which migrate poleward. An unusually extended
migration of either of these rain belts may bring them close together,
leaving but a small part, if any, of the intervening desert actually
rainless. The Arabian desert occupies a somewhat similar position. Large
variations in the annual rainfall may be expected towards the equatorial
margins of the sub-tropical belts.

The main features of the sub-tropical rains east of the Atlantic are
repeated on the Pacific coasts of the two Americas. In North America the
rainfall decreases from Alaska, Washington and northern Oregon
southwards to lower California, and the length of the summer dry season
increases. At San Diego, six months (May-October) have each less than 5%
of the annual precipitation, and four of these have 1%. The southern
extremity of Chile, from about latitude 38 deg. S. southward, has heavy
rainfall throughout the year from the westerlies, with a winter maximum.
Northern Chile is persistently dry. Between these two there are winter
rains and dry summers. Neither Africa nor Australia extends far enough
south to show the different members of this system well. New Zealand is
almost wholly in the prevailing westerly belt. Northern India is unique
in having summer monsoon rains and also winter rains, the latter from
weak cyclonic storms which correspond with the sub-tropical winter
rains.

[Illustration: FIG. 9.--Annual March of Temperature for selected
Sub-tropical Stations. C, Cordoba; A, Auckland; Ba, Bermuda; Bd,
Bagdad.]

From the position of the sub-tropical belts to leeward of the oceans,
and at the equatorial margins of the temperate zones, it follows that
their temperatures are not extreme. Further, the protection afforded by
mountain ranges, as by the Alps in Europe and the Sierra Nevada in the
United States, is an important factor in keeping out extremes of winter
cold. The annual march and ranges of temperature depend upon position
with reference to continental or marine influences. This is seen in the
accompanying data and curves for Bagdad, Cordoba (Argentina), Bermuda
and Auckland (fig. 9). The Mediterranean basin is particularly favoured
in winter, not only in the protection against cold afforded by the
mountains but also in the high temperature of the sea itself. The
southern Alpine valleys and the Riviera are well situated, having good
protection and a southern exposure. The coldest month usually has a mean
temperature well above 32 deg.. Mea