in the mid-to-high latitude regions of the world during the winter months when there's little wind and cloud cover. It occurs mostly at night time.

Upper cold pool

Abnormally cold air in the upper atmosphere. This can produce heavy rain when it interacts with a intense surface low pressure system or other type of rain-bearing weather system.

Urban heat island effect

Is formed when there's a steep horizontal temperature gradient located at the boundary between a rural landscape and the suburban zone. In some cities this temperature can be great as 4°C per kilometre. Cities are naturally much warmer than rural areas throughout the night particularly during the cooler months of the year when the winds are light and the sky is cloud free. On such nights, heat is released more slowly from urban surfaces (roads and high rise buildings) than it is from vegetation. Additionally, artificial heating within urban areas increases also from industrial activity/heating and domestic heating. It is well known that urban warmth is more pronounced in higher latitude cities (e.g. cities far away from the equator).

Volatile organic compounds (VOC's)

Are chemical compounds found in both indoor products (e.g. wet paint, new furnishings, building materials, floor coverings, as well as burnt food and heated cooking oils) and outdoor environments (e.g. traffic emissions as well industrial pollutants) that evaporate into the air. These chemical compounds accumulate in an indoor setting due to the lack of ventilation and moderate-to-high indoor emission rates. VOC's are one of the most common indoor chemical airborne pollutants that we breathe and is known to cause sick building syndrome and other respiratory health problems. Of course, the concentrations of VOC's varies from home-to-home due to occupant behaviour and varying weather conditions at different latitudes.

Volatile organic compounds that occur outdoors are normally much lower in concentration since wind and other meteorological factors allow for easier dispersal into the atmosphere.

The main health concern is there are some VOC's that are carcinogenic (e.g., aldehydes, formaldehyde and acrolein).

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