Bureau - Glossary of Terms (original) (raw)

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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.

Percentages

One useful way of representing rainfall across a wide area like Australia is in terms of percentages of a mean rainfall over some base period. Percentages between 0% and 100% indicate lower than average rainfalls, while percentages greater than 100% indicate higher than average rainfalls. In some northern parts of the country, dry season rainfalls are often very low, and weeks or even months may go by without any rainfall being recorded. In these places and times of the year, the monthly rainfalls may be below average (that is, percentages less than 100%) in rather more than 50% of years.

The percentage of mean rainfall maps on this web site are all generated from monthly means calculated for each calendar month using the years 1961 to 1990.

Note that the month to date rainfall percentages maps compare the rainfall recorded in the month to date with the mean rainfall for the entire month. Consequently early in the month, the rainfall percentages will typically be quite low, unless the rainfall is unusually heavy at the start of the month.

Likewise, the year to date rainfall percentages maps compare the rainfall recorded in the year to date with the mean rainfall for the entire year.

The southern wet season is defined as the months April to November (inclusive). The southern wet season to date rainfall percentages compare the rainfall recorded in the southern wet season to date (that is, since the start of April) with the mean rainfall for the entire season.

The northern wet season is defined as the months October to April (inclusive). The northern wet season to date rainfall percentages compare the rainfall recorded in the northern wet season to date in a manner analogous to those for the southern wet season.

It is quite difficult to tell from a rainfall percentages map if the rainfall has been highest or lowest on record. To see how the rainfall in the current period compares with previous years, the rainfall deciles maps should be consulted.

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. (See Drizzle, Rain).

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 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 is likely to occur at a particular location. Such a flood would result from the most severe combination of critical meteorological and hydrological conditions.

Probable Maximum Precipitation (PMP)

The theoretically greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of year.

Probabilities, or Probabilistic Forecasts

An attempt to convey the uncertainty in a forecast by expressing its likelihood of occurrence as a probability, or percentage. Akin to odds in the gambling industry. 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.

Probability of Precipitation Forecast

As part of the NexGenFWS, rainfall forecasts for major centres are presented as chance of any rain and rainfall amount for day one of the forecast.