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### Theory of Flight

Atmosphere
The invisible ocean that stairs around the earth is known as atmosphere. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention "greenhouse effect", and reducing temperature extremes between day and night. The atmosphere contains various types of gases. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%.

Structure of Earth
The earth is surrounded by 5 belts of air called the atmosphere belt. Earth's atmosphere is divided into five main layers. These layers are : Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

Information obtain : Wikipedia & Theory of Flight course notes
Image obtain          : Wikipedia & Google image

1. Pressure
• The earth is surrounded by a belt of air called the atmosphere, which is about 320 km in depth. This belt of air is heavy.
• This weight exerts pressure which measured at the earth's surface, is about (1.0132*10^5 N/m^2) also known as ( 1013.2 millibars or 14.7 lbs sq.in).
• The higher one goes up through the atmosphere, the less the weight of the air above, therefore, the less pressure it exerts.
• Therefore :- AIR PRESSURE DECREASES WITH ALTITUDE
2.  Density
• Density is a measure of the thickness of the air.
• Density = Mass / Unit Volume
• As pressure decreases with altitude, so, as height increases, the air will be able to expand, decreasing the MASS per unit volume. The air, therefore, becomes less dense.
• Conclusion :- AIR DENSITY DECREASES WITH ALTITUDE
3.  Temperature
• The heat radiated by the sun passes through the Earth's atmosphere without appreciably raising the temperature.
• The Earth absorbs the heat, raising the temperature of the air in contact with it, which expands and rises.
• The rising column of air will further expand, decreasing its temperature as it does so.
• Therefore :- AIR TEMPERATURE DECREASES WITH ALTITUDE
• Under standard condition, temperature decreases approximately 1.98 degree celsius for each increase of 1,000 feet of altitude of 38,000 feet.
• Above 38,000 feet, the Ambient temperature remains at -56.3 degree celsius.
• The standard temperature at sea level is 15 degree celsius.
4.  Humidity
• Humidity is a condition of moisture or dampness
• The maximum amount of air, that an air can hold, depends on the temperature.
• The higher the temperature, the more water vapour it can hold.
• The density of air varies with humidity.
SUMMARY
• Pressure, density, and temperature all decreases with an increase in altitude (and so does humidity)