The Physics of Fog

While the weather conditions that lead to the formation of fog are usually quite benign, fog itself can be very disruptive. In particular the aviation and marine industries are often interested in how fog or mist will affect the visibility for their journeys, though fog also affects road-users.

Inversions

How often have you gone to sleep on a calm night under clear skies, only to wake up and find the whole valley is full of fog?  This makes for great photos if you live above the cloud, as shown below from January 28th 2017 in Crofton Downs, Wellington, but it’s not so nice for the people living beneath the cloud.  Often the top of the fog is a smooth, flat surface, and is due to an ‘inversion’.  In this blog post we unravel what an inversion is, and why it leads to valley fog like this.

Wellington Airport Sea Fog

The annual average fogs for Wellington Airport is 6 fogs per year, much rarer than at Christchurch (50 days per year) or Dunedin Airport (64 days per year) or Hamilton (92 days per year), yet it is very disruptive when it does roll in off the sea and can last for hours at a time.  Interestingly, Wellington gets more fog days in summer while fogs occur in winter for many other airports. Wellington airport is a hub for domestic commercial air traffic, therefore even short closures have major flow on effects for passengers and other airports around the country.

Rugby Weather: Fog

Rugby and fog do not go well together. Fortunately, in the days when rugby was played in the afternoon, they did not meet up much, except for the occasional test in Scotland when the All Blacks disappeared into the “gloom” as they scampered in a late try.

However, the introduction of night-time rugby gave fog a chance to get on the field for some game time. Especially in the United Kingdom, where evening games sometimes have to be cancelled when fog turns up.

Layers and streets of cloud

Clouds come in many different types and are characterised and named according to both their shape and height in the atmosphere.  While a single snapshot in time at a given location may only contain one type of cloud, there are many days when multiple cloud types can be observed in the sky at once. The satellite images below is a nice example of different cloud types sitting at different levels above Waikato. Here's what the satellite image looked like Saturday afternoon 3 July 2010, for southwestern parts of Waikato near Kawhia Harbour:

Ridge-Top Winds

Written by Erick Brenstrum, Meteorologist.

I started tramping as a teenager with the expectation of rain in the hills about two days out of three. So we were always prepared to change plans if confronted by a river in flood. Likewise, conditions on the tops could send us scurrying back below the bush line, as the wind over the ridge crests was sometimes strong enough to throw an adult carrying a heavy pack off their feet.