What Is the Atmosphere Made Of?
WHAT IS THE ATMOSPHERE MADE OF?
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WHAT'S IN THAT STUFF YOU'RE BREATHING?
There are many things in the atmosphere, but basically it is made up of two different elements which are gases. Measured by volume (not weight) completely dry air contains approximately 78% nitrogen and 21% oxygen with about 1% of everything else. The extra 1% or so of total air content consists of  many different things. It is mostly argon plus small amounts of carbon dioxide and a number of other gases. However, air is not normally dry so you have to factor in an average of about 1% or more of water vapor. In addition to those things, air also contains dust, pollen, spores, ash, and various chemical pollutants. If you consider the air by mass or weight instead of volume then some of those gases like water vapor and carbon dioxide which are heavier would consist of a larger percentage of the air. The same would be true of particles and other pollutants in the air.
TO THE EDGE OF SPACE
All of those gases and particles are not evenly distributed in the atmosphere, And the atmosphere is not evenly distributed by mass or weight. The densest and heaviest part of the atmosphere is near the Earth. The atmosphere becomes thinner as you go higher above the Earth's surface. About 50% of the mass of the atmosphere is located below an altitude of about 18,000 feet (5.6 km). About 90% of the atmosphere's mass is below an altitude of 52,000 feet (16 km). Almost all of the atmosphere's mass (99.99997%)  is located below 330,000 feet or 62 miles (100 km). That altitude of 100 km is considered by many people to be the edge of space - the boundary between the Earth's atmosphere and outer space. At that altitude the atmosphere is so thin that no vehicle with wings can actually fly there. You have to be in a rocket traveling fast enough to orbit the Earth to stay that high above the earth. That altitude of 100 km or 62 miles above sea level is called the Karman Line. But even above that altitude the atmosphere still exists even though it is very, very thin and we consider that area to be space. It is in that area that the Space Shuttle and the International Space Station orbit the Earth.
A FIVE-LAYER CAKE
The atmosphere of the Earth can be divided into five layers. The bottom layer is where we live it's called the troposphere. It begins at the surface of the earth and goes up from there to about 23,000 feet (7 km) at the poles or 56,000 feet (17 km) at the equator. The air in the troposphere is mostly warmed by heat coming from the surface of the Earth. For that reason the troposphere is warmest close to the earth and the temperature decreases with altitude. Above the troposphere is the stratosphere which extends to about 170,000 feet or 32 miles (51 km). As you go higher in the stratosphere the temperature actually increases because the heat there comes directly from the sun. Above the stratosphere is the mesosphere. It extends up to 260,000 to 280,000 feet or 50 to 53 miles (80 to 85 km). It's in the mesosphere that meteors burn up when they enter the atmosphere, and it is where the Space Shuttle has to be protected by a heat shield as it makes re-entry. The temperature decreases as you go higher into the mesosphere. In fact when you reach the top of the mesosphere the average temperature there is 121° below zero Fahrenheit (-85°C). Water vapor there freezes forming ice clouds known as Noctilucent clouds. Above the mesosphere is the thermosphere where the temperature actually increases as you go higher. The temperature of atmospheric molecules in the thermosphere can rise to as high as 2730°Fahrenheit (1500°C)!  However, the molecules are so far apart that their temperature is meaningless. The International Space Station orbits in the thermosphere. The very outermost layer of the Earth's atmosphere is called the exosphere. Here the molecules are so far apart that they can travel hundreds of miles before bumping into one another. The gases in the exosphere are mostly hydrogen and helium. Once you reach the height of the thermosphere and exosphere you are considered to be in space. In fact the United States definition of an astronaut is anyone who has flown more than 50 miles (80 km) above sea level.
IF YOU CAN'T STAND THE (LOW) PRESSURE, STAY OFF THE HIGH MOUNTAIN
Because most of the mass of the atmosphere is located within a few thousand feet of the Earth's surface, that is the area where the air is the most dense and where the the atmospheric pressure is the greatest. Air presses on everything, including our bodies. If we drive up or down a mountain or ascend or descend in an airplane our ears pop because of the change in air pressure. Air pressure can also change when a low pressure system enters the area where we live and the barometer (an instrument which measures air pressure) indicates the pressure has dropped. That usually means that the storm may be approaching. An altimeter is an instrument used by pilots or mountain climbers to tell how far they are above sea level by measuring the air pressure. In commercial airplanes which fly at around 33,000 feet (10 km) the cabin is pressurized to make up for the fact that the outside air pressure is very low. As people climb mountains the air becomes thinner making it harder for them to breathe and making them tired and weak. People who climb very tall mountains, such as Mount Everest which is 29,029 feet (8848 m) above sea level, take oxygen along to help them breathe in the thin air.
PROTECTING HARMFUL STUFF CALLED OZONE SO OZONE CAN PROTECT US FROM THE HARMFUL STUFF THAT CREATES OZONE
The ozone layer is an important part of the atmosphere located in the lower part of the stratosphere between about 49,000  and 110,000 feet or 9.3 to 22 miles (15 to 35 km). Ozone is a molecule of oxygen containing 3 atoms instead of the usual 2 atoms. The ozone layer plays an important part in protecting us from harmful ultraviolet (UV) radiation from the sun. The ozone layer absorbs 97 to 99% of the most damaging high-frequency UV rays. There is ozone throughout the atmosphere, but about 90% of it is in the ozone layer. When ozone is in the lower atmosphere where we live, it's considered pollution and has bad effects on people and plants. Ozone in the air we breathe causes headaches, burning eyes  and breathing problems, and it can permanently damage our lungs. However, in the ozone layer high above, it protects us from the harmful UV rays. Some man-made chemicals such as CFCs cause harm to the ozone layer and for that reason that have been banned in most countries. People have done that to protect the ozone which protects us. It is interesting that the ozone layer is actually created and sustained by the UV radiation from the sun – the same UV rays that the ozone layer protects us from. The design of the atmosphere is amazing.
IONIC LIGHT SHOW
The ionosphere is a part of the atmosphere from about 31 to 620 miles (50 to 1000 km) above the Earth. That would include the mesosphere, the thermosphere, and the exosphere. In this area the molecules of the atmosphere are ionized by solar radiation, meaning that they have electrons stripped away. These ionized molecules can reflect radio waves that are transmitted from the earth and therefore cause changes in worldwide radio reception. It is also the ionosphere which causes auroras. There is Aurora Borealis also known as the Northern Lights and Aurora Australis or the Southern Lights. These are natural light displays in the sky usually seen at night in regions near the Arctic and Antarctic circles. The auroras are caused by the ionized nitrogen or oxygen molecules colliding with solar wind particles that are traveling along the Earth's magnetic field lines. (Solar wind is a gas of free electrons and positive ions emitted by the outer layer of the Sun, called the corona. The solar wind travels through space in all directions and reaches the Earth.) The auroras can be green red or blue with green being the most common of all. They are very beautiful to watch but are usually seen only by people who live in the far northern or southern parts of the world.
ELECTRIC ATMOSPHERE
There is another thing which can be seen in our atmosphere - electricity. The atmosphere between the surface of the earth and the ionosphere can act like a large electrical circuit. During thunderstorms we often see lightning in which millions of volts of electricity move between clouds or between clouds and objects on the earth. The electrical voltage is produced when tiny droplets of water vapor pick up small electrical charges and then they condense together to form larger droplets while combining their electrical charges until finally the voltage is high enough to cause an electric spark to jump through the air. The spark causes a flash of light and as it heats the air it also causes a loud noise we know as thunder. Because of the massive amount of power it releases, lightning can do great damage to objects or buildings or people. You can get an idea of how far away a lightning strike is by counting the time between the flash and the sound of the thunder. Light travels at the speed of 186,000 miles per second, so the light is seen almost instantly. Sound at sea level travels at about 1100 feet per second so it takes about 5 seconds for sound to travel 1 mile. If the time between the flash of the lightning and the sound of the thunder is 5 seconds, then you know that lightning strike was about a mile away. If it's two or 3 seconds, you know the lightning is about half-a-mile away and you should take shelter, if you haven't already. If the time is 1 second or less, then you should thank God you're still okay.
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