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Sydney disappears under a blanket of fog

Sydneysiders have been given a reprieve from pounding rain, instead waking up to thick, grey fog.

Much of the city vanished under a blanket of grey before 8am today, which Bureau of Meteorology duty forecaster Andrew Haigh said was caused by wet and clear conditions.

"When the sky is clear and the wind is light overnight, those are favourable conditions for fog because the surface of the earth and the air near the surface has a chance to cool down more than it otherwise would," Mr Haigh said.

"In addition to that we had the rain, so we had high humidity near the ground in the evening, so all that combined to drop the temperature enough so that relative humidity got to 100 per cent.

"When that happens any further cooling will result in condensation of moisture and that condensation can be droplets of water in the air, which is fog."


Mr Haigh said it was possible the fog was quite thick in the western suburbs early this morning.

From Pyrmont, the Sydney Harbour Bridge, the Anzac Bridge and the city's skyline could not be seen.

The days of wet conditions have also put Warragamba Dam again at full capacity.

The Sydney Catchment Authority said it had started spilling, with water flowing through the central drum gate.

It is the second time this year that the dam has overflowed, with capacity reached for the first time in 14 years last month.

NSW State Emergency Service spokesman Phil Campbell said the overflow would cause some minor flooding of the Hawkesbury River near Richmond, but locals should not be concerned.

Local roads and the Yarramundi Bridge would be flooded and farmers living along the river had been warned to move their livestock and equipment to higher ground, he said.


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