Air

Air surrounds the earth just above the land and sea. It is
made up of a mixture of gases that we cannot see. Water in
the form of vapor is mixed with the gases. Air also carries
tiny particles of solid material such as dust, but the
water and the solid particles are not considered part of
the air. 

The gases in air are nitrogen (78%) and oxygen (21%) The
remaining 1% of air consists almost entirely of the gas
argon. Air also contains very small amounts of several
other gases. They include neon, helium, krypton, xenon,
hydrogen, azone, carbon dioxide, nitrous oxide, and
methane. 

Air often contains many solid particles which are usually
invisible except when present in great numbers. Some of
these are salt from the oceans, pollen from plants, and
tiny living things called microbes. 

Characteristics of air are pressure and resistance,
movement, and air compression. Air pressure is caused by
the weight of air from the top of the atmosphere as it
presses down upon the layers of air below. The upper
atmosphere has less pressure than the air near the earth,
because there is less air above it. Air pressure at sea
level averages about 15 pounds on every square inch. This
weight changes a little from day to day along with the
weather. Air is a little sticky and resists the motion of
objects going through it. The faster an object moves
through the air, the more air resistance it meets. Air
resistance also creates heat. 

The air moves constantly because of the motion of the
earth, the gravitational attraction of the moon and sun,
the heating of the sun and the vibration or movement of
molecules of the gases that make up the air. 

Compressing air can alter the speed of molecules. The speed
increases when the pressure becomes stronger and the air
becomes warmer. Lowering the pressure has the opposite
effect. 

Scientists have divided the atmosphere of the earth and the
space just beyond it into four main layers. From the ground
up, these layers are the troposphere, stratosphere,
mesosphere, and thermosphere. 

The troposphere is the air layer that extends from the
earth up to the area where the temperature stops becoming
lower. On the average, the troposphere extends about 6
miles over the North and South Poles and about 19 miles
over the equator. Most of the air, moisture, and dust of
the atmosphere is in the troposphere. Clouds and weather
are usually confined to this level of the atmosphere.
 
The stratosphere extends from the troposphere to about 30
miles above the earth. It has a lower layer of nearly
constant temperature and an upper layer in which the
temperature increases with increasing altitude. 

The mesosphere lies above the stratosphere. It begins about
30 miles above the earth and extends upward to about 50
mile. The temperature of the air in the mesosphere
decreases from about 28 degrees F. at it base to about -135
degrees F. near its top, called the mespause. 

The thermosphere begins about 50 miles above the earth and
continues into the depths of space. Over 99 % of the
earth's atmosphere lies in the layers below the
thermosphere, and thus the air in the thermosphere is
extremely thin. The temperature climbs rapidly because the
thin air is fully exposed to the sun's radiation. The
aurora borealis (northern lights) and aurora australis
(southern lights) also occur in the thermosphere.
 
Air has no color, no smell, and no taste. Yet air is just
as real as water or land. Without air, there could be no
living plants or animals on the earth. Air covers a larger
area than all the oceans of water put together. It is
hundreds of miles deep and covers all the earth.