Fog in the country, smog in the town

Posted in Geography, London, Science, Sea on Friday, 27 April 2012

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This edited article about fog originally appeared in Look and Learn issue number 695 published on 10 May 1975.

London fog, picture, image, illustration

A scene in London fog, once known as a pea-souper

With very little warning, fog will settle over land or sea. At sea, ships have to reduce speed and run the risk of collision, and on land, rail and road transport is reduced to a crawl.

It has been estimated that one day of heavy fog in a city like London costs over one million pounds. This huge bill is made up of delays to transport, working time lost through people being late for work, charges for extra lighting, and damage to goods.

If fog could be done away with, the gain to a country’s health and wealth would be enormous.

A great deal of Britain’s fog begins far out in the Atlantic. There currents of air warmed by the Gulf Stream take up water moisture from the sea.

The water is in the form of vapour, rather like the steam produced by water boiling in a kettle. But unlike steam from a kettle, the water vapour is invisible.

When the stream of warm air carrying the water vapour meets a cold layer of air or passes over cold ground, the water vapour in the air condenses. This means that it turns into water again.

The same thing happens when steam from a kettle strikes against the comparatively cold wall of the kitchen and trickles down as water.

If the current of air carrying the water vapour is close to the ground, the water vapour condenses into very tiny droplets. These then hang suspended on and above the surface of the ground as mist or fog.

Fog produced by warm, moisture-laden air coming into contact with colder ground is called advection fog. Advection is a word used by meteorologists, as weather scientists are called, and means the transfer of heat by air moving across ground.

Advection fog occurs mostly during autumn and winter, when the land is then cold in comparison with the warm air blowing in from the sea.

Another kind of fog is caused by what is called radiation.

On cloudless nights the earth radiates or releases its heat very quickly into the atmosphere. Then as the earth cools, it cools the air in contact with it.

If the air has soaked up a lot of the moisture from the ground, it begins to lose some of the vapour as droplets of water. These partly condensed drops of water then hang in the air as fog.

There must be some solid particles in the air for any kind of fog to form. This is because the tiny condensed droplets must have something to cling to.

Over the sea, particles of salt from the spray provide the solid to which the water in fog clings. Over the land, there are always dust specks, even in the pure air of the countryside.

There is a great deal of difference between fog in the country and the fogs that blanket cities and towns. Fogs in the country and at sea are “clean,” although they can be very dense.

But the dark fogs of cities and manufacturing towns are “dirty” because of the large amount of soot from chimneys mixing with the water vapour.

The density of any kind of fog depends upon the number of water droplets in every cubic inch of air.

When there are only a thousand droplets to the cubic inch of air, the fog is very light.

In a very dense fog, when you cannot see through it all, there are probably a million droplets of water to the cubic inch.

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