Foggy mornings in parts of eastern NSW this week are a sure sign that winter is on the way.
Fog shrouded large areas of eastern NSW on Wednesday morning, dropping visibility to 500 metres in Bathurst and Goulburn and down to just 100 metres in parts of Sydney.
This thick fog caused some transport delays, both on the ground and in the air.
While fog can form at any time of year, it generally follows a seasonal trend and most commonly occurs during the cooler months of autumn and winter in southeastern Australia.
Fog forms when air near the ground cools down enough to reach a level called its 'dew point' temperature.
This is simply the temperature at which airborne water vapour will condense into small water droplets, causing fog.
The dew point temperature is higher when there is more moisture in the air, for example after recent rain. The higher the dew point, the less the air needs to cool down to produce fog.
Recent showers in eastern NSW had increased the amount of moisture sitting near the surface on Wednesday morning.
This raised the dew point temperature, making it easier for fog to form as mercury dropped overnight.
The lower minimum temperatures that occur during autumn and winter also make fog occur more frequently.
Fog in NSW is most common about the ranges and away from the coastal fringe.
According to the Bureau of Meteorology, airports located near the Great Dividing Range experience around 40 fog days per annum.
Locations on the western slopes see less fog, with around 25 fog days occurring on average each year.