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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. |
