Clearing the air around fog
Good morning Auckland, what’s up with the pea soup?
Aucklanders who have been staggering to work in thick morning fog may get more of the same mist-making conditions for weeks to come.
Niwa meteorologist Ben Noll said this week’s run of foggy mornings in the Super City had been “impressive” — and enough to cancel and delay flights in and out of Auckland.
So what is fog? We could think of it as a blanket of microscopic water droplets suspended in the air.
These tiny water droplets scatter any light that passes through or past them, meaning that objects in the fog become hard to see.
Auckland’s current pea-soup mornings have two principle ingredients: radiation fog and sea fog.
Radiation fog — the most common type in New Zealand, according to MetService — usually forms overnight or early morning during the coldest hours of the day, before dissipating after the sun comes up.
It primarily forms over land but has been observed to form over shallow inlets and harbours as well.
Radiation fog requires a mix of clear skies, light winds and enough moisture in the lowest layer of the atmosphere, near the ground.
These conditions are most usually met when New Zealand has high pressure over the country — as it does now — but there are other situations where these conditions can be met too.
On a clear night with light winds, the air can cool down enough to reach its dew point, or 100 per cent relative humidity, causing water vapour in the air to start to condense into fog.
At sea, fog forms differently. “This happens when you’ve got cool air blowing over a relatively warm sea surface,” Noll said.
“Our sea surface temperatures are still in the teens, but we get some cold temperatures or cool air flows that come off the land towards the sea.
“So if you are tracking at 5C on land, but your adjacent water is 15C, that’s a pretty big difference in temperature and that can lead to the
formation of fog over the sea.”
Around New Zealand, high pressure to the east of the country typically offers a good platform for sea fog to form — with northeasterly winds bringing air down from the subtropics, across cooler waters.
If the high pressure stays in place for a long time, called a blocking high, then the sea fog can persist over the water for many days.
Noll said Auckland had been hit by a hybrid of the two types, which wasn’t unusual given the city’s maritime geography.
“We are talking about a thin piece of land with the sea on both sides, and with relatively mild ocean temperatures,” he said.
“In the context of this winter, we did have a very dry June, so fog was slightly less likely during that month as there would have been less ground moisture.
“But the fact July has been wetter, in western areas in particular, that’s recharged a lot of that groundwater and given a pretty good base to create moisture that goes into the formation of fog.
“We didn’t have that low-level moisture early in the season, and that tips the odds toward having more fog in July — and it could very well continue into the month of August.”
He encouraged motorists to drive to the conditions and be prepared to react quickly.
In the shorter term, MetService forecaster Andrew James said, a ridge of high pressure that had been dominating the country, and fuelling fog-favourable conditions, was expected to hang about over the weekend. “But the winds do pick up over coming days, so it’s looking less likely that we’ll keep getting this fog for now.”
[The fog] could very well continue into the month of August. Ben Noll, Niwa meteorologist