Our best summer weather (warmest, and most sunny and settled) seems to occur most often in February - late summer. Warm humid air comes from the north. When it encounters the cold water of Cook Strait the moist air is cooled and water condenses forming mists, which build up into fog if there isn't any wind to dissipate it. (In case you wondered, the difference between fog and mist is that fog is denser and reduces visibility to less than 1 km whereas mist reduces visibility to 1-2 km.)
Sure enough, with our hottest weather this summer came a spell of dense fog, disruptions of air traffic, and interesting light effects. This was the view at Lyall Bay yesterday.
Wellington's south coast, beside Cook Strait, is where the warm hits the cold. So it is a particularly good place to experience the fog. Our airport is on the south coast too and normally from Lyall Bay you can watch the planes taking off on the nearby runways. But with yesterday's fog even the practice launching of a Lyall Bay Surf Lifesaving Club rescue boat was barely visible. Nevertheless, it was a hot summer day and people were not deterred from their beach pleasures.
The fog softens and obscures almost everything - a swimmer who is not far away looks like a white smudge. Boys play, walkers walk and optimistic surfers are looking for the waves.
The moist air swirled and shifted, so the brightness and visibility kept changing. I noticed that there was an arc of lighter fog, looking just like a rainbow but with only a hint of colour at the edges.This is a view from Waitaha Cove, looking back over Lyall Bay. Above, the bright blue sky of a hot summer day, below the dense shifting fog. Believe it or not, there is a boat - I think a Coast Guard one - about a third of the way along from the left hand edge of the bow.
I thought of this as a "fog-bow" or "mist-bow" and when I checked The Cloud Collector's Handbook, http://cloudappreciationsociety.org/collectors-handbook/ , I discovered that this phenomenon really is called a fogbow. As with a rainbow, a fogbow is formed by sunlight shining on water droplets. The water droplets of fog are much smaller than the raindrops which cause rainbows. The small size of fog's water droplets limits refraction of light within them, but it does occur in the larger raindrops. This means that the reflected light from fog does not have the distinctive colour separation seen in a rainbow, and there is very little colour in a fogbow. In fact they are sometimes called white or albino rainbows.
So, I have learned something new, and have another tick in my cloud "collection" (fog is described as a low-level Stratus cloud.) And the winds picked up - it is Wellington after all - and the fog has been rolled, as it were.