Atmosphere
From Pilots Almanac
The atmosphere is classified into layers based upon the characteristics each layer exhibits. Most weather occurs in the troposphere and most flying occurs in the troposphere and stratosphere.
The atmosphere is comprised of a mixture of gases referred to as "air". It is about 80 percent nitrogen and 20 percent oxygen. Water vapor and many other gases constitute the remainder of the gas mixture. Note that the molecular weight of water vapor is less than that of both oxygen and nitrogen. For this reason, moist air is less dense than relatively drier air. Airfoils and air-breathing engines exhibit reduced performance in moist air because of this reduction in density.
Air is made up of matter and has weight. Since air is gaseous, it is compressible. This means that the air pressure nearer the surface of the earth is greater than the air pressure in the stratosphere.
Air exerts pressure on everyone and everything. At the earth's surface the pressure is 14.7 pounds per square inch. That does not sound like much, but it means the air pressure per square foot is 2116.8 pounds. Increase the air pressure and the air's density is increased. Because of air's compressibility flight conditions will vary depending upon the altitude. This is due to the air density. More molecules in the air will generate greater lift with less thrust. Fewer molecules in the air will require greater thrust to generate adequate lift.
The decrease in air density with increase in altitude also affects people physiologically. Decrease the air pressure and the oxygen pressure is also decreased. The rate at which our lungs absorb oxygen depends on the partial pressure exerted by the amount of oxygen in the air. Since our atmosphere is about 1/5 oxygen, the oxygen pressure at any given altitude will be about 1/5. Under normal conditions our lungs function under 3 pounds per square inch of oxygen pressure. As an airplane climbs higher into the troposphere, it will encounter less oxygen. Without supplemental oxygen the people on board such a flight will suffer from hypoxia, a deficiency in oxygen. The symptoms of hypoxia are a feeling of acute exhaustion with an immediate impairment in vision and judgment resulting in unconsciousness and death if the proper amount of oxygen is not soon administered. Prolonged flights at or above 10,000 feet and even short flights above 12,000 should use auxiliary oxygen.
The ocean of air we live in can be calm, delightfully warm and pleasant or it can be turbulent and rainy as with a thunderstorm, hurricane or tornado. The air temperature varies from below -100 Celsius to above 1500 Celsius (-150 Fahrenheit to 2700 Fahrenheit). These variations are caused by the uneven heating of the earth's surface by the sun's energy as well as how the earth reacts to this energy. The characteristics of a substance (for example water or land) will affect the amount of heat absorbed or released by that substance. Let's say we have a land surface and water surface of equal temperature and we apply an equal amount of heat to each. The land surface will become hotter at a much faster rate than the water surface. The opposite is true when both substances release the same amount of heat. Under equal heat loss the land will become colder at a faster rate than the water.
Layers of the Atmosphere
Atmospheric Gases
The atmosphere consists of a number of gases with four making up 99.998% of the constituency. The most common, nitrogen (78%), dilutes oxygen and prevents rapid burning at the earth's surface. Living things need it to make proteins. Oxygen (21%) is used by all living things and is essential for respiration. It is also necessary for combustion or burning. Argon makes up slightly less than 1% of atmospheric gas. Carbon dioxide, used by plants in photosynthesis to make oxygen, makes up about 3/100 of a percent of the total atmospheric gas. It acts as a blanket and prevents the escape of heat into outer space.
