Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.

76. The Character of the Air. The atmosphere which envelops us at
all times extends more than fifty miles above us, its height being far
greater than the greatest depths of the sea. This atmosphere varies
from place to place; at the sea level it is heavy, on the mountain top
less heavy, and far above the earth it is so light that it does not
contain enough oxygen to permit man to live. Figure 40 illustrates by
a pile of pillows how the pressure of the air varies from level to
level.

[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]

Sea level is a low portion of the earth's surface, hence at sea level
there is a high column of air, and a heavy air pressure. As one passes
from sea level to mountain top a gradual but steady decrease in the
height of the air column occurs, and hence a gradual but definite
lessening of the air pressure.

[Illustration: FIG. 41.--The water in the tube is at the same level as
that in the glass.]

77. Air Pressure. If an empty tube (Fig. 41) is placed upright in
water, the water will not rise in the tube, but if the tube is put in
water and the air is then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what happens when we
take lemonade through a straw. When the air is withdrawn from the
straw by the mouth, the pressure within the straw is reduced, and the