Unlike this weekend, last weekend was calm with light winds and clear skies across NSW, so it was highly likely that there would be fog in the valleys near Sydney on the Sunday morning. And so it proved on arrival at the Burragorang Valley, just like last year in our story about the River of cloud just west of Sydney.

Image: Thick fog covering the Burragorang Valley, west of Sydney.

With an earlier arrival than last time, there was even thicker fog so that the other side of the valley was almost not visible. And there was something strange below hovering over the fog. A very large shadow with a rainbow-like halo!

Image: Mountain spectre and glory projected onto the fog below.

The large shadow is called a mountain spectre because it has been commonly seen in mountainous areas in Europe, and the halo is also known as a glory. The phenomenon is caused by sunlight interacting with the water droplets of fog or mist between an object and the sun, which enlarges the object's shadow and causes the glory at the top of the shadow.

Image: Sunlight passing through mist at the top of the valley.

Here is what the scene looked like from the Himawari-9 satellite. Many areas of fog can be seen branching out across the different valleys. The Burragorang Valley is to the west of Camden.

Image: Himawari-9 visible satellite image at 8am Sunday 28^th April

And here is the synoptic chart from that morning, showing the high pressure ridge that led to light winds, clear skies, and cool overnight temperatures. These are the conditions that are favourable for the formation of fog.

Image: BOM MSLP analysis at 10am Sunday 28^th April