Issued

Introduction

For most of southern Australia, the weather is usually warmer at the start of the year (January and February), when days are longer. For northern Australia though, the hottest time of the year is often earlier. So, why are there differences and when is the actual hottest time of the year?

For much of Australia, the hottest week typically occurs in December or January.

The hottest day or night in any particular year depends on individual weather systems. But that is set against a seasonal cycle—as we know, some parts of the year are warmer than others.

Hottest day

The hottest day of the year typically occurs in October or November across the tropical north of the continent, and generally shifts into December and then January moving south (see Figure 1).

Map of when to expect the hottest day of the year
Figure 1: When to expect the hottest day of the year (i.e. mean time of the year for the highest daily maximum temperature). The hottest day of the year usually occurs in October or November across the tropical north of the continent, and generally shifts into December and then January moving south.

For Australia's tropical north, the hottest day of the year is closely linked to the wet and dry seasons, which are driven by the solar cycle. The longest day of the year occurs at the summer solstice and in the Southern Hemisphere it occurs each December between the 20th and 22nd.

As the days get longer, the sun heats the Australian land area more quickly than the surrounding ocean. This difference in heating draws in warm, moist air over northern Australia and eventually establishes westerly winds that bring cooler, rainy conditions with the onset of the Monsoon. For Darwin, this often occurs around Christmas Day, but there is considerable variability from one year to the next. The onset of the northern wet season means that the hottest days in the tropical north often occur in the build-up period, before the start of the southern summer.

Across southern Australia, the hottest days are usually those with northerly winds bringing down hot air that has been heated in the country's interior. Australia's continental interior needs the long days of summer to heat up and it takes some days or weeks for the land to catch-up with the solar cycle. The subtropical ridge also moves south as the summer solstice approaches, typically bringing more sunny days over central and southern Australia.

The hottest day has been as early as September or October over much of the north, even as early as August in a few places (see Figure 2).

Figure 2: Earliest time of the year that the highest daily maximum temperature occurs (using data from July 1981 to June 2011). It can be as early as September or October over much of the north, even as early as August in a few places.

There is a small patch in northern coastal New South Wales in which the highest temperature of the year occurred as early as August. This patch, coloured pale orange on Figure 2, dates from the August 2009 heatwave (see Special Climate Statement 18: Exceptional winter heat over large parts of Australia). For example, Yamba on the North Coast of New South Wales reached 36.1 °C on 24 August 2009, exceeding the maximum temperatures reached at the site during the summer months that year.

Figure 3: Daily maximum temperatures at Yamba Pilot Station in 2009.

The hottest day has been as late as March around the southern coast, even as late as April in a few places in some northern coastal areas (see Figure 4). The latest hot days in the north occur after the withdrawal of the summer Monsoon, which is often uncomfortably hot. Being close to the equator, the sun is highest in the sky near to the solar equinox.

Figure 4: Latest time of the year that the highest daily maximum temperature occurs (using data from July 1981 to June 2011). It can be as late as March around the southern coast, even as late as April in a few places in some northern coastal areas.

While Figure 1 shows when the hottest day of year has been on average across Australia, we've seen that the actual timing 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 the dates are from one year to the next. The timing of the hottest day is particularly variable, relative to the rest of the country, over southeast Queensland and a narrow strip along the New South Wales coast (see Figure 5).

Figure 5: Variability in the timing of the hottest day of the year (units are days).

Another way of looking at this is to understand at what time of the year the temperature first rises above 35 °C. Days with maximum temperatures above 35 °C are already present at the start of September across much of the continental north, but arrive progressively later moving south and east (see Figure 6).

Figure 6: Mean arrival time for the first maximum temperature above 35 °C, based on all September to February days between 1981–82 and 2010–11. Grey colouring indicates areas where daily maximum temperatures above 35 °C were not observed.

Warmest night

Compared to the hottest day, the warmest night of the year usually occurs a little earlier in the season. The warmest night of the year occurs, on average, during December or January over most of Australia (see Figure 7).

As with the hottest day, the warmest night of the year usually occurs earlier for much of northern Australia compared with the south. In some areas of the Kimberley and the Northern Territory, the warmest night of the year has often already occurred by the start of December. As you move south over the continent, the average date of the warmest night moves into January, and along the southwest coast of Western Australia and in northern Tasmania it is typically during February. These locations have climates strongly influenced by the surrounding oceans. The heat capacity of the oceans means that sea surface temperature lags the solar cycle by a number of months.

Map of when to expect the warmest night of the year
Figure 7: When to expect the warmest night of the year (i.e. mean time of the year for the highest daily minimum temperature).

The year-to-year variability in the timing of the warmest night of the year is largest in coastal regions, particularly along southern Australia and in an area around Darwin and Joseph Bonaparte Gulf in the north (see Figure 8).

Figure 8: Variability in the timing of the warmest night of the year (units are days).

The warmest night of the year has occurred as early as September or October across much of northern Australia and in isolated patches of the south. For most of the country though, the earliest date has been in November or December (see Figure 9).

Figure 9: Earliest time of the year that the highest daily maximum temperature has occurred, using data from July 1981 to June 2011.

The warmest night of the year has occurred as late as April in many areas of southern Australia, and even as late as May in southwest Western Australia. The latest date for the warmest night is generally earlier in the season across most of northern Australia, including some isolated patches in both the Kimberley and the Northern Territory that haven't had a warmest night later than January (see Figure 10).

Figure 10: Latest time of the year that the highest daily maximum temperature has occurred, using data from July 1981 to June 2011.

Summary

In southern Australia, the hottest day and night usually occur during January, several weeks after the December summer solstice. In northern Australia, the hottest day of the year is typically earlier, before the start of the wet season. However, every year is different and in most areas we can get the hottest temperatures at any time from spring to autumn.

Further information