Some 440 years ago tomorrow, on August 19th, 1561, "the galleys neared the port of Leith, and entered fog".
"Borne up the Firth of Forth on a fresh east wind," the narrator goes on, "the fog settled for miles along the shoreline, heavy and impenetrable.
"All night long the ships floundered outside the harbour, announcing their positions by the beat of drums, until at last the pall lifted slightly with the dawn, and the three vessels crept silently into port to drop anchor".
The event was the ill-fated arrival in Edinburgh of Mary, Queen of Scots, the first time since childhood that she set foot in her inhospitable kingdom. And the fog in question is a well-known phenomenon on the east coast of Scotland; it is called a haar.
With an easterly air-flow in summer, it often happens that a warm body of moist air, with a temperature of 25 degrees or more over northern Europe, is carried westwards across the North Sea.
As it moves towards Britain, its temperature falls steadily by contact with the cold water, down to 13 or 14 degrees.
At this relatively low temperature, the air is overburdened with the moisture it was able to hold effortlessly when it was warmer; condensation takes place, and the result is a haar in the easterly breeze playing on the Scottish coast.
Until recently, although it was easy to predict the conditions conducive to a haar, it was hard to specify exactly when and where it might occur. Nowadays, however, satellites make the task a little easier, two types of satellite image being used in combination for this purpose - visible and infra-red. The visible picture records more or less what might be seen by an ordinary black-and-white camera. The clouds are a brilliant white, and so is the haar, and for this reason, it is often difficult to distinguish fog from cloud.
In the case of infra-red pictures, the sensor reacts to temperature rather than to visible light; cold objects, like clouds, appear white, and those at higher temperatures black.
On an infra-red picture, the haar can hardly be seen at all, because there is little or no contrast between its temperature and the sea a few feet below it.
By comparing the two pictures the forecaster can identify the haar on the visible image for what it is. Then having pinpointed the areas where the fog is present, and analysed the local winds, it is possible to forecast accurately the areas where the haar is likely to be carried during the next few hours.