Weather Elements

Precipitation:

Any or all of the forms of water, 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.

Showers:

Usually begin and end suddenly. Relatively short-lived, but may last half an hour. Fall from cumulus clouds, often separated by blue sky. Showers may fall in patches rather than across the whole forecast area. Range in intensity from light to very heavy.
See also: Clouds and precipitation.

Rain:

In contrast to showers, rain is steadier and normally falls from stratiform (layer) cloud. Liquid water drops greater than 0.5 mm in diameter. Rain can range in intensity from light to very heavy.
See also: Clouds and precipitation.

Drizzle:

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

Frost:

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

Fog:

Suspension of very small water droplets in the air, reducing visibility at ground level to less than a kilometre.

Mist:

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

Haze:

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.

Dusty:

Dust is raised by the wind to moderate heights above the ground. The visibility is reduced at eye level.

Thunderstorms:

Thunderstorms are one or more convective clouds in which electrical discharge can be seen as lightning and heard as thunder by a person on the earth's surface.
A severe thunderstorm produces one or more of :-

• hail at the ground with diameter of 2 cm or more;
• wind gusts at the ground of 90 km/h or more;
• tornadoes; or
• very heavy rain likely to cause flash flooding.

Tornado:

A tall, rapidly rotating column of air between 5 and 1000 metres in diameter which is attached to the base of a cumulonimbus or large cumulus cloud and which is capable of producing damage at the earth's surface. Tornadoes may form water spouts when they occur over water.

Blizzard:

Violent and very cold wind which is laden with snow, some part, at least, of which has been raised from snow covered ground.

Change:

Signified by a transition between two airmasses over a relatively short time period, usually when a cooler air mass replaces a warmer airmass over an area. A change may or may not be accompanied by rain, and is characterised by a rapid change in wind direction usually from warm north to northwesterly to cooler south east to southwesterly. A change differs from a sea breeze in that it is most often associated with the passage of a front or low pressure trough and affects a large area over a period of a day or more, as distinct from a sea breeze, which characteristically only affects areas up to around 60 km inland from the coast for a period of hours.

Windy:

A prolonged period of average wind speeds exceeding 40km/h during the day.

Time

Timings refer to Local Time in each State/Territory.

Early in the morning: 

Expected to occur before 7am.

In the morning: 

Expected to occur between 1am and 11am.

In the late morning: 

Expected to occur between 9am and midday.

During early afternoon: 

Expected to occur between 12pm and 4pm.

During the afternoon: 

Expected to occur between 1pm and 9pm.

In the evening: 

Expected to occur between 6pm and midnight.

Later in the evening: 

Expected to occur after 9pm.

Wet or Dry

Wet

Precipitation:

Any or all of the forms of water, 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.

Showers:

Usually begin and end suddenly. Relatively short-lived, but may last half an hour. Often, but not always, separated by blue sky.

Rain:

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

Drizzle:

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

Dew:

Deposit of water drops on objects at or near the ground, produced by condensation of water vapour from the surrounding air.

Thunderstorms:

Thunderstorms (commonly shortened to Storms) are one or more convective clouds in which electrical discharge can be seen as lightning and heard as thunder by a person on the earth's surface.

Thundery rain:

Rain with embedded thunderstorms.

Fine:

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.

Dry

Dry:

Free from rain. Normally used when preceding weather has also been relatively dry, and dry weather is expected to continue for at least a day or so.

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.

Distribution of Showers or thunderstorms over time for a location

Shower or two:

Not many, less than a few. Used to describe showers occurring infrequently.

Few:

Not many, amounting to a small number. Used to describe a small number of showers over a time period.

Distribution of Showers or thunderstorms over States or Territories

Isolated:

Showers which are well separated in space (ie. generally covering 10 to 25% of the area) during a given period.

Local:

Restricted to relatively small areas.

Patchy:

Usually associated with rain. Occurring irregularly over an area (ie. generally covering 10 to 25% of the area).

Scattered:

Irregularly distributed over an area (ie. generally covering 25 to 55% of the area). Showers which, while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.

Widespread:

For rain: Occurring extensively throughout an area (ie. generally covering 25 to 55% of the area).

For showers: Occurring extensively throughout an area (ie. generally covering over 55% of the area).

Likelihood of showers, rain or thunderstorms

Possible, Chance and Risk:

These terms may be used interchangeably for location forecasts. Indicates that the weather element is expected to occur over the surrounding area, but due to the random nature of showers and thunderstorms, there is only a chance of that weather element occurring at the forecast location. Risk is generally associated with thunderstorms.

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

These terms are used in text forecasts to indicate the likelihood of receiving a measurable amount of rainfall (0.2 mm) during the day for a location.

Chance of rain	Terminology used
0%, 10%	No mention of rainfall in forecast
20%, 30%	Slight (20%) chance of rainfall in forecast Slight (30%) chance of rainfall in forecast
40%, 50%, 60%	Medium (40%) chance of rainfall in forecast Medium (50%) chance of rainfall in forecast Medium (60%) chance of rainfall in forecast
70%, 80%	High (70%) chance of rainfall in forecast High (80%) chance of rainfall in forecast
90%, 100%	Very high (90%) chance of rainfall in forecast Very high (near 100%) chance of rainfall in forecast

Intensity of Precipitation

Drizzle Intensity

Light:

Up to 0.2 mm per hour. Can be felt on the face but is not visible. Produces little run off from roads or roofs. Generally visibility is reduced, but not less than 1000 m.

Moderate:

0.2 mm to 0.4 mm per hour. Window and road surfaces streaming with moisture. Visibility generally between 400 and 1000 m.

Thick or Heavy:

Over 0.4 mm per hour. Visibility reduced to less than 400 m.

Rain or Showers Intensity

Light:

Up to 2 mm per hour. Individual drops easily identified, puddles form slowly, small streams may flow in gutters.

Moderate:

2.2 mm to 6 mm per hour. Rapidly forming puddles, down pipes flowing freely, some spray visible over hard surfaces.

Heavy:

6.2 mm to 50mm mm per hour. Falls in sheets, misty spray over hard surfaces, may cause roaring noise on roof.

Violent:

Over 50mm mm per hour. Gutters and downpipes overflowing, spray to height of several centimetres over hard surfaces, may cause roaring noise on roof.

Hail Intensity

Light:

Sparse hailstones of small size, often mixed with rain.

Moderate:

Particles numerous enough to whiten the ground.

Heavy:

A proportion of the hailstones exceed 6 mm diameter.

Snow Intensity

Light:

Gives a water equivalent of up to 2 mm per hour.

Moderate:

Gives a water equivalent of 2.2 mm to 6 mm per hour.

Heavy:

Gives a water equivalent of more than 6 mm per hour.

Cloud Cover

Clear:

Free from cloud, fog, mist or dust haze.

Sunny:

Little chance of the sun being obscured by cloud.
Cloud expected to cover approximately 0 to 15% of the sky for the majority of the day. (Note: 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 the sky may consist of cirrus cloud.)

Mostly sunny

Predominantly more clear sky than cloud.
Cloud expected to cover approximately 15 to 30% of the sky for the majority of the day. For example, during the day the sun would be obscured by cloud for short periods of time.

Partly cloudy

Predominantly more clear sky than cloud.
Cloud expected to cover approximately 30 to 65% of the sky for the majority of the day. For example, during the day the sun would be regularly obscured by cloud for short periods of time.

Cloudy:

Predominantly more cloud than clear sky.
Cloud expected to cover approximately 65 to 100% of the sky for the majority of the day. For example, during the day the sun would be obscured by cloud for substantial periods of time.

Overcast:

Sky completely covered with cloud.
Cloud expected to cover 100% of the sky for the majority of the day.

Notes on terms used in cloud forecasts

Forecasts of cloud cover normally give an average, if no significant variations are expected. A clear day, for example, may at some times see a few cloud patches.

Forecasters expecting significant variations in cloud amount may use such terms as sunny periods, sunny breaks, cloudy periods, cloudy at times, mostly/mainly sunny, mostly/mainly cloudy.

If expecting a major change in cloud cover, they usually indicate a distinct trend, e.g. becoming sunny or cloud increasing.

Apparent Temperature/Heat Index :

An adjustment to the ambient temperature based on the current humidity and wind speed, designed to be a measure of the discomfort caused to an appropriately dressed adult, walking outdoors, in the shade by the current wind and humidity levels. For calm wind conditions, if the current humidity is higher than the reference humidity then the Apparent Temperature will be higher than the current Temperature; if the current humidity is lower than the reference humidity, then the Apparent Temperature will be lower than the current Temperature. In cold, windy conditions, the Apparent Temperature can also be used as a measure of Wind Chill.

The Apparent Temperature used by the Bureau is the Steadman Apparent Temperature. For more information see http://www.bom.gov.au/info/thermal_stress/

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.

Temperature

Temperature terms like 'hot' or 'cool' represent different temperature ranges; for summer (October - March) and winter (April - September); and for different parts of Australia. For example, a cool winter day for Darwin, would be a warm winter day for Melbourne. The tables below match the temperature terms with summer and winter for different Australian regions.

October - March: Maximum daytime temperature

Descriptive term	Inland plains	Coast and ranges south of tropics	Coastal tropics
Very hot	more than 40 °C	more than 37 °C	-
Hot	35-39 °C	32-37 °C	35-39 °C
Warm	30-34 °C	27-31 °C	Not used
Mild	25-29 °C	22-26 °C	-
Cool	20-24 °C	16-21 °C	-
Cold	less than 20 °C	less than 16 °C	-

October - March: Minimum overnight temperature

Descriptive term	South of 33° S plus ranges	North of 33° S except ranges and tropics	Tropics
Hot	more than 22 °C	more than 25 °C	-
Warm	18-21 °C	21-24 °C	-
Mild	15-17 °C	18-20 °C	-
Cool	10-14 °C	13-17 °C	less than 20 °C
Cold	less than 10 °C	less than 13 °C	-

April - September: Maximum daytime temperature

Descriptive term	South of 33° S plus ranges	North of 33° S except ranges and tropics	Tropics
Warm	more than 20 °C	more than 25 °C	-
Mild	16-20 °C	20-24 °C	-
Cool	13-15 °C	15-19 °C	20-25 °C
Cold	10-12 °C	10-14 °C	less than 20 °C
Very cold	less than 10 °C	less than 10 °C	-

April - September: Minimum overnight temperature

Descriptive Term	South of 33° S plus ranges	North of 33° S except ranges and coastal tropics	Coastal Tropics
Mild	more than 10 °C	more than 15 °C	-
Cool	5-9 °C	10-14 °C	13-17 °C
Cold	1-4 °C	5-9 °C	less than 13 °C
Very cold	less than 1 °C	less than 5 °C	-

NOTES:
1. No international definitions have been laid down for the terms hot, warm, etc. The above table has therefore been developed in the Bureau to serve as a general guide only.
2. In using the table, forecasters use their discretion and take account of factors such as incidence of cloud/sunshine, wind (chill factor), location and time of year.
3. In some cases, terms such as 'warmer' may be used to contrast the expected weather with the weather recently experienced.
4. 'Mild' need not be used in the strict dictionary sense (calm, fine) when indicating its assigned temperature ranges. Nevertheless, 'mild' is not used when the weather departs significantly from the dictionary definition e.g: gales or heavy rain.

Wind

The wind is a continuous succession of gusts and lulls and changes of direction. Reported wind speed and direction are 10 minute averages. Usually only the 10 minute mean wind speed is forecast, unless the gusts are expected to be a significant feature.

Fresh, gusty southwest winds indicates that the mean wind speed will be between 30 and 39 km/h (17 - 21 knots) and the mean wind direction will be from the southwest, but that there will also be gusts to speeds significantly higher than the mean.

Wind direction:

Direction is based on true north orientation. Direction is where the wind is blowing from, for example a northerly wind is blowing from the north. Some forecasts may use abbreviations to describe the wind direction.

Gust:

A gust is any sudden increase of wind of short duration, usually a few seconds.

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.

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.

Windy:

A prolonged period of average wind speeds exceeding 40km/h during the day.

Tending:

A gradual change.

Shifting:

A relatively abrupt change.

Wind speed descriptions

(Derived from the Beaufort Wind Scale.) Wind speeds are given as the equivalent speed, averaged over 10 minutes at a standard height of 10 metres above open flat ground or sea. Wind speed is given in km/h in land areas and knots in marine areas.

	Units in km/h	Units in knots	Description on Land	Description at Sea
Calm	0	0	Smoke rises vertically	Sea like a mirror.
Light winds	19 km/h or less	10 knots or less	Wind felt on face; leaves rustle; ordinary vanes moved by wind.	Small wavelets, ripples formed but do not break: A glassy appearance maintained.
Moderate winds	20 - 29 km/h	11-16 knots	Raises dust and loose paper; small branches are moved.	Small waves - becoming longer; fairly frequent white horses.
Fresh winds	30 - 39 km/h	17-21 knots	Small trees in leaf begin to sway; crested wavelets form on inland water	Moderate waves, taking a more pronounced long form; many white horses are formed - a chance of some spray
Strong winds	40 - 50 km/h	22-27 knots	Large branches in motion; whistling heard in telephone wires; umbrellas used with difficulty.	Large waves begin to form; the white foam crests are more extensive with probably some spray
	51 - 62 km/h	28-33 knots	Whole trees in motion; inconvenience felt when walking against wind.	Sea heaps up and white foam from breaking waves begins to be blown in streaks along direction of wind.
Gale	63 - 75 km/h	34-40 knots	Twigs break off trees; progress generally impeded.	Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well marked streaks along the direction of the wind.
	76 - 87 km/h	41-47 knots	Slight structural damage occurs -roofing dislodged; larger branches break off.	High waves; dense streaks of foam; crests of waves begin to topple, tumble and roll over; spray may affect visibility.
Storm	88 - 102 km/h	48-55 knots	Seldom experienced inland; trees uprooted; considerable structural damage.	Very high waves with long overhanging crests; the resulting foam in great patches is blown in dense white streaks; the surface of the sea takes on a white appearance; the tumbling of the sea becomes heavy with visibility affected.
	103-117 km/h	56-63 knots	Very rarely experienced - widespread damage	Exceptionally high waves; small and medium sized ships occasionally lost from view behind waves; the sea is completely covered with long white patches of foam; the edges of wave crests are blown into froth.
Hurricane	118 km/h or more	64 knots or more		The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.

Wind direction descriptions

Abbreviation	Wind Direction
N	North
NE	Northeast
E	East
SE	Southeast
S	South
SW	Southwest
W	West
NW	Northwest

Sea and Swell

Direction:

Direction is based on true north orientation.

Inshore:

The coastal waters zone within 12 nautical miles of the coast.

Offshore:

The coastal waters zone between 12 and 60 nautical miles from the coast.

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.

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. A lookup table has been developed to outline how the Combined sea and swell is calculated.

Sea

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

Swell

Description	Wave Length	Period	Wave Height
Low swell of short or average length	0 - 200 m	Less than 11 sec	0-2 m
Long, low swell	over 200 m	Greater than 11 sec	0-2 m
Short swell of moderate height	0-100 m	Less than 8 sec	2-4 m
Average swell of moderate height	100-200 m	Greater than 8 sec, less than 11 sec	2-4 m
Long swell of moderate height	over 200 m	Greater than 11 sec	2-4 m
Short heavy swell	0-100 m	Less than 8 sec	over 4 m
Average length heavy swell	100-200 m	Greater than 8 sec, less than 11 sec	over 4 m
Long heavy swell	over 200 m	Greater than 11 sec	over 4 m