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Peebles and Selkirk
Climate and Rainfall

By climate we mean the prevailing weather of a country; by weather, the state and behaviour of the atmosphere. These depend mainly upon temperature; and temperature is determined by latitude, altitude, season, prevailing winds, and proximity to the sea. Bulk for bulk, warm air is lighter than colder air; while water vapour is twice as light as air. Hence dryness, as well as temperature, affects the weight of the atmosphere. Warm and dry air may therefore be heavier than colder air. Air in motion will also naturally exercise less pressure than stationary masses of air.

In an area of low pressure the wind flows outwards in great spirals with a direction contrary to that of the hands of a clock. Such a condition of low pressure is called a cyclone. Cyclones accompany, like eddies in a river, the great drift of westerly and south westerly winds which are the prevailing winds in our islands. From barometric readings, therefore, collected from various quarters, it is possible to plot out regions of cyclonic disturbance and so to foretell changes and disturbances in the weather. So also a region in which the pressure is high will, generally speaking, be one towards which winds will move in the same direction as the hands of a clock. Such a condition of high atmospheric pressure is called an anti-cyclone.

The region where the pressure is greatest in the Northern Hemisphere is along latitude 350 N.; and it is this belt of high pressure that has most influence on the climate of Great Britain, and, therefore, of Peebles and Selkirk. From the region of high pressure streams of air flow northwards to the North Pole and southwards to the Equator. But owing to the rotation of the earth from west to east, the winds become south-west winds and north-west winds respectively. It is with the former that we are concerned. These south-west winds, or "variable westerlies," are the prevailing winds of Great Britain, and consequently of Peebles and Selkirk. Records of winds give the following percentages for west, southwest, and south winds in Selkirkshire: Tinnis, for 25 years, 53.4; Bowerhope, near St Mary’s, for 10 years, 60.9; Thirlestane, for three years, 60.5; and in Peeblesshire, at Stobo Castle, for five years, 51.39.

Seeing that the "westerlies" blow from a region of high pressure to one of low pressure they are said to follow the fall of the barometric gradient. That is to say, the winds should cut the lines of equal pressure at right angles, but, owing to the earth’s rotation the winds are deflected, and so they cut the isobars at an acute angle. Roughly speaking, therefore, the isobars coincide in direction with that of the prevailing winds. The most important point to notice in connexion with the isobars is that as they pass over the Irish sea and St George’s Channel, they curve downwards, and, as they pass over land, they curve upwards, the curve increasing in proportion to the width of the passage over the sea, or over the land.

The pressure within the counties is greatest in May and June, mostly in the latter month, and least in October. The barometer over a period of 40 years has stood highest at Galashiels with an average of 29.953, compared with readings taken at North Esk, the Glen, Stobo, Bowhill. Other causes than that of elevation may, of course, have determined these means, and the lower temperature of Bowhill is no doubt due to a more south-westerly exposure than Stobo; but the regularity of the variation is sufficiently striking.

Since the sun is the predominating influence which determines annual temperature, the isothermals—lines of equal temperature will follow mainly an east and west course, and the temperature will decrease as we pass northwards. The average rate of decrease in Great Britain is one degree for every 116 geographical miles. The "westerlies" bring heat and moisture to our shores, and, without the influence of the surrounding sea and these warm south-west winds, the climate of Great Britain would be so extreme that in January the temperature of Peebles would be equal to that of Greenland, or, in other words, drop 20°. Peebles and Selkirk being inland counties do not benefit to the same extent from these warm westerlies as the western seaboard counties. Edinburgh, although lying to the north, has a mean annual temperature 2° higher than that of Peebles and Selkirk, due to the proximity of Edinburgh to the sea; and to the greater elevation of Peebles and Selkirk, the temperature falling, on an average, 1° for every 270 feet of elevation. Within the counties themselves the variations in temperature depend mainly upon elevation and situation as regards the "westerlies." The highest stations will be the coldest, and the most westerly, other conditions remaining the same, the warmest.

The average annual rainfall of the British Isles is about 394 inches. The driest part of the year in Scotland is generally April. The heaviest period of rainfall in Scotland is more irregular, occurring sometimes in winter and sometimes in summer. In Peebles and Selkirk, taking the results of 26 stations in 1909, we find that 14 places had their lowest rainfall in November. In 1910, out of 28 stations, all had their lowest rainfall in September. In 1909, out of 26 stations, 22 had their greatest rainfall in October. In 1910, out of 28, 17 had their greatest rainfall in August. North Esk reservoir with a record of 40 years gives a mean rainfall of 3976 inches. The Glen for 20 years gives 40~60 inches; and the stations on the Talla catchment area for 15 years give from 62.70 at Talla Linns Foot up to 75.17 inches at Gameshope Farm. The highest mean fall in Selkirkshire is Borthwick Brae, with 44.29. But the influence of position with respect to hills is greater than that of altitude. In Peeblesshire and Selkirkshire the hills in the 60-inch zone are the highest in the Southern Uplands. The whole south-western portion of Selkirk, including Ettrick village and St Mary’s Loch, lies within the 50-inch zone.

Peebles and Selkirk, therefore, have a less rainfall than the Western Highlands; but they have a greater rainfall than all the eastern counties of Scotland from Roxburgh to Sutherland. Most of the south of Scotland has a rainfall exceeding 40 inches, whereas roughly one-third of Scotland is embraced within the 30- to 40-inch zone. The crowding of the isohyets indicates a rapid change from one zone to another; and from the Grey Mare’s Tail to Jedburgh, a distance of only 30 miles, we pass through five different zones of from 60 to 30 inches. As most of the river valleys run from south-west to northeast, the rain-bearing winds will bring moisture to both sides. Hence the hills are "the greenest that e’er the sun shone on." A Yarrow legend that the deluge came from the south-west, is no doubt due to the fact that all great rain storms and floods would come from that quarter.

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