Analysis: Why so foggy?
A fortnight ago we looked at the reason behind the cool green water over the Christmas and New Year break, but a flow on effect of this extended upwelling event was multiple accounts of coastal sea fog. This summer, we've had more than I've seen in a decade on Sydney's Northern Beaches.
Sea fog can form in many different ways, but our current episodes are due to the localised cold water upwelling events.
You've probably heard the word dew point thrown around when listening to the weather. It's the temperature at which cooling air becomes saturated resulting in condensation or dew forming on grass and other objects.
We often see the result of this early in the morning. Offshore wind that blows from inland usually has less moisture and a low dew point (say for example 10 degrees). This means if the overnight air temperature stays above 10 degrees dew won't form, but if it dips to 10 or below you'll be walking out to moist dewy grass.
Onshore winds, however, are moist as they blow across the ocean, and this increases the dew point to, let's say, 20 degrees. This means that overnight air temperatures only need to drop to 20 degrees to create dew, which we see happening frequently. This is why when the East Coast is under a persistent north-east wind pattern we'll see dewy mornings.
One key to dew formation is also the lack of any major wind to disturb the formation of condensation on surfaces. If it's windy we'll see dew formation suppressed.
Also linked to the dew point is sea fog. But how does that form?
For sea fog to form we need the air to cool down to its dew point, and over the last month the persistent upwelling events along the NSW coast have been the catalyst.
The cold water cools the warm humid air above it, causing what many in on the West Coast of the United States know as a Marine Layer. It's a temperature inversion where warm air sits on top colder air, opposite to what's usually seen overnight with colder air aloft warmer surface air.
The marine layer feeds on itself with the cold water cooling the humid air causing it to become denser and stay trapped on the ocean surface. This is also clearly visible if you get any elevation on top of the marine layer, with the sea fog hugging the earth's surface, while above it's clear and sunny. Similar to dew formation, the wind needs to be relatively light.
The eventual breakdown of the marine layer is usually caused by wind turbulence reaching down to the surface level and breaking up the fog.
Sea fog clearing early afternoon December last year.
We've seen varying cases of this fog over the past month in Sydney and the East Coast, with some mornings starting completely socked in, while others have seen the fog move in through the day. These events where the fog has moved in through the day are a case where the sea fog has formed offshore or from another location and then moved in with the developing sea breeze or gradient wind, dissipating sometime afterwards as the wind continues to increase.
Victoria saw its coast blanketed in fog this morning and this was a result of a local inversion across Bass Strait. The inversion, along with humid air, no wind and sea temperatures of 20 degrees (close to the local dew point) resulted in heavy sea fog throughout Bass Strait.
Strong and persistent NE winds are forecast for the Sydney region over the next few days and we'll see this bring the water temperatures down again, but whether we'll see local winds light enough for sea fog is to be seen.