Explainer - Have you had a flight delayed by fog recently? You're not alone. What is fog, and will there ever be a way to beat it?

As the country settles into midwinter, foggy weather is a common morning sight - and while it may be pretty in pictures, it can cause havoc for commuters and air travellers with delayed and cancelled flights.

What causes fog, and how do airports know when it's too dangerous to fly? Here's what you need to know.

A foggy start to the day by the waterfront in Auckland's CBD.

EDWARD GAY / RNZ

What is fog, anyway?

"It's basically having a cloud on the ground," said MetService communications meteorologist Alywn Bakker.

"Fog is what happens, essentially, when the air gets so much moisture in it that it can't hang on to it anymore, and it starts to sort of condense out."

The most common kind of fog in New Zealand is what's known as "radiation fog."

Don't worry, despite the name, radiation fog won't turn you into the Incredible Hulk. It gets its name from changes in radiative heat during the course of a day, and typically forms overnight or in the early morning during the coldest hours of the day, then disperses once the sun comes up.

It's "where everything's really still and cold," Bakker said.

"There's not a lot of wind. It's very moist and everything starts to cool down at ground level."

Clear skies are better conditions for radiation fog, as cloudy nights tend to absorb heat emitted by the Earth's surface and bounce it back down, keeping the land from cooling quite as much.

This handy MetService graphic shows how that works:

Fog forms when the air temperature and the dewpoint temperature get close together. The dewpoint is the point at which condensation occurs - when air becomes saturated with water vapour.

"If those two temperatures are very close together, that's when you're going to be getting fog," Bakker said.

There are several other kinds of fog - such as advection fog, which forms when warm moist air moves over a cool surface, and often leads to sea fog.

There's also freezing fog, which is seen in places like Central Otago and can cause hoarfrosts and black ice.

You can even sometimes see the rare fogbow, created from fog droplets instead of rain droplets like rainbows are - one was photographed in Australia just recently.

Multiple flights have been delayed by fog at Auckland Airport in recent weeks.

Supplied

Why does fog keep causing flight delays and cancellations?

Low visibility is the obvious answer, and while technology can help navigate the haze, nature is still in the driver's seat.

"We understand fog-related delays can be disruptive for travellers, particularly when plans are time-sensitive," an Auckland Airport spokesperson told RNZ.

"Safety comes first in low-visibility conditions, and we appreciate people's patience while the airport, airlines and Airways work through the impacts."

Airways New Zealand is a state-owned enterprise that manages air traffic, including air traffic towers across the country.

An Airways spokesperson said tools such as meteorological observations, forecasts and visibility measuring equipment are used to monitor fog. Some runways use Runway Visual Range equipment, which measures visibility along the runway.

"Low Visibility Procedures are typically triggered when runway visual range falls below 1200 metres, or when fog prevents air traffic control from maintaining visibility of key operational areas of the airfield," the Auckland Airport spokesperson said.

"What this means is aircraft movements are managed more cautiously, with greater spacing and more controlled movement around the airfield. This can reduce the number of flights able to arrive and depart in a short period."

Airways doesn't make the call on whether airlines delay or cancel flights, and said its role is to "provide air traffic management services and ensure aircraft can operate safely within the prevailing conditions."

"When visibility is reduced, Airways may implement low visibility procedures and manage air traffic accordingly. Airlines then make decisions about delays, cancellations or diversions based on a range of factors, including weather conditions, aircraft capabilities, crew requirements and their own operational considerations."

Fog doesn't affect all flights equally. Aircraft and airlines have varying levels of certification and capability to operate in low visibility.

"Different aircraft are approved to operate in different weather conditions depending on their onboard navigation systems and whether they are operating under visual flight rules or instrument flight rules," the Airways spokesperson said.

"Some aircraft are approved to operate in lower visibility than others, which can influence whether flights continue operating as planned or experience delays," the Auckland Airport spokesperson said.

An aircraft's ability to operate in low visibility also depends on the equipment available at the airport, the Airways spokesperson said.

Auckland Airport has New Zealand's most advanced low-visibility landing capability, a Category III Instrument Landing System.

"This is one of the most advanced landing systems available," the Airways spokesperson said.

"The system combines radio guidance, specialised runway lighting and ground infrastructure to help suitably equipped aircraft land safely in very low visibility conditions. While it doesn't eliminate the impact of fog, it can help reduce disruption by allowing some aircraft to continue operating safely when visibility is significantly reduced."

Auckland Airport also recently completed a major upgrade of its airfield lighting control system and said it "is continuing to work with Airways on opportunities to further improve efficiency during low-visibility operations."

Fog watches can also affect ferry travel if visibility becomes too hazardous for boats.

Drivers should be cautious in the fog.

RNZ

What should I do if I'm driving?

For drivers, the NZ Transport Agency recommends extreme caution in foggy conditions. And watch how you use your headlights.

"If you drive with your headlights on full beam in fog, the light will just reflect back on you. Dip your lights - it'll be much easier to see," NZTA's website advises.

If your car has fog lights, use them - but only use them in fog or other reduced visibility conditions such as snow.

"You can't use your fog lights when the weather is clear, even at night. The law is very clear on this," NZTA has cautioned.

That's because front fog lights produce very intense lights that can dazzle other drivers. The Automobile Association (AA) notes that "They give off a short, wide beam of light that is designed to shine through the fog, lighting up the area directly in front of the vehicle and helping the driver see the sides of the road better."

"They are aimed as low as possible because fog itself often hangs no higher than a couple of feet above the road, and if a fog light is aimed high, it will produce glare in the fog and will blind oncoming drivers."

Rear fog lamps have a low beam of red light to help vehicles behind you see better in snow or fog.

Are some parts of the country worse for fog than others?

Auckland may seem very foggy some mornings, but Hamilton has it beat.

"Hot spots" for fog include Hamilton and Dunedin, Bakker said.

"You'd be expecting between 10 to 12 fog events a month at Hamilton compared to Auckland" which would normally have about three, he said.

Rivers are actually big generators for fog.

"Anywhere that is near a river that really ups your fog risk," Bakker said.

"Rivers, they're a good moisture source, but they don't really contribute to keeping the place warm. Whereas the sea really does help contribute to keeping the place warm. It's why Auckland is pretty mild."

OK, but will we ever find a way to beat our nemesis, fog?

Many companies have investigated ways to combat fog - a few years ago, the Christchurch start-up company Pyper Vision was reportedly looking at developing a spray that could absorb moisture. The company continues to work on fog forecasting technology advances according to its website.

Humans have been looking at ways to beat fog for a long time - during World War II, Britain developed the FIDO system - Fog, Intensive, Dispersal Of - a network of pipes and petrol burners capable of clearing fog from runways to help fighter planes take to the sky.

"That would drive the fog off," Bakker said. "You know, you're heating the place up, but it also creates a bunch of updrafts, which makes it really difficult to land."

Despite all the technological advances of the modern world, fog is still a bit of a mystery.

"Fog is really quite difficult to predict," Bakker said.

"The thing about fog is, it's a really finely balanced set of circumstances. So you need the right temperature, you need the right wind speed, you need the right moisture. And if any of those is off, you won't get it."

Auckland Airport's spokesperson said safety remains the top priority when dealing with the unpredictable nature of fog.

"Fog itself can't be prevented, so the focus is on forecasting, preparation and having the right infrastructure in place when visibility drops. While weather conditions can still cause delays and cancellations, these investments help support safe flight operations during low visibility conditions."

But while having your flight delayed is no fun, on the relative scale of weather disaster possibilities fog remains fairly chill.

"Fog is irritating but low impact," Bakker said.

"It annoys airports and it's not great to drive in, but otherwise it's not that bad."