Bureau - Glossary of Terms (original) (raw)

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M

The terms and definitions found in the glossary relate to the science of meteorology.

For descriptions and definitions of words used in the Bureau's forecasts and warnings, please refer to Weather Words.

Mean

A mean or average value over a stated period is the arithmetic mean. This is obtained by totalling the individual values and then dividing this 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.

Median

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.

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.

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 wind. 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.

Parts of the North-Queensland coast also receive significant rainfall throughout the cooler months. In the Top-End of the NT, the Bureau considers the wet season as being from 1 October to 30 April, while in some other parts of tropical Australia, particularly in WA, the wet months are often only from about January to March.

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.