t is often
difficult to determine just what one is the chief factor in causing the
direction of the wind at any particular time. There are very many
instances, however, where the cause can be traced without difficulty;
many of these have already been mentioned and there are many more that
might be. Of course, as has been often stated, there is only one remote
cause for all winds, and that is the sun, coupled with the movements of
the earth. But there are certain local conditions that are continually
modifying the phenomena of air movement. The velocity of winds as they
occur from day to day varies very greatly with the height above the
surface of the earth; ordinarily the velocity at 1000 feet above the
earth will be more than three times greater than it is at 50 or 60 feet
above, and even at 60 feet the velocity is much greater than at the
surface of the earth. This is due partly to the retarding effect of
friction caused by contact of the air with the earth's surface, but more
particularly by trees, inequality of surface, and other obstructions on
the earth.

There is a variety of wind called mountain winds that arise from
different causes. As has been stated in a former chapter, under ordinary
conditions the air is more dense at sea-level than at any point above,
and the density is constantly changing from denser to rarer the higher
we ascend. Suppose at a certain point, say halfway up a mountain side,
the air has a certain density, and if it is at rest the lines of equal
density or pressure will seek a level, just as water would under the
same conditions. Suppose we start at a given point on the side of a
mountain and run out on a level till we are 100 feet in a perpendicular
line above the side of the mountain, the air contained within those
lines will be in the shape of a triangle. If now the sun shines upon the
side of the mountain the air is warmed and expands according to a
well-known law, and the amount of expansion will depend upon the depth
of the volume of air; hence the point of greatest expansion in our
figure will be where the air is 100 feet deep, and will gradually
decrease as we go toward the mountain till we come to the point where
our horizontal line makes contact with the mountain side. At that point,
of course, there is no expansion, because there is no depth of air; and
the effect will be that the expanded air will overflow toward the
mountain, and be deflected up its sloping side. If we apply th