ATMOSPHERE

The atmosphere is an ocean of air surrounding the earth and extending upwards for about 500 miles. The flight of all objects using fixed or moving wings to sustain them is confined  to the lower layers of the atmosphere. The ability of the body to fly successfully is therefore closely associated with the atmosphere , its properties and peculiarities.

Composition of air :

Since air is a fluid having a very low internal friction between its molecules, it can be considered within limits as an ideal fluid. Air is a physical mixture of a number of separate gases They are Nitrogen, Oxygen, Argon, Carbon dioxide,Hydrogen,Neon and Helium. For all practical purposes the atmosphere can be regarded as consisting of 21% oxygen and 79% Nitrogen.

Density of air :

The weight of air is usually considered negligible for all practical purposes. It is true that the density of air i.e. mass per unit volume is comparatively low, yet it is the very important property which makes all flight possible. 

Pressure of the atmosphere:

The average pressure at sea level is about 14.7psi/sq.in. The higher we ascend the less will be the the air above and hence less will be the pressure.

Temerature changes in the atmosphere:

An important factor that affecting the atmosphere is the temperature . The air in contact with the earth is heated by conduction and radiation, As a result the density becomes less and the air therefore rises. In rising the pressure drop allows the air to expand and the expansion in turn causes the drop in temperature. Both on account of expansion and because of the greater distance from the heating effect of the earth, the temperature is lower at high altitude than at sea level. the rate of fall  in temperature  is about one degree fahrenheit for every 300 ft. This rate of fall in temperature continues up to a height 36000 ft and then suddenly constant. The portion of the atmosphere where the temperature drops is known as the "TROPOSPHERE" and the portion where the the temperature remain constant is called the "STRATOSPHERE".

Adiabatic lapse rate of temperature in troposphere is 6.5 degree C/Km or 1.98 degree fahrenheit/1000Ft.At standard sea level condition the temperature at sea level is 15 degree C or 59 degree F and the temperature at tropopause , at an average height of  36000 ft is -56.5 degree C or -70 degree F approx.

Humidity and its effect:

Humidity is a conditions of moistures and dampness. The amount of water vapor that the air can holds depends upon on the temperature . Higher the temperature of the air , more the water vapor that it can absorb. Weight of the water vapor is approximately 5/8 of that dry air and hence the air containing water vapour is not as heavy as dry air.  On damp days the density is less than that on the dry days. For this reason the aircraft requires a longer run way for take-off or landing on a damp day than it does on a dry day.

Invertion of temperature :

It often happens that as we begin to climb, the temperature rises instead of falling , this is called an "inversion of temperature'.

The instrument which indicates the speed of an aeroplane is known as "air speed indicator" and the instrument which is used to measure the height of an aeroplane is called "altimeter."