ontinental climate is much greater than that of the more humid,
cloudier and cooler marine climate. Both amount and frequency of
rainfall, as a rule, decrease inland, but the conditions are very
largely controlled by local topography and by the prevailing winds.
Winds average somewhat lower in velocity, and calms are more frequent,
over continents than over oceans. The seasonal changes of pressure over
the former give rise to systems of inflowing and outflowing, so-called
continental, winds, sometimes so well developed as to become true
monsoons. The extreme termperature changes which occur over the
continents are the more easily borne because of the dryness of the air;
because the minimum temperatures of winter occur when there is little or
no wind, and because during the warmer hours of the summer there is the
most air-movement.

[Illustration: FIG. 3.--Annual March of Air Temperature. Influence of
Land and Water. (After Angot.)

M, Madeira.
Bd, Bagdad.
V, Valentia.
N, Nerchinsk.]

_Desert Climate._--An extreme type of continental climate is found in
deserts. Desert air is notably free from micro-organisms. The large
diurnal temperature ranges of inland regions, which are most marked
where there is little or no vegetation, give rise to active convectional
currents during the warmer hours of the day. Hence high winds are common
by day, while the nights are apt to be calm and relatively cool.
Travelling by day is unpleasant under such conditions. Diurnal cumulus
clouds, often absent because of the excessive dryness of the air, are
replaced by clouds of blowing dust and sand. Many geological phenomena,
and special physiographic types of varied kinds, are associated with the
peculiar conditions of desert climate. The excessive diurnal ranges of
temperature cause rocks to split and break up. Wind-driven sand erodes
and polishes the rocks. When the separate fragments become small enough
they, in their turn, are transported by the winds and further eroded by
friction during their journey. Curious conditions of drainage result
from the deficiency in rainfall. Rivers "wither" away, or end in sinks
or brackish lakes.

Desert plants protect themselves against the attacks of animals by
means of thorns, and against evaporation by means of hard surfaces and
by a diminished leaf surface. The life of man in the desert is likewise
strikingly controlled by the climatic peculiarities of strong sunshine,
of heat