Scotland's weather: Why does it always rain on us?

ONCE it was only the British who talked about the weather day and night, but in this age of global warming it's a subject very much on everyone's agenda. Professor Alastair Dawson is the assistant director of the Aberdeen Institute for Coastal Science and Management, and the author of several acclaimed publications on climate change. In a brand new book, So Foul and Fair a Day, he explores the influence that climate conditions have had on Scotland from its earliest days right up through the present, demonstrating that it is impossible to understand Scottish history without taking its weather into account as well.

• So Foul and Fair a Day: A History of Scotland's Weather and Climate is out now from Birlinn, 20.

THE BEN NEVIS OBSERVATORY AND CLEMENT WRAGGE

A PROPOSAL had been circulating to construct a weather observatory on the top of Ben Nevis. If scientists could understand the links between weather at low and high altitudes, then there was a possibility that improvements in weather forecasting could be made, to the benefit of all. Accordingly, plans were drawn up in the late 1870s by Thomas Stevenson, the lighthouse engineer and father of the writer Robert Louis Stevenson, for a new weather observatory on the top of Scotland's highest mountain.

The initial idea was that the costs could be borne through public subscription, but by 1880 it had become clear that insufficient funds were available. To the rescue came Clement Wragge, above, who offered his services to the Scottish Meterological Society to climb the mountain every day during the summer of 1881 to make observations at the summit, at the same time as observations made in Fort William by his wife.

The process was repeated the following summer from June to November. Were it not for the drive and stamina of Clement Wragge and the widespread interest generated by his detailed weather observations, a subsequent appeal to the public for construction funds might not have succeeded.

As it turned out, sufficient money was raised and the observatory was officially opened in 1883. The observatory was continuously manned and weather readings were taken every hour – sometimes in horrendous weather.

A road was constructed up to the summit and people using it were charged one shilling. There was even a restaurant erected on the summit by one of the Fort William hotel-keepers, and people could shelter there overnight during the summer months. In 1893, the Scots explorer Speirs Bruce, as part of his preparations for his 1902 voyage to Antarctica in the Scotia, was asked to be a member of the staff of the Ben Nevis observatory. Like many others, he was extremely concerned at the closure due to lack of funds of the observatory in 1904.

Soon after Clement Wragge learned he was not to have employment linked to an observatory, he emigrated to Australia and eventually became head of the Australian weather bureau. He became famous because of his idea to give names to the biggest tropical hurricanes (Katrina, etc), but in Clement's case he elected to use names of people he did not like.

THE GLENCOE MASSACRE

ON 12 December, 1691, the exiled King James VII & II gave permission for the Jacobite chiefs to submit to King William by the end of the year. The news arrived in Glencoe in mid December, thus giving the chiefs of the MacDonalds of Glengarry and the MacIains of Glencoe the near-impossible task of braving west Highland blizzards to reach Inveraray in sufficient time. Their journey through snow, ice and atrocious weather conditions took them first to Inverlochy and then to Inveraray, and they finally arrived a day late. They were not able to meet Campbell of Ardkinglas until well after the deadline, on 5 January, 1692. This was seen as an opportunity to make an example of the MacDonalds and simultaneously eliminate some enemies, and led to the infamous massacre at Glencoe in February 1692.

ICE AND THE LOCH NESS MONSTER

OF ALL the millions of words that have been written about the Loch Ness monster, very few consider the constraints placed by the last great ice sheets on the existence of a monster. Clearly no monster of an age equivalent to the dinosaurs could possibly survive at least 20 separate ice ages, when on each occasion a gigantic ice sheet removed all vestiges of Loch Ness from its path, although there may have been times at the ends of ice ages when the retreat of the ice was accompanied by the filling of the loch basin with fresh water.

We can be pretty sure that this was the case when the last Scottish ice sheet melted. We can also be fairly certain that the loch filled with fresh water derived from the melting ice. We also know from the assemblages of glacial landforms and sediments that make up the landscape between the northern end of Loch Ness and Inverness that, after the ice melted, the sea was never high enough to enter Loch Ness from the Moray Firth – nor did the sea ever enter Loch Ness from the south.

Loch Ness, therefore, filled with fresh water during the melting of the last ice sheet. The freshwater loch was still there during the last expansion of glacier ice during the Younger Dryas (a rapid climate-change event). We know that the northern limit of the Younger Dryas ice-cap reached only as far north as Fort Augustus.

So during the 1,200-year period of cold climate, the loch received a continual influx of fresh water draining from the ice edge at Fort Augustus. Equally, large volumes of fresh water continued to drain northwards out of Loch Ness.

Then, towards the close of this period of cold climate, came the big flood originating from the drainage of the ice-dammed lakes in glens Roy and Spean. After the flood came the final melting of the Younger Dryas ice-cap, part of which also drained into Loch Ness. When the ice finally melted away, the amounts of water draining into the loch reduced substantially.

So where does this leave Nessie? It is clearly impossible for such a creature to have entered the loch from the sea. Also, any creature would have to have evolved independently within the loch, starting from the time when the last Scottish ice sheet melted. That gives it perhaps 13,000 years – 16,000 years at best – to weave its mysterious and magical spells.

WHY IT RAINS MOSTLY IN THE WEST

ONE of the most striking aspects of Scotland's weather is the comparatively large amount of rainfall in the west of the country compared with the east. A year spent on Skye, for example, would settle any debate regarding the truth of this statement.

By contrast, rainfall in Fife is relatively low by Scottish standards.

Even so, the people of Tiree or the Uists, for example, might disagree with this statement and point to the fact that, though they are located far to the west, the weather on those islands, although windy, is relatively dry.

The explanation lies in what happens to moisture-laden air when it is forced to rise. The first port of call for cyclones tracking eastwards across the North Atlantic is the west of Scotland. As the air reaches the mountains of the western and northern Highlands it is forced to rise, and as it rises it is cooled.

The rate at which air is cooled with increasing altitude is well known – for every 100m increase in altitude, the air temperature falls by 0.65C. As the air cools, it shrinks and becomes less able to contain moisture. So as the air rises (known as adiabatic ascent) over the mountains of western Scotland, much of the moisture previously contained within the air is condensed out as rainfall. This type of rainfall is known to meteorologists as orographic rainfall.

Once the air has passed eastwards across the highest mountain ranges it is relatively dry. It is also progressively warmed as it descends to lower altitudes (known as adiabatic descent) in the course of its overall movement from west to east. Curiously, air warms at a faster rate as it descends that it cooled as it rose.

In this way, the air that is moved from west to east across the Scottish land mass during the passage of an Atlantic cyclone is much drier in the east of the country than in the west.

But although we have a simple explanation for Portree being much wetter than St Andrews, do we also have an explanation for why Tiree and the Uists, for example, are so dry?

The answer is actually very straightforward – many low-lying areas on the west coast are also relatively dry because the air in the eastward-tracking Atlantic cyclones is not forced to rise as it passes over these areas.

CROMARTY HERRING AND WHALES

THE cold waters that marked the start of the 18th century were associated with the arrival of the herring in huge numbers, and with them came the whales. Historical documents for Shetland show a concentration of reports of whales for the two decades between 1720 and 1740. These reports include whales driven ashore in Unst in 1720, a shoal at Hillswick in July 1731 coincident with the arrival of the herring and a large whale at Bressay in 1734 (reduced to "ten barrels of oil and some whalebone"), and culminate with 276 whales at Urafirth during 1740-41.

During the 1710s, a spectacular sight was to be seen in the Moray Firth during the summers. In the middle of the July, huge numbers of herring could be seen swimming up the firth, often pursued by whales and porpoises. The herring would head for sandy banks, where they would spawn before leaving again at the start of September. This annual migration, linked to the southward push of cold polar water in the North Atlantic, became known as the "har'st of the herring-drove".

So great were the numbers of fish that some beaches were entirely covered, with not enough salt to cure all the fish. When this happened, people took the rest of the dead fish as manure for the fields, causing a vile stench. The whales attracted lots of attention and often local fishermen would take to their yawls to try to catch a whale – mostly unsuccessfully.