What is air? Air is a mixture of gases that circle Earth, kept in place by gravity. Air makes up Earth's atmosphere. The atmosphere of our home planet is made up of three primary gases. These gases are oxygen, nitrogen, and argon. In addition to these three gases, there are many other gases in the atmosphere, in trace or small amounts. Additionally there are small particles, or particulates floating in the atmosphere, such as dust, water, and pollen The air we breathe is 78 percent nitrogen gas, 21 percent oxygen, 0.9 percent argon, and 0.03 percent carbon dioxide, along with water vapor- floating molecules of water. As you move up in altitude through the atmosphere, the concentration of air molecules decreases. Some people call this "thin air". The air is thinner higher in the atmosphere because there is lower pressure the higher you go up.

Air is important for almost all life on Earth including plants and animals. Plants need gasses from air to do photosynthesis. Animals need to breath air to get the oxygen they need to survive. There is a special layer of air molecules high in the stratosphere layer of Earth’s atmosphere, called the ozone layer. The composition of the atmosphere is different in the ozone layer. There are more ozone molecules than anywhere else. Ozone molecules help block some of the Sun’s strongest rays. Currently, scientists are monitoring this layer. It has recently become so thin at the South Pole where the molecules are being destroyed that we call it a “hole”.

What is Weather? Weather is the state of the atmosphere at a given time and place. Most weather takes place in the troposphere, the lowest layer of the atmosphere. Weather is measured and described in a variety of ways by meteorologists, scientists who study and predict weather. Air temperature and pressure, the amount and type of precipitation, the strength and direction of wind, and the types of clouds are all described in a weather report. Weather changes each day because the air in our atmosphere is always moving, distributing energy from the Sun. In most places in the world, the types of weather events also vary throughout the year as seasons change

Air Pressure When your friend squeezes your arm, you feel pressure! That's because molecules collide with each other and things like your arm, the ground, or a tree. They exert a force on those surfaces. Molecules in Earth’s atmosphere constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure. Molecules that are packed closely are at higher pressure than molecules that are more spread out. The molecules inside this balloon are at a higher pressure than the molecules outside the balloon.

What's a high pressure system? Sometimes, high in the sky, air slows its forward motion and piles up. This is called convergence. The piled-up air sinks. When the sinking air reaches the Earth’s surface, it spreads out. This is called divergence. In the Northern Hemisphere it spreads out with clockwise surface rotation and is called a high pressure system.

What's a low pressure system? High in the sky, air moves away from an area faster than air enters an area. When air spreads out (divergence) at high altitudes, air from below flows upward to fill the space, making a low pressure system. The system has surface convergence and anti-clockwise rotation in the Northern Hemisphere.

Air Pressure on-line Experiments Students log onto Explore “Treehouse Weather Kids” Seasons and Temperatures Air Pressure and Wind Clouds and Moisture Rock ‘n Roll Weather

What are the characteristics of each layer of the atmosphere? In order to better understand our atmosphere scientists have divided it into several layers. These layers are divided based on the characteristics of the gases found at that height. Imagine a layered cake. A layered cake is made out of layers of different ingredients. The first layer might be cake mix. On top of that a cook could put a layer of pudding. On top of that layer they perhaps could put another layer of cake mix, followed by a layer of whip cream, and so forth. Each layer in our atmosphere is referred to as a sphere.

What are the characteristics of each layer of the atmosphere? The Earth’s atmosphere extends outward to a distance of approximately 6,000 miles. However, the bulk of that atmosphere is compressed into the first 16 miles. The higher that one travels the less compressed the atmosphere becomes, and the less pressure there is exerted on the items it that atmosphere. Imagine that you are swimming in a giant pool of steel marbles. The deeper you go into the marbles the heavier the marbles become. This is because there are more marbles on top of you, and more weight pushing down on your body. The higher you go, the lighter they become.

. Imagine how quickly the gases in our atmosphere are mixed together. It only takes a few days for particulates from one part of the Earth to be blown all over the planet. Dust particles from the Sahara Desert are blown into the atmosphere, and end up all over the Earth!

The Troposphere The troposphere is the lowest layer of Earth's atmosphere. The troposphere starts at Earth's surface and goes up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. Higher up it gets colder. Air pressure and the density of the air are also less at high altitudes. The layer above the troposphere is called the stratosphere.

The stratosphere is the second layer (going upward) of Earth's atmosphere. It is above the troposphere and below the mesosphere. The ozone layer is within the stratosphere. The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere.

The mesosphere is a layer of Earth's atmosphere. The mesosphere is above the stratosphere layer. The layer above the mesosphere is called the thermosphere. The mesosphere starts at 50 km (31 miles) above Earth's surface and goes up to 85 km (53 miles) high. As you get higher up in the mesosphere, the temperature gets colder. The top of the mesosphere is the coldest part of Earth's atmosphere. The temperature there is around - 90° C (-130° F)!

Mesosphere Scientists know less about the mesosphere than about other layers of the atmosphere. The mesosphere is hard to study. Weather balloons and jet planes cannot fly high enough to reach the mesosphere. The orbits of satellites are above the mesosphere. We don't have many ways to get scientific instruments to the mesosphere to take measurements there. We do get some measurements using sounding rockets. Most meteors burn up in the mesosphere

You know that waves can form in the ocean or other bodies of water. But did you know that there are waves of air in the atmosphere? Some of these waves start in the lower atmosphere, the troposphere and stratosphere, and move upward into the mesosphere. The waves carry energy to the mesosphere. Most of the movement of air in the mesosphere is caused by these waves.

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