down to
about latitude 50 deg. N. Over no small part of Siberia minimum
temperatures below -70 deg. may be looked for every winter. Thorshavn
and Yakutsk are excellent examples of the temperature differences along
the same latitude line (see fig. 11). The winter in this interior region
is dominated by a marked high pressure. The weather is prevailingly
clear and calm. The ground is frozen all the year round below a slight
depth over wide areas. The extremely low temperatures are most trying
when the steppes are swept by icy storm winds (_buran_, _purga_),
carrying loose snow, and often resulting in loss of life. In the North
American interior the winter cold is somewhat less severe. North
American winter weather in middle latitudes is often interrupted by
cyclones, which, under the steep poleward temperature gradient then
prevailing, cause frequent, marked and sudden changes in wind direction
and temperature over the central and eastern United States. Cold waves
and warm waves are common, and blizzards resemble the buran or purga of
Russia and Siberia. With cold northerly winds, temperatures below
freezing are carried far south towards the tropic.

The January mean temperatures in the southern portions of the
continental interiors average about 50 deg. or 60 deg. In summer the
northern continental interiors are warm, with July means of 60 deg. and
thereabouts. These temperatures are not much higher than those on the
west coasts, but as the northern interior winters are much colder than
those on the coasts, the interior ranges are very large. Mean maximum
temperatures of 86 deg. occur beyond the Arctic circle in north-eastern
Siberia, and beyond latitude 60 deg. in North America. In spite of the
extreme winter cold, agriculture extends remarkably far north in these
regions, because of the warm, though short, summers, with favourable
rainfall distribution. The summer heat is sufficient to thaw the upper
surface of the frozen ground, and vegetation prospers for its short
season. At this time great stretches of flat surface become swamps. The
southern interiors have torrid heat in summer, temperatures of over 90
deg. being recorded in the south-western United States and in southern
Asia. In these districts the diurnal ranges of temperature are very
large, often exceeding 40 deg., and the mean maxima exceed 110 deg.

The winter maximum rainfall of the west coasts becomes a summer maximum
in the interiors. The change is grad