It was an upside-down night of weather in Victoria, with the mercury hovering well above zero overnight across the higher parts of Victoria’s alpine region, while numerous locations near the coast were slightly colder for most of the night.

For example, at 4 am this Tuesday, July 7:

The ski resorts of Mt Buller, Mt Hotham and Falls Creek were 4.8°C, 3.5°C and 4.0°C respectively.

Locations only a few metres above sea level like Melbourne Airport, Geelong and the Melbourne CBD were 2.9°C, 2.9°C and 4.6°C respectively.

That’s right: the Melbourne CBD was colder at 4am on Tuesday morning than Mt Buller, the nearest major ski resort to the Victorian capital. Why the unusual disparity?

The answer is that there was a strong temperature inversion.

What is a temperature inversion?

We all know that the atmosphere becomes colder as altitude increases (until you hit the stratosphere). That’s why it’s usually colder in the mountains than lower down.

But sometimes, the temperature profile temporarily flips, with relatively cooler air near ground level sitting beneath a layer of warmer air.

That’s called a temperature inversion, and it’s what occurred in parts of Victoria overnight.

How do temperature inversions happen?

One of the most common ways for a temperature inversion to develop is by radiative cooling of the air near the ground.

This typically happens on clear and calm nights, when the ground gets colder at night when heat radiates into space.

Clear and calm nights typically occur near the centre of a strong high pressure system – and it’s no exaggeration to say that southeastern Australia has one of those in place at the moment.

Indeed, the high with a central pressure of 1044.5 hPa centred over Tasmania on Monday set a new provisional high pressure record for the Australian region.

Relatively mild easterly winds played a part in the mild overnight temperatures in the Victorian Alps last night too. The influence of that Tasman Sea air contributed to an unusually warm layer of air forming from around 1500-2500m above the surface.

Image: 850 hPa temperature and wind for Victoria at 7 am (AEST) on Tuesday, July 7, 2026. Source: Weatherzone.

As the image above shows, temperatures at the air pressure level of 850 hPa in the atmosphere (an elevation of about 1500 m) were virtually the same in the Melbourne area as in the mountains to the northeast of the city. Most nights (without an inversion in place), you’d see a stark contrast.

If you look closely at the wind barbs, the "feathers" show the southeasterly wind flow that contributed to relatively warm air at higher elevations. Winds were light, which meant the air in in the atmosphere couldn’t "mix" and prevent an inversion from occurring.

What effects do temperature inversions have on surface weather?

Apart from making the surface cooler than the layer of air above it, temperature inversions can also cause valley fog – and that’s what happened in parts of Victoria last night as the valley air condensed to its dew point, forming fog.

Image: From about Geelong west, the 9 am satellite image showed cloud, but around Port Phillip Bay and greater Melbourne, as well as in the high country valleys in the state’s northeast, it was fog. Source: Weatherzone.

One unwanted effect of temperature inversions at this time of year is that they prevent snowmaking at the alpine resorts, with overnight temperatures remaining too warm up high.

Indeed, the 6:15 am Mt Hotham snow report began by saying "This morning we have a temperature inversion with the current temp sitting at a balmy +4.2 degrees."

With the snowpack still very light for this time of year in the Australian Alps, temperature inversions are far from ideal for resorts looking to bolster their natural snowpack with snowmaking.

Please check the Weatherzone snow page for the latest forecasts, live cam images and more throughout the 2026 snow season.