tempers the heat of the low latitudes. High mountain peaks, even on the
equator, can remain snow-covered all the year round.

No general law governs the variations of relative humidity with
altitude, but on the mountains of Europe the winter is the driest
season, and the summer the dampest. At well-exposed stations there is a
rapid increase in the vapour content soon after noon, especially in
summer. The same is true of cloudiness, which is often greater on
mountains than at lower levels, and is usually at a maximum in summer,
while the opposite is true of the lowlands in the temperate latitudes.
One of the great advantages of the higher Alpine valleys in winter is
their small amount of cloud. This, combined with their low wind velocity
and strong insolation, makes them desirable winter health resorts.
Latitude, altitude, topography and winds are the determining factors in
controlling the cloudiness on mountains. In the rare, often dry, air of
mountains and plateaus evaporation is rapid, the skin dries and cracks,
and thirst is increased.

Rainfall usually increases with increasing altitude up to a certain
point, beyond which, owing to the loss of water vapour, this increase
stops. The zone of maximum rainfall averages about 6000 to 7000 ft. in
altitude, more or less, in intermediate latitudes, being lower in winter
and higher in summer. Mountains usually have a rainy and a drier side;
the contrast between the two is greatest when a prevailing damp wind
crosses the mountain, or when one slope faces seaward and the other
landward. Mountains often provoke rainfall, and local "islands," or
better, "lakes," of heavier precipitation result.

Mountains resemble marine climates in having higher wind velocities than
continental lowlands. Mountain summits have a nocturnal maximum of wind
velocity, while plateaus usually have a diurnal maximum. Mountains both
modify the general, and give rise to local winds. Among the latter the
well-known mountain and valley winds are often of considerable hygienic
importance in their control of the diurnal period of humidity,
cloudiness and rainfall, the ascending wind of daytime tending to give
clouds and rain aloft, while the opposite conditions prevail at night.

_Supan's Climatic Provinces._--The broad classification of climates into
the three general groups of marine, continental and mountain, with the
subordinate divisions of desert, littoral and monsoon, is convenient for
purposes o