Winter solstice is on Saturday but it won't bring Australia's coldest weather
This Saturday is the 2025 winter solstice, Australia's shortest day of the year.
How many hours of daylight you experience depends entirely on latitude — Hobart will see only 9 hours and 1 minute while Darwin will see 11 hours and 24 minutes.
For remaining capitals, running south to north:
The variation in the length of day through the year, and indeed the very existence of seasons, is due to the Earth's tilt of 23.5 degrees.
As we orbit the Sun, this tilt leads to changes in the length of day and the angle of the Sun viewed from the ground — it's higher in summer and lower in winter.
Saturday, the winter solstice, marks the day when the Sun is furthest north, tracking directly over the Tropic of Cancer.
The exact time of this year's solstice is 12:42pm for eastern states and just after midday for the NT and SA, meaning the longest night of the year will follow immediately after.
For WA, a 10:42am solstice will result in Friday night being the longest of the year.
From this position over the Northern Hemisphere, Australia is not only in Earth's shadow for the longest period, but critically for temperatures, it's also when the Sun's noon position is lowest in the sky.
From a simple energy perspective, the winter solstice therefore has the least amount of incoming heat directly from sunlight.
However, for nearly the whole of Australia, the coldest weather does not arrive when the days are shortest.
For the vast majority of the country, the coldest month is July, and for some locations, the coldest day on average is not until early August, even though days are longer and the Sun's angle is higher.
The cause of the temporal anomaly between the shortest day and coldest temperatures is due to the different heat capacities of substances.
Put simply, land regions warm up and cool down much faster than oceans.
This property is demonstrated daily as air temperatures fluctuate sharply between day and night, while water temperatures remain steady regardless of the hour.
Water, therefore, is said to have a greater heat capacity than land, which leads to an annual minimum temperature for waters surrounding Australia occurring mostly between July and September — well after the Southern Hemisphere's shortest day in June.
The warmer oceans earlier in winter act as one giant heater — they transfer energy into the atmosphere and delay the coldest weather until well after incoming heat from the sun reaches a minimum on June 21.
This delay is called seasonal lag, and it also occurs after the summer solstice in December, with the hottest weather in southern Australia not arriving until January.
The coldest weather in any given year is determined by the exact timing of weather systems; however, on average, due to seasonal lag, it occurs a few weeks after the winter solstice.
The delay is greatest in regions where the climate is influenced by neighbouring oceans — so coastal areas of southern Australia have a longer temperature lag than inland regions.
For maximum temperatures, the absolute peak lag is about five weeks in south-west Tasmania and on the southern WA coast near Albany.
For Sydney, Melbourne, Brisbane, Perth and Adelaide the coldest day of the year — on average — lands in the second or third week of July.
Unsurprisingly, parts of the central interior, well away from oceans, have a lag of only a few days.
The seasonal changes in daytime temperatures across northern Australia are more intricate due to the contrast in cloud cover through the year — there are even small regions that have their average lowest winter maximums a few days before the winter solstice.
The national variability in minimum temperatures is far more uniform, with well over 90 per cent of Australia, including the northern tropics, having its coldest night between July 11 and 21.
Seasonal lag is even greater in some Northern Hemisphere coastal regions, depending on the time of year.
San Francisco's warmest month of the year is not until September, a full three months after the summer solstice, during which time the length of day has already shortened by more than two hours.
Of course, thanks to the natural variability of weather, the actual date of the coldest weather will vary considerably from year to year — even though this is far more likely to occur in winter.
According to the Bureau of Meteorology, for the northern tropics, the coldest day of the year across a 30-year period can occur in most months.
For most of southern and central Australia, the range is from April or May to anytime between August and November.
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