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44
I. PHYSICAL GEOGRAPHY.
next come Storlien, with 874 mm, Björkholm (57°20’ N. Lat.; 12°22’ E. Long.),
821 mm, and Gothenburg, 779 mm, whereas at Karesuando’ the rainfall is only
307 mm and at Kalmar only 393 mm.
The yearly precipitation in Sweden shows considerable variations. At
Uppsala, where the observations seem to be trustworthy from 1836 inclusive, the
maximum was 812 mm, in 1866, and the minimum 312 mm, in 1875, the
average for the 77 years, 1836—1912, amounting to 544. The highest maximum
hitherto observed with certainty in Sweden is 1 256 mm, at Riksgränsen in
1905; then comes 1 209 mm at Hilleshult, in Halland, in 1910, while the
lowest amount of precipitation known is 172 mm, at Karesuando, in 1891.
These variations show a remarkable periodicity, the length of which varies
between 4 and 8 years. The precipitation is, furthermore, subject to considerable
variations of local and accidental nature. The monthly precipitation observed at
a single station has varied between zero and 395 mm, droughts of several weeks’
duration being sometimes followed by wet periods of about the same length.
The precipation in twenty-four hours, also, varies between very wide limits: 30
to 50 mm falls nearly every year at some stations in summer and autumn in
the form of rain, but also, in snowy winters, in the form of snow. The
absolute maximum for 24 hours hitherto observed in Sweden is 187 mm of rain and
90 mm of snow. The rain sometimes comes as violent showers; sometimes steadily
and heavily, sometimes as a mere drizzle.
Hail is not rare during the summer; it occurs during thunderstorms,
according to Hildebrandsson, on an average in the proportion 1:11. Hailstorms in
Sweden, however, are less numerous and destructive than they are in more
southerly countries, the same thing being the case with thunderstorms, while both
decrease, in general, as one goes towards the north and the east, and from
the interior of the country towards the coast. Thunderstorms are most frequent
in Älvsborg Län, and occur least frequently in that of Jämtland.
It is of great interest to know the proportion of the precipitation that remains
on the ground as snow. In Skåne it forms only 9 % of the annual total, while
in the north of Lappland it amounts to 36 %.
The yearly period of the precipitation is everywhere very strongly
marked. In most of the provinces, the principal maximum occurs during
the latter part of the summer, in addition to which there is, in some places,
a secondarsr maximum in October, which, in exceptional cases, may be
the principal maximum, the precipitation in the latter part of the summer
becoming the secondary one instead. The minimum occurs everywhere, in
February, March, or April. The climate of Sweden is distinguished by
summer and autumn rainfall, with a relative scarcity af rain from the
latter part of the winter to the early part of the summer, this being the
case especially in the littoral and the islands of the east of Sweden. During
the period of vegetation, this distribution of the precipation constitutes
a great inconvenience for agriculture, which is often hampered by want of
rain in the spring and by too much rain in the latter part of the summer
and in the autumn. These conditions are illustrated by the two maps
on pp. II, 38 and 40 the former of which shows half the total amount of
precipitation during the months of May and June, and the second
one-third of the total precipitation during July, August, and September — all
averages for the years 1880—1909. The differences become still more
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