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.