Hoar frosts, freezing fog and black ice are predicted to make for a gnarly mix of roading conditions in the Mackenzie Basin area for the remainder of this week.   

Forecasts supplied to New Zealand Transport Agency Waka Kotahi (NZTA) show temperatures may drop to -5 deg C in places and bring very frosty mornings and icy roads over the next two days. Freezing fog is forecast in the Mackenzie Basin mainly south of Irishman Creek (see map below), which could persist all day tomorrow and Thursday, along with hoar frosts and the risk of black ice where moisture remains on roading surfaces.

The risk of hoar frost and black ice is forecast to continue through to midday on Friday.

Further north, frost/ice can also be expected to impact State Highways 1, 7, 73, 75 and 77 through the remainder of this week. 

NZTA contractors will be out laying grit or anti-icing agents to help keep people safe on icy highways. Drivers are urged to reduce their speeds, increase their following distances, and avoid sudden braking or turning movements on icy roads. Ice is more likely to form in places such as bridges and shaded areas, and black ice can be harder to spot.

Staying as visible as possible, with headlights on in foggy conditions, also helps to keep people safe.

• Hoar frost happens when water vapour transitions directly from a gas into solid ice depositing onto objects, typically under clear conditions.
• Freezing fog is where microscopic, supercooled water droplets exist in the air at temperatures below zero deg C. Because these liquid droplets remain unfrozen in the air, they freeze instantly upon contact with any surface that is at or below the freezing mark, depositing a layer of ice.
• Black ice is a thin, highly transparent layer of ice that forms on roads, making it appear as the dark colour of the road surface beneath it.