Fog Types - Cabbage Tree

NZ Fog Types — cabbagetree.blog

Reading the Fog

A guide to New Zealand fog types · with notes from Toko Mouth

New Zealand sits where cold Southern Ocean air frequently meets warm subtropical moisture, making fog a regular visitor along the coast. At Toko Mouth, where the Tokomairiro River meets the Pacific, several distinct fog types occur — each with its own character, formation mechanism, and behaviour.

Radiation fog

Overnight / Dawn

Radiation Fog

The most common fog type at Toko Mouth. On clear, calm nights, the ground radiates heat to space and cools rapidly. If the air near the surface cools to its dew point, water vapour condenses into a shallow fog layer — often just 10–50 metres deep. It forms in hollows and river valleys first, then spreads across low-lying ground.

Typical depth

10–100m

Season

Autumn / Winter

Clears by

Mid-morning

Toko MouthThe Tokomairiro River valley channels radiation fog towards the coast on still nights. You'll often see the fog bank sitting inland while the beach stays clear — until the wind drops and it spills out over the bar.

Sea fog

Coastal / Persistent

Sea Fog (Advection Fog)

Forms when warm, moist air moves over a cooler sea surface. As the air chills to its dew point, fog develops and can be thick and persistent. Unlike radiation fog, sea fog can form at any time of day and doesn't need clear skies. The cold Southland Current off the Otago coast is a key driver of sea fog along this coastline.

Typical depth

50–300m

Season

Spring / Summer

Clears when

Wind strengthens

Toko MouthSummer sea fog off the Otago coast arrives with the nor'easter — warm moist Pacific air cools as it crosses the Southland Current. It can blanket the coast while the hills are in full sunshine.

Valley fog

Inland drainage

Valley Fog

A variant of radiation fog that occurs when cold, dense air drains downslope into valleys during clear nights — katabatic flow. The Tokomairiro River catchment, with its broad flat valley floor and surrounding hills, is well set up for this. Valley fog tends to be denser and longer-lasting than open-ground radiation fog.

Typical depth

20–200m

Season

Autumn / Winter

Clears by

10am–noon

Toko MouthValley fog from the Tokomairiro catchment sometimes flows down to the coast and merges with radiation fog near the estuary. Driving the valley road early on a calm winter morning, you can move in and out of dense fog patches within metres.

Frontal fog

Pre-frontal

Frontal Fog

Associated with approaching warm fronts. As warm air overrides cold air, precipitation falls through the colder surface layer. This rain evaporates and saturates the cold air, producing fog at ground level. Frontal fog can persist over a wide area and is often accompanied by light drizzle.

Typical depth

100–500m

Season

Year-round

Clears when

Front passes

Toko MouthFrontal fog here arrives as the cloud base descends ahead of a slow warm front tracking from the Tasman. Less common than radiation or sea fog but more disorientating — the coast disappears into murk while the wind is already building.

Hill fog

Orographic

Hill Fog (Orographic Fog)

When moist air is forced to rise over hills or ranges, it cools and condenses. The cloud that forms at the hilltops is essentially fog — the difference between fog and cloud is just your altitude relative to the base. The Lammerlaw and Waipounamu Ranges above the Tokomairiro catchment regularly produce hill fog during northwesterly and northeasterly flow.

Altitude

Above cloud base

Season

Year-round

Clears when

Wind drops

Toko MouthOn days with moist NW or NE flow, the Lammerlaws are often capped in cloud visible from the coast. The valley below stays clear while above 400m the hills are in dense fog.