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CLIMATE.
41
The ice-covering of the lakes remains, on an average, about 115 days in the
southern part of Sweden, 150 days in Central Sweden, and 200 days, or still
longer, in the North. The break-up of the ice occurs in April in Southern
and Central Sweden, whereas in the North it takes place as låte as in May or
June. The waters freeze in November or December and in the north-eastern
part of Norrland as early as in October.
The ice-conditions in the seas washing the coasts of Sweden is a very
important matter for navigation. Along the west and south coasts,
ocean-navigation is endered somewhat difficult by ice — mostly drift ice — only in the most
severe winters. Stationary fields of sea-ice, visible from the outermost
lighthouses such as those of Vinga or Väderöbod, are rare; as a rule, they last only
a few days, and it is very seldom that they are passable, as they were, for
example, in February and March, 1888. In some severe winters the Sound
has been covered with ice thick enough to drive across. The northern and
central parts of the Baltic, from the Aland Sea to the southern part of Gottland,
Öland, and the mainland of Kalmar Län, is covered with ice outside the belt
of coast islands only during very cold winters, and then, as a rule, only for a
few days or weeks, and for a distance of but some miles from the land. Drift
ice, hpwever, is commoner, often covering a considerable part of the surface of
the Baltic, and forming enormous masses of pack-ice. Only during the mildest
winters can navigation in these waters continue through the whole season without
interruption. During very severe winters the Ålancl Sea (the strait between the
Baltic and the Gulf of Bothnia) is covered with trafficable ice.
The Sea of Bothnia (the southern part of the Gulf of Bothnia) is frozen
over every winter along the coast, but seldom, if ever, in its central parts.
Navigation there is interrupted, on an average, from the end of November to the
beginning of May, by field- or drift-ice, with some differences for different
harbours, and with variations in respect to time amounting to 6 or 8 weeks,
according to the severity of the winter. The Qvarhen (the strait between the
northern and the southern parts of the Gulf of Bothnia) is, as a rule, covered
with trafficable ice every third or fourth winter. The Gulf of Bothnia proper
is, as a rule, covered with ice every winter. The waters along the coast begin to
freeze, on an average, about the middle of November; sometimes towards the
end of October, or at the beginning of December. The ice ordinarily begins to
break up during the latter half of May; it seldom does so at the beginning of
the month, but sometimes it happens as låte as the early days of June.
Fogs, of greater or lesser density, are not rare in Swedish waters. In the
Gulf of Bothnia and the northern part of the Baltic, as far south as to the
island of Gottland, we find the maximum of fogs, which amounts to 4 to 8
times a month during April or May; in the southern part of the Baltic and in
the western seas the maximum occurs during the winter and amounts to 7 to
10 times per month.
Precipitation (rain, snow, hail, etc.) is, next to temperature, the most
important climatic factor, for sufficient moisture and a sufficiently high
temperature are the essential conditions of vegetation. The precipitation of
moisture, too, provides springs, lakes, and rivers with their supplies of water,
without which the waters in question would soon disappear in consequence
of evaporation and escape. In Sweden, precipitation is considerably
in excess of evaporation, and the excess is carried off by the rivers of
the country.
The following table, which has been calculated by means of the ob-
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