he air.

_Humidity, Cloudiness and Fog._--The absolute humidity must be low in
polar latitudes, especially in winter, on account of the low
temperatures. Relative humidity varies greatly, and very low readings
have often been recorded. Cloudiness seems to decrease somewhat towards
the inner polar areas, after passing the belt of high cloudiness in the
higher latitudes of the temperate zones. In the marine climates of high
latitudes the summer, which is the calmest season, has the maximum
cloudiness; the winter, with more active wind movement, is clearer. The
curve here given illustrates these conditions (fig. 14). The summer
maximum is largely due to fogs, which are produced where warm, damp air
is chilled by coming in contact with ice. They are also formed over open
waters, as among the Faeroe Islands, for example, and open water spaces,
in the midst of an ice-covered sea, are commonly detected at a distance
by means of the "steam fog" which rises from them. Fogs are less common
in winter, when they occur as radiation fogs, of no great thickness. The
small winter cloudiness, which is reported also from the antarctic zone,
corresponds with the low absolute humidity and small precipitation. The
coasts and islands bathed by the warm waters of the Gulf Stream drift
usually have a higher cloudiness in winter than in summer. The place of
fog is in winter taken by the fine snow crystals, which often darken the
air like fog when strong winds raise the dry snow from the surfaces on
which it is lying. Cumulus cloud forms are rare, even in summer, and it
is doubtful whether the cloud occurs at all in its typical development.
Stratus is probably the commonest cloud of high latitudes, often
covering the sky for days without a break. Cirrus cloud forms probably
decrease polewards.

[Illustration: FIG. 14.--Annual March of Cloudiness in Polar Latitudes
(marine type).]

_Cyclones and Weather._--The prevailing westerlies continue up into the
margins of the polar zones. Many of their cyclonic storms also continue
on to the polar zones, giving sudden and irregular pressure and weather
changes. The inner polar areas seem to be beyond the reach of frequent
and violent cyclonic disturbance. Calms are more common; the weather is
quieter and fairer; precipitation is less. Most of the observations thus
far obtained from the Antarctic come from this marginal zone of great
cyclonic activity, violent winds, and wet, disagreeable, inhospitabl