The study of climatic factors and, particularly, any tendency to even a minor change, is of great importance to Australia's economic development. This information is, and will be, available only through the interest and public spirit that have inspired a large number of observers. These observers have been on station properties, at Post Offices, on lighthouses, and, in all walks of life, to record weather reports carefully and regularly through the years - their services merit the warmest appreciation.

The Commonwealth Bureau set up its headquarters in 1908 in Melbourne, on the corner of Victoria and Drummond Streets, Carlton, on the northern fringe of the city. In addition to administering the meteorological service throughout Australia, this office continued the meteorological functions of the State Observatory. In subsequent years, it became obvious that the needs of Victoria required the special attention of a single unit separated from the Commonwealth administration. This led to the emergence of the office of the Deputy Director (Victoria) and has continued to meet the growing demand for weather forecasts and a wide range of climatological data

Overview

The State of Victoria experiences a wide range of climatic conditions. These range from the hot summer of the Mallee to the winter blizzards of the snow-covered alps, and from the relatively dry wheat belt to the wet eastern elevated areas from which so many of our permanent streams spring.

The average pattern is complicated by the vagaries of the season. In exceptionally wet years, the Mallee may suffer from surplus rains with normally dry water-courses two kilometres wide, and all farming operations brought to a stand-still. In other years, stream flow from the eastern highlands can be drastically reduced.

Climate zones

The climate of Victoria is characterised by a range of different climate zones.

There are the warm and dry grasslands of the State's northwest, covering the Mallee, and much of the Wimmera and Northern Country.

The climate of the less elevated parts of the Northeast is classified as temperate with no dry season and a hot summer.

By contrast, the climate of the State's mountainous regions, as well as that of South Gippsland, the Otways, and the exposed coast of the far southwest, is classified as temperate with no dry season and a mild summer.

Most of the rest of Victoria experiences a climate classified as temperate with no dry season and a warm summer.

The exception is an area covering much of the far southwest of the State, excluding the exposed coast. This area experiences a climate classified as temperate with a distinctly dry and warm summer.

Daytime temperatures

February is the hottest month of the year, with January only slightly cooler. Average maximum temperatures are in the low 20s along the coast. Over the elevated areas that form the Central Divide and the Northeast Highlands, maximum temperatures decrease steadily with height, being as low as in the mid-teens in the very highest localities. Apart from these mountains, there is a steady increase from the coast to the north, until, in the extreme north, an average in the low 30s is reached.

Temperatures fall rapidly during the autumn months and then more slowly with the onset of winter. Average maxima are lowest in July; the distribution during this month again shows lowest values over the elevated areas, but a significant feature is that apart from this orographically induced area, there is practically no variation across the State. Day temperatures on the coast average in the low-teens in July; much the same value is recorded over the wheat belt, and temperatures are only one or two degrees higher in the far northwest under conditions of few clouds and relatively profuse winter sunshine.

Conditions of extreme summer heat may be experienced throughout the State except over the Alpine area. Most inland places have recorded maxima in the mid-40s with an extreme for the State of 50.8°C at Mildura in the far northwest.

Usually, such days are the culmination of a period during which temperatures gradually rise. Relief comes sharply in the form of a cool change with rapid temperature drops of more than 15°C at times. However, such relief does not always arrive so soon, and periods of two or three days, or even longer, have been experienced when the maximum temperature exceeds 40°C.

On rare occasions, extreme heat may continue for as long as a week with little alleviation. In 1908, there was an exceptional heatwave when the temperature in Melbourne rose to 39.9°C on 15 January, and then over the subsequent five days reached 42.8°C, 44.2°C, 40.0°C, 41.1°C and 42.7°C, respectively.

Overnight temperatures

Night temperatures, as gauged by the average minimum temperature, are, like the maximum, highest in February. Values are below 10°C over elevated areas, but otherwise they are mostly between the low-teens and the mid-teens.

In mid-winter, average July minima are between 6°C and 8°C along exposed parts of the coast and in Melbourne's Central Business District; elsewhere, there is a steady decrease from between 4°C and 6°C over the plains, to below zero over the higher alps.

With the exception of the exposed coast, all parts of Victoria may experience severe frost. Although most of these occurrences are restricted to the winter season, spring frosts constitute a serious hazard to agriculture and in some years a late frost may result in serious frost damage.

Frequencies are highest, and occurrences usually more severe, in elevated areas and valleys conducive to the pooling of cold air. At a large number of inland stations, extremes stand at below -5°C. In a few susceptible areas, notably deep mountain valleys and on the highest alps, minima have been below -10°C.

The lowest temperature recorded in Victoria was -12.8°C at Hotham Heights on both 30 July 1931 and 13 August 1947. By contrast, the lowest temperature recorded at Cape Otway is exactly 0.0°C.

Usually, the coldest nights follow a burst of showery weather from the south or southwest and occur once the State comes under the stabilising influence of the following anticyclone.

Rainfall

Different synoptic systems produce rainfall in different parts of Victoria. The most reliable rainfall occurs in the Western District, where the passage of cold fronts, especially in winter, bring frequent light to moderate falls.

The topography plays a considerable part in increasing rainfall in some areas, such as the Otway Ranges, where sloping valleys funnel winds. The Otway Ranges also produce a distinct rain-shadow to the east, with markedly lower average rainfalls between the ranges and the western side of Port Phillip Bay.

South of the Great Divide, the amount of rainfall produced by frontal systems gradually decreases eastwards, with the result that the East Gippsland District gets little rain from this source. The rainfall over East Gippsland is less reliable, with lengthy dry periods broken by relatively heavy rainfall events caused by low pressure systems off the NSW south coast or in eastern Bass Strait.

Inland, the main source of rain is from bands or areas of cloud from eastward moving frontal systems that cross the Southern Ocean. The frequency of these systems and the amount of rain associated with them varies from season to season. A weak monsoon season over northern Australia is sometimes associated with a dry period over inland Victoria. Irrigation water collected in the upper reaches of the Murray and Goulburn Rivers provide major supplementation to the water needs of agriculture and horticulture in the dry north and northwest.

Thunderstorms are another important source of rainfall, particularly during the spring and summer months. However, rainfall from thunderstorms is frequently very local in nature.

There are marked variations in the rainfall from one year to the next. There are three major causes of these variations. The first of these is the "El Niņo - La Niņa" phenomenon. The second of these is what is known as the "Indian Ocean Dipole". The third is the "Antarctic Circumpolar Wave".

El Niņo - La Niņa

The "El Niņo - La Niņa" phenomenon operates over the tropical Pacific Ocean and represents an oscillation between two main states, "El Niņo" and "La Niņa".

During the La Niņa state, the trade winds blow across the Pacific Ocean from east to west and the sea surface temperature progressively becomes warmer along this east to west track. As a result of evaporation from the ocean surface, these trade winds accumulate more and more moisture as they blow towards the Australasian / Indonesian region. By the time the trade winds reach our region, they contain so much water vapour that even relatively weak low pressure systems are able to liberate the vapour in the form of heavy rain and thunderstorms.

During the El Niņo state, the warm water is located, not over the Australiasian / Indonesian region but, over the central Pacific Ocean. As a result, the trade winds reverse and below towards the warm water and deny the Australiasian / Indonesian region its moisture. Then, even strong low pressure systems are unable to produce much rain from the relatively dry air over the region.

Indian Ocean Dipole

The "Indian Ocean Dipole" operates over the ocean areas to the northwest and west of Australia.

When the (usually) very warm waters off the northwest coast are even warmer than normal and the (usually) cool waters off the west coast are even cooler than normal, the resulting increase in temperature contrast between these two regions leads to a strengthening of the jet streams.

As a consequence of the stronger jet streams, vertical motions in the atmosphere are also stronger, and each front that moves across the continent is accompanied by a thicker cloud band than usual and also heavier rain than usual.

By contrast, when the (usually) very warm waters off the northwest coast are cooler than normal and the (usually) cool waters off the west coast are warmer than normal, the resulting decrease in temperature contrast between these two regions leads to a weakening of the jet streams.

As a consequence of the weaker jet streams, vertical motions in the atmosphere are also weaker. Each front that moves across the continent is then accompanied by a thinner cloud band than usual and also lighter rain than usual.

Antarctic Circumpolar Wave

The Southern Ocean contains a large current which flows continuously around the Southern Hemisphere, carrying most of the water with it. It takes about eight or nine years for this current to transport water completely around the world.

Carried along with this current are two large areas of relatively warm water, alternating with two large areas of relatively cold water. As these relatively warm and cold zones of water pass to the south of Australia, they impact upon the amount of moisture in winds from the Southern Ocean and, hence, the amount of rainfall accompanying these winds.

To illustrate, when the Southern Ocean water is warm, evaporation is greater and rainfall is higher.

Conversely, when the Southern Ocean water is cool, evaporation is less and rainfall is lower.

Evaporation

Evaporation is determined by measuring the amount of water evaporated from a free water surface exposed in a pan. Evaporation from a free water surface depends on a number of climatic elements, mainly temperature, humidity, and wind. Evaporation data are useful in water conservation studies and estimating potential evapotranspiration for irrigation and plant growth studies. In Australia, where surface water storage is vital over large areas, evaporation is a highly significant element.

Evaporation varies markedly with exposure of the instrument. Sheltering from wind and shading of pans cause local variations in measured evaporation of as much as 25%. Instruments near expanses of water such as coastal inlets, rivers, reservoirs or irrigation systems may record lower evaporation than the surrounding country due to local effects on meteorological elements, notably humidity. Such reductions are about 5% to 10%.

The so-called "Class-A" pan instruments have a wire mesh bird guard, which reduces the measured evaporation, and an estimate of the unguarded average "Class-A" pan evaporation may be derived by applying a 7% increase to the guarded pan value.

In Victoria, average monthly evaporation follows seasonal changes in solar radiation, giving highest evaporation in the summer and lowest evaporation in winter.

Annual "Class-A" pan totals vary from over 2000 mm in the northwest of the State, to between 1400 mm and 1600 mm over much of the northern plains and between 1200 mm and 1400 mm in the south, to below 1000 mm over the higher ranges.

Annual "Class-A" pan totals vary from over 2000 mm in the northwest of the State, to between 1400 mm and 1600 mm over much of the northern plains and between 1200 mm and 1400 mm in the south, to below 1000 mm over the higher ranges.

Wind

Wind varies from day to night, from season to season and also from place to place.

One example of the diurnal variation is the sea breeze, which brings relief on many hot days along the coastline. The sea breeze is not as important in Victoria, from the point of view of physical comfort, as it is in Queensland or along the northwest coast of Western Australia. Conditions in these latter areas would be almost unbearable at times were it not for the relief afforded almost every day by this breeze.

Warrnambool, along the State's west coast, is one city markedly affected by the sea breeze phenomenon. On summer mornings, 29% of winds are offshore (from the NW, N, or NE). By the afternoon, as a result of the onset of the sea breeze, only 7% of winds are offshore.

Another example of the diurnal variation is the valley or katabatic breeze, which brings cold air down valleys during the night. The valley breeze is well developed in many hilly area of Victoria, being the result of differential cooling after sunset. It often springs up during the night, often suddenly, and continues after sunrise until the land surfaces are sufficiently heated again.

Wangaratta, in the State's northeast, is one city markedly affected by the katabatic phenomenon. Wangaratta is in the Ovens River valley, which descends from the mountains to the SSE of the city. In the morning, 33% of winds are from the S or SE, as a result of the katabatic. Only 13% of winds are from this direction in the afternoon.

Seasonal variations in wind are not as marked over Victoria as they are over the extreme north of the continent. In the north, the southeast trade winds alternate with the moist northwest monsoonal winds.

The predominant wind stream over Victoria is of general westerly origin, although it may arrive over the State from the northwest or from the southwest. There are wide variations from this general description, however, and many northerlies or southerlies are experienced. Furthermore, the hot northerlies which bring heatwave conditions from the interior are well known, whilst in winter and spring exceptionally cold weather may accompany a southerly wind stream.

Wilsons Promontory, an exposed locality along the South Gippsland coast, illustrates some of these features. At all times of the year, both in the morning and in the afternoon, prevailing winds are from the W. However, the second and third most frequent winds are from the NE and SW in spring, summer and autumn, from the NW and NE in winter.

Easterly winds are least frequent over Victoria. Usually, they are associated with anticyclones over Tasmania and fine weather over most of the State. Under special circumstances, however, they can be gale-force and accompanied by heavy rain.

Indeed, coastal gale-force winds are a feature of Victoria's climate. They occur most frequently between June and November when large pressure gradients between high pressure over the continent and deep depressions to the south produce gale-force and occasionally storm-force westerly winds. In such situations in winter, these winds manifest themselves as blizzards in the alpine regions.

Fortunately, Victoria does not experience the tropical cyclones, which cause so much damage to places on the Queensland coast. The nearest approach to these are smaller rotating storms or closed depressions, which occasionally pass down the New South Wales coast and less frequently enter the waters of eastern Bass Strait and lash the Gippsland coast.

The sensitive equipment required to measure extreme wind gusts has been installed at a number of sites. However, it is likely that, even with a good network, the strongest possible gusts may have "escaped" detection by the anemometers.

A number of tornadic squalls have been experienced, and from the severe local damage, engineers have estimated wind strengths to be well over 100 knots. It is considered that any place in Victoria could feasibly experience, at some time, a local gust over 100 knots.

Humidity

The relative humidity is the percentage of (invisible) water vapour in the atmosphere compared to the maximum amount that could be held.

The relative humidity in the mid-afternoon (3pm) is close to the daily minimum relative humidity. This is because the amount of water vapour in the atmosphere usually varies little through the day, but the maximum amount of water vapour that could be held increases with rising temperature. In winter, the relative humidity at 3pm varies from 70 to 80% along the coast, to around 60% on the northern plains. In mid-summer, the relative humidity at 3pm is also around 70 to 80% along the coast, but is around 30% on the northern plains.

The relative humidity in the early morning, usually the time of minimum temperature, is close to the daily maximum relative humidity.

The mid-morning (9am) relative humidity is usually close to the daily average relative humidity, because the temperature at that time is close to the average temperature. In winter, the relative humidity at 9am varies from 80 to 90% in most districts. By contrast, in mid-summer, the relative humidity at 9am varies from 70 to 80% along the coast, to around 50% on the northern plains.

Sometimes, the early morning temperature falls so far that it reaches the temperature at which the atmosphere is holding as much water vapour as it possibly can. In such circumstances, water vapour in the atmosphere will begin to condense and dew, mist and fog will begin to form. This temperature is therefore called the dewpoint temperature.

With onshore winds from a relatively cool ocean surface, coastal regions experience fewer extremes of temperature, but generally have high humidity.

When temperatures are high, and there are N to NW winds from the dry interior, humidity is low.

Thunderstorms

The average number of days on which thunderstorms occur in Victoria each year varies from about 10 along parts of the coast to in excess of 30 about the highlands. The Melbourne region would typically experience thunderstorms on about 15 days per year. The majority of these storms have life cycles of between 40 minutes and one hour.

Under certain conditions, severe thunderstorms develop and produce damaging phenomena such as tornadoes, destructive wind gusts, large hailstones and flash flooding. Many severe thunderstorms produce very heavy rain and local flooding, but it is those that produce large hail and tornadoes which are responsible for the majority of severe damage. Severe thunderstorms are most likely to occur during the late spring and early summer months with the majority developing during the afternoon and evening. Thunderstorms can generate tornadoes, with around one significant event recorded every two years.

Dust

A dust storm may be defined as a mass of moving air that contains large amounts of dry opaque particles in suspension. Turbulence, brought about by strong surface heating, enables dust to be mixed into the atmosphere and lifted vertically. Thus, dust storms have their maximum frequency in the summer months when surface heating is strongest and the soil normally driest. However, dust may be lifted in winter by strong winds blowing over dry country.

Victoria, particularly the northwest of the State, does experience bad storms on occasions during dry years. The intensity of dust storms has been reduced in recent years with improved methods of cultivation, soil conservation and pasture improvement, which particularly included the growing of lucernes on lighter soils.

The frequency of dust storms is related to the nature of the season, being higher in dry seasons than in wet. An isolated wet month is not necessarily dust free but a succession of wet months reduces the frequency and intensity of dust storms because of the binding effect of plant growth.

Dust storms occur mainly during the daylight hours but the phenomenon has occasionally been observed at night. Dust particles may float in the wind stream for days or may be brought down by precipitation. The phenomenon of red rain may be hundreds of miles from the source of the dust.

Under the worst conditions observed, dust storms of moderate to severe intensity have occurred in the far north on four to six days per month, during the summer months. These frequencies could recur with extreme climatic conditions, such as occurred in the ninety months from 1895 to 1902 when Victoria experienced its most intensive recorded drought.

In bad seasons dusty conditions develop with nearly every change of weather. The most common change is that associated with a wind shift from north or northwest to west or southwest and on such occasions the storm may take the form of a moving wall of dust from some westerly point and extending in a line in a roughly north-south direction. Most dust storms are associated with winds from between northwest and southwest and on most occasions a sustained wind speed of approximately 25km/hour is sufficient to raise a dust storm from dry soil.

Another bad form of dust storm though less frequent, is associated with persistent northerly winds in which local dust and wind borne dust from districts further north combine. Such storms are more prolonged than the former types and usually occur in the spring or autumn. Local dust may be lifted by winds from any direction on reaching a force of about 20km/hour. Therefore, local dust may be fairly frequent, provided dry particles are available.

Bushfires

Bushfires can occur in almost any part of Australia whenever a period of little or no rain enables grassland vegetation and forest litter to become very dry.

When these conditions are combined with strong and gusty winds from the dry interior of the continent, fires can become virtually impossible to control. This happens during summer over the southeastern States and, as a result, this area is regarded as one of the most fire hazardous in the world, ranking with California and southern France.

In Victoria, these conditions develop on a number of occasions almost every summer with the strong and hot northerly winds ahead of summer cool changes. Fires become particularly dangerous when the strong northerly winds are followed by a wind shift to southwesterlies bringing a cool change. Such wind changes create enormous fire hazards as the original wild fire's elongated flank becomes a wide head of fire on a fresh path of destruction.

The variety of climatic conditions and vegetation across Victoria results in varying degrees of threat from bushfires.

The first written record of a fire causing major damage in Victoria was on 6 February 1851 (Black Thursday). Other major conflagrations occurred in 1939 (Black Friday), 1969 (Lara) and 1983 (Ash Wednesday). It is difficult, however, to compare the severity of different bushfires, since the tragic consequences cannot be quantified.

Snow

Snow in Victoria is confined usually to the Great Dividing Range, which at intervals during the winter may be covered to a considerable extent, especially over the elevated eastern section. Snow has been recorded in all Districts except for the Mallee, the Wimmera North, and the Lower North. Snow has been recorded in all months over the higher Alps, but main falls occur during the winter months.

Heaviest falls in Victoria are confined to sparsely populated areas, whereas severe winter storms that occur, for example, in some North American cities, may cause general community disorganisation. The Victorian snow season can also not be compared with the long and cold winters, for example, of eastern Europe, northern Asia, or the northern interior of North America, where a snow cover forms early in winter and persists throughout the season.

Drought

Droughts have shaped the history of the settlement in Victoria. In fact, they have been recorded as an agricultural hazard from the middle of the previous century, when population was extending into the drier parts of the State.

A deficit in rainfall is the major factor in determining the occurrence of drought, but seasonal rainfall alone does not always give a true indication of the severity of drought effects. In fact, the rainfall in any particular season may be close to average, but if it all fell in one event at the wrong time in relation to the growing cycle of a particular crop, then that rain would be of little use. Other indicators used to measure drought, such as stock numbers, can vary due to non-meteorological conditions, such as economic factors.

The areas of Victoria most commonly affected by periods of low rainfall are those to the north of the Great Dividing Range and the East Gippsland District, where rainfall is less reliable. The remaining southern districts often obtain a measure of relief from showers produced by passing cold fronts. Even there, however, occasional severe droughts occur.

Floods

All districts of the State have experienced some flooding. Occurrences are, generally speaking, more frequent, and tend to be more serious, in southern, eastern, and northeastern rivers.

Apart from the heavier falls, which are an intrinsic characteristic of the climate of these areas, frequency may be increased in many instances by valley contours, and damage is often greater because of the higher density of adjacent property and crops.

Flash flooding

These floods result from relatively short intense bursts of rainfall, commonly from thunderstorms, and can occur in almost all parts of Victoria.

Because of the speed with which it occurs, this type of flooding poses the greatest threat to loss of life and property as well as causing major social disruption. Flash floods tend to be localised and it is difficult to provide an effective advance warning of impending occurrence.

Inland river flooding

Inland flooding typically occurs along large, long, flat rivers such as the Murray River and the lower reaches of its larger Victorian tributaries such as the Goulburn, Loddon, Campaspe, Avoca and Ovens Rivers, as well as the lower reaches of the Wimmera River.

Flood-affected areas often do not experience the rainfall that leads to the flood as it generally results from large volumes of water produced by heavy rainfall in the upper reaches of the river. It is also often coupled with flooding in tributary streams.

Flood-waters tend to spread over the countryside inundating large areas, often for weeks, with potential for major losses of stock and damage to crops and pasture, as well as extensive damage to road and rail links. Communities can be isolated for long periods. Because of the great length of the river and shallow gradients, flood peaks move slowly and can usually be predicted with a high level of accuracy, allowing effective local action to reduce damage and losses.

Mountain and coastal rivers

Mountain and coastal river flooding generally occurs more quickly than inland river flooding. The head-waters reaches of inland rivers, such as the Grampians area of the Wimmera River and the upper areas of rivers flowing northwards off the Great Dividing Range to the Murray River, experience flooding of this type. So also do streams in hilly or mountainous areas, and rivers draining to the Victorian coast through Gippsland and the Central District.

These rivers are steep, particularly through Gippsland, and drain quickly. When heavy rain falls, river levels rise rapidly. For example, the Snowy and Avon Rivers are regarded as the wildest rivers in Victoria when in flood, but carry little water at other times. Flooding often only lasts a few days, but flows are fast and debris loads high. These floods are potentially very damaging and can pose a significant risk to loss of life and property. This is because there is generally not much time available to take preventative action such as removal of stock and evacuation of goods and people.

Table Climate at various Victorian locations.