Issued

Introduction

For most of Australia, weather in the middle of the year is generally cooler, and days are shorter. But when is the coldest time of the year?

On average for much of Australia, the coldest week occurs in July.

The coldest day or night in any particular year depends on individual weather systems. But that is set against a seasonal cycle—some parts of the year are, on average, cooler than others. The shortest day of the year occurs at the winter solstice and in the Southern Hemisphere it occurs each June between the 20th and 22nd. However, it takes some days or weeks for the land and oceans to catch-up with the solar cycle as these store and release heat through the year. That means temperatures are usually at their lowest in July and August.

As an island continent, the surrounding oceans also play a large part in determining when the coldest day and night of the year will be. Water cools down and warms up much more slowly than air, so sea surface temperatures continue to decrease around Australia into spring. Sea ice around Antarctica reaches its peak extent in late September, nearly three months after the darkest days of winter and close to the spring equinox. For southern parts of the country, all that cold water and ice means that even as the sun begins to warm the Australian land mass, southerly winds can still transport cold air over Australia, leading to late-season cold spells.

In parts of Australia's tropical north, the coldest day of the year can occur in almost any month—even during summer—usually in response to heavy rain.

Coldest night

The mean time for the lowest minimum temperature is in July across almost all of Australia (see Figure 1). In Tasmania's southeast, around Hobart, the coldest night is typically in late June, but in parts of southwest Western Australia it isn't until the first week of August.

Figure 1: When to expect the coldest night of the year (i.e. mean time of the year for the lowest daily minimum temperature). The mean time for lowest daily minimum temperature is in July (blue shades) across almost all of Australia.

While Figure 1 shows when the coldest night of year has been on average across Australia, actual conditions can vary widely from year to year. We can measure this variability in timing from one year to the next and calculate the standard deviation, which is a measure of how far from average are the dates from one year to the next (see Figure 2).

Regions particularly exposed to the westerly winds, such as southwest Western Australia, coastal South Australia, western Victoria, and western Tasmania, see a wide variability in the timing of the coldest night. The variability in the timing is relatively low over much of the continental interior, where the weather tends not to vary strongly.

Figure 2: Variability in the timing of the coldest night of the year (units are days).

The coldest night can arrive as early as May over much of the country, or even April in a few places in the southeast (see Figure 3).

Figure 3: Earliest time of the year that the lowest daily minimum temperature has occurred, using data from January 1981 to December 2010. The lowest minimum temperature can arrive as early as May over much of the country, or even April in some locations, for example in western Victoria.

For most of Australia, the latest time the coldest night of the year has occurred is in August or September, but there are isolated areas in both northern and southern Australia in which the lowest temperature of the year occurred as late as October (see Figure 4).

Figure 4: Latest time of the year that the lowest daily minimum temperature has occurred, using data from January 1981 to December 2010. For most of the country, the lowest daily minimum temperature can be as late as August, but around southern coastal parts it can arrive as late as September or October.

One of those areas (coloured red on the map in Figure 4) in western Victoria dates from a cold snap in October 2006. Westmere on the Western Plains of Victoria recorded its lowest temperature on record of −4.6 °C on 22 October 2006. On that same morning, temperatures at Ararat dropped to −4.5 °C and Ballarat Aerodrome recorded −3.6 °C (see Figure 5). It was the coldest day of the year in both parts of western Victoria and in the State's northeast.

Figure 5: Daily minimum temperatures across Victoria on 22 October 2006. Temperatures over large areas of western Victoria and in the State's northeast dropped below zero.

Another way of looking at this is to understand at what time of the year the temperature first drops below 5 °C. In the elevated parts of the southeast that is likely to occur by the start of March, but across most of the middle of the country such cold nights don't arrive on average until May or June, and not until July or August further north. In the tropical north, temperatures have never been recorded below 5 °C (see Figure 6).

Figure 6: Mean arrival time for the first minimum temperature below 5 °C, based on all March to August days between 1981 and 2010. Grey colouring indicates areas where daily minimum temperatures below 5 °C were not observed.

Coldest day

Compared to the timing for coldest nights, there is a larger spread in the average timing of the coldest day of the year across Australia. The tendency is for the coldest day to be in June in the north of the country and in July in the south and east (see Figure 7).

Figure 7: When to expect the coldest day of the year on average, or mean time of the year for the lowest daily maximum temperature. The mean time for lowest maximum temperature is in July across most of southern and eastern Australia, but tends to be earlier in the tropical north and west.

The year-to-year variability in the timing of the coldest day of the year is very large across much of the tropical north (see Figure 8).

Figure 8: Variability in the timing of the coldest day of the year (units are days).

For most of the southern half of the country, the coldest day of the year is rarely before the start of May or after the end of September (see Figure 9). Some areas in the southern coastal fringes and through South Australia have seen their coldest day in a year in October (see Figure 10).

In the tropical north, the coldest day can occur at almost any time of the year. Cloudy skies and rain can bring low temperatures during the wet season (October to April), but there can also be surges of cool southeast winds at the start of the dry season (May to September).

Figure 9: Earliest time of the year that the lowest daily maximum temperature has occurred, using data from January 1981 to December 2010.
Figure 10: Latest time of the year that the lowest daily maximum temperature has occurred, using data from January 1981 to December 2010.

The large area showing a coldest day of the year in November extending from the Kimberley in the northwest through central Northern Territory (shaded red on Figure 10) dates from 18 November 1981. Cold Antarctic air had made its way to northern Australia over the preceding days and interacted with tropical moisture to produce several days of rain and kept daytime temperatures more than 15 degrees cooler than average (see Figure 11).

Figure 11: Maximum temperature anomalies on 18 November 1981 (using the 1961–1990 average). Temperatures were more than 15 °C cooler than average across much of central and northwest Australia, making it the region's coldest day of the year.

Summary

In most parts of Australia, the coldest night and day typically occur during July, several weeks after the June winter solstice. However, every year is different and in most areas we can get the coldest temperatures at any time from autumn to spring.

Further information

Find more information about average temperatures at your favourite location by searching for monthly climate statistics on Climate Data Online.

Maps of average temperatures across Australia can be found on the Average conditions page of the Bureau's website.

Read our Blog—Solstices and equinoxes: the reasons for the seasons