Fog bound

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Land’s End airport fog-bound through advection fog.

THREE weeks after I returned from Sicily, I had planned a three-day visit to the Isles of Scilly, a group of small granitic islands just 40 kilometres offshore from Land’s End.

The latter place is the most south westerly point in England and very near where I was born in West Cornwall. The Isles of Scilly can be reached by plane
(15 minutes), ship (three hours plus) or hydrofoil (two hours).

Having been tossed around on a ship when travelling to these islands as a boy and experiencing severe sea sickness, I opted for the short plane flight.

My early life was spent in this sub-tropical part of the UK with palm trees growing in my front garden and sub-tropical plants of many varieties adorning public parks.

The winter climate was mild, with what I called “warm rain.” Ground frosts and sub-zero temperatures and snow were seldom seen but when they occurred, it was always with a vengeance. What I forgot were the number of foggy days in autumn and winter.

This recent visit by me saw my plane fog- bound at Land’s End airport for three consecutive days. No flights at all! Why? I was fog-bound and, most regrettably, abandoned my short holiday.

What exactly is fog? It is caused by tiny water droplets suspended in the air. Essentially, fog is a cloud at ground level, thereby reducing visibility to less than 1,000 metres.

Thick fog is when visibility drops below 180 metres. In my case, very dense fog enveloped Land’s End airport, reducing visibility at times from 50 to five metres, causing all flights to be cancelled.

The thickest and most persistent fogs occur in industrial and urban areas where millions of pollution particulates provide condensation nuclei upon which the water droplets grow, hence smog!

For fog to form, there is a combination of atmospheric moisture, light winds and a certain temperature, known by meteorologists as “fog-point” or “dew-point.”

Fog-point is simply the temperature at which the air must reach to allow its moisture content to condense into a visible cloud.

This will depend on several factors: the time of the year, atmospheric pressure conditions, the amount of cloud cover, the strength of the wind, the relative humidity of the air-mass and as importantly the geographical location and topography of the land.

Various types of fog

Tropical areas are not immune to fog formation, as we in Kuching well know, and especially if travelling to work at 6am.

Many a day, when I worked in Kuching, I had to dip the headlights of my car to avoid reflection off dense fog banks. The type of fog formed depends essentially on how the cooling that caused the condensation occurred.

Radiation fog

This fog-type develops when the skies are clear at night with wind speeds as low as one to five kilometres per hour. Heat from the land is radiated back into space and so the land surface cools.

In northern latitudes, in wintertime, when the sun is low in the sky, this type of fog may persist all day and particularly along river valleys, reservoirs and lakes.

A gentle breeze allows these minute water droplets to be stirred up to form a shallow layer of radiation fog. Lakes and rivers cool by a very small amount at night (less than one degree Centigrade) whereas the nearby land freely loses its heat.

During the day, the surface layer of this fog is “burnt off” by the sun’s direct rays but remember that, as it is daytime, the rays penetrate through this low cloud layer to heat up the surface below. Slowly the moisture evaporates into the air and, thus, the fog disperses.

Advection fog

Sometimes, referred to as “fret”, “haar” or simply sea fog, depending on where you are in the UK, it commonly occurs when moist air passes over a cool surface such as a cyclonic warm front moving over a snow-covered landscape.

It is most common at sea, when moist tropical air moves over cooler waters.

With the wind blowing onshore, the sea fog is transported over the coastal land area. This is what caused the cancellation of my flights at Land’s End airport.

Photos, that we often see of San Francisco’s Golden Gate Bridge shrouded in fog, are the result of warm moist Pacific air blowing eastwards over the cold California Ocean Current.

The persistence of this type of fog depends much upon the temperature of the land surface, which, if much cooler, the fog lingers for longer. Such blanket fog can disrupt ships entering ports and even work on offshore oil platforms.

Upslope fog

This is often referred to as hill fog and is caused by winds blowing moisture laden air up a slope. This orographic uplift of air causes the air to cool as it rises and fog forms.

At dawn, from the summit of a mountain, one can look down on the radiation fog in nearby valleys and also see upslope fog developing.

Low’s Peak on Mt Kinabalu, at 4,101 metres above sea level, offers a superb location to observe such phenomena as dawn breaks.

Evaporation fog or steam fog

Often forming over heated outdoor swimming pools, lakes and reservoirs which are overlain by a layer of much colder air, steam fog, from above, looks almost like a layer of snow.

The relatively warmer water evaporates into the lower air layer, causing it to rise and mix with the colder air passing over its surface.

This warm, moist air cools as it mixes with the colder air, causing condensation and, thus, this fog-type forms.

Freezing fog

This is usually formed when liquid fog droplets freeze on a cold surface, forming white rime. Upslope fog can turn into this on mountain tops, thus creating very difficult and slippery mountaineering conditions.

“Every cloud has a silver lining.” Above, I have highlighted the detrimental effects of fog on road, aircraft and shipping transport but fog, also, has beneficial properties in bringing life to some areas of our world.

The Coast Ranges in California are renowned for their Redwood forests which receive up to 40 per cent of their moisture from coastal advection fog as it condenses on their leaves and branches through fog-drip.

In the coastal Namib Desert of southwest Africa, advection fog brings life supporting water inland.

This fog is caused by warm, moist Southern Atlantic air, blowing over the cold Benguela Ocean Current and it condenses on fog nets, erected by the local people to catch the water droplets and feed them into water tubs for storage.

In such a desert environment, with some of the tallest sand dunes (reaching up to 240 metres in height) in the world, insect, animal and plant life may be found — thanks to wet fog!

The Welwitshia plant (Welwitshia mirabilis) springs readily to mind. It is named “mirabilis” from the Lain meaning “wonderful”.

A truly miraculous plant to survive in such arid conditions, where the average annual rainfall varies from five to 85 millimetres per annum, it looks no more than a dense, wizened long-leafed mass of dried grass.

When, at certain times of the year, advection fog arrives, it bursts into life through fog-drip, producing as many as 100 flowers in a season.

Interestingly, the Namibian coastline is known as the “Skeleton coast” owing to the large number of shipwrecks, caused by fog!

Fog also caused the demise of many a ship around the rugged coastline of West Cornwall and on the Isles of Scilly before fog horns were invented.

Whilst fog of any sort annoys most of us when our visibility is reduced, to some humans, plants and animals it is a life giving source of water.

Fog still remains one of the most difficult weather phenomena for meteorologists to predict with great accuracy as so many variables need to be taken into account.