and in Alaska. Between these two regions of lower pressure the
divide extends from North America to eastern Siberia. This divide has
been called by Supan the "Arktische Wind-scheide." The pressure
gradients are steepest in winter. At the pole itself pressure seems to
be highest in April and lowest from June to September. The annual range
is only about 0.20 in.

The prevailing westerlies, which in the high southern latitudes are so
symmetrically developed, are interfered with to such an extent by the
varying pressure controls over the northern continents and oceans in
summer and winter that they are often hardly recognizable on the wind
maps. The isobaric and wind charts show that on the whole the winds blow
out from the inner polar basin, especially in winter and spring.

_Rain and Snow._--Rainfall on the whole decreases steadily from equator
to poles. The amount of precipitation must of necessity be comparatively
slight in the polar zones, chiefly because of the small capacity of the
air for water vapour at the low temperatures there prevailing; partly
also because of the decrease, or absence, of local convectional storms
and thunder-showers. Locally, under exceptional conditions, as in the
case of the western coast of Norway, the rainfall is a good deal
heavier. Even cyclonic storms cannot yield much precipitation. The
extended snow and ice fields tend to give an exaggerated idea of the
actual amount of precipitation. It must be remembered, however, that
evaporation is slow at low temperatures, and melting is not excessive.
Hence the polar store of fallen snow is well preserved: interior
snowfields, ice sheets and glaciers are produced.

The commonest form of precipitation is naturally snow, the summer limit
of which, in the northern hemisphere, is near the Arctic circle, with
the exception of Norway. So far as exploration has yet gone into the
highest latitudes, rain falls in summer, and it is doubtful whether
there are places where _all_ the precipitation falls as snow. The snow
of the polar regions is characteristically fine and dry. At low polar
temperatures flakes of snow are not found, but precipitation is in the
form of ice spicules. The finest glittering ice needles often fill the
air, even on clear days, and in calm weather, and gradually descending
to the surface, slowly add to the depth of snow on the ground. Dry snow
is also blown from the snowfields on windy days, interfering with the
transparency of t