Zeynep AVCI

                            

              ATMOSPHERIC CONDITIONS

Structure of the Atmosphere:

      The atmosphere is an envelope of air that surrounds the Earth and rests upon its surface. It is as much a part of the Earth as the seas or the land, but air differs from land and water as it is a mixture of gases. It has mass, weight, and indefinite shape. The atmosphere is composed of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases, such as argon or helium. Some of these elements are heavier than others. The heavier elements, such as oxygen, settle to the surface of the Earth, while the lighter elements are lifted up to the region of higher altitude. Most of the atmosphere’s oxygen is contained below 35,000 feet altitude.

Internatıonal Standard Atmosphere:

     The Internatıonal Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscocity of the Earth's atmosphere change over a wide range of altitude or elevations. It has been established to provide a common reference for temperature and pressure and consist of tables of values at various altitudes , plus some formulas by which those values were derived. The international Organization for Standardization (ISO) publishes the ISA as an international standard, ISO 2533:1975. Other  standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States Government, publish extensions or subsets of the same atmospheric model under their own standard-makig authority.
       
         The ISA model divides the atmosphere into layers with linear temparature distributions. The other values are computed from basic physical constant and relationship. Thus the standard consist of a table of values at various altitudes , plus some formulas by which those values were derived. For example, at sea level the standard gives a pressure of 1013.25 hPa and a temperature of 15 degrees Celsius, and initial lapse rate of - 6.5 C /km. The tabulation continues to 11 km where the pressure has fallen to 226.32 hPa and the temperature to -56.5C/km. Between 11 km and 20 km the temperature remains constant.

International Standard Atmosphere  (ISA) and Pressure Setting:

          Based on the International Standard Atmosphere for dry air (ICAO 1964), which is defined as under; 

1. At mean sea level (msl), the pressure= 1013.25 hPa and temperature= 15.0 degC

2.From msl to 11 km, a decrease in temperature (or lapse rate) of 6.5 degC/km

3.From 11 km to 20 km, the temperature is  held to be isothermal (not changing) at a value of -56.5 degC

4.From 20 km to 32 km, an increase in temperature of about 1 degC/km

     

For practical aviation purposes , İ.E Flight Weather Charts, the following relationships are used:

       ( Below F050, height usually expressed in altitude , above mean sea level , rather than pressure altitude - see definitions below )

                            PRESSURE SETTING DEFINITIONS;

QFE:

 The pressure corrected to the official airfield elavation. An altimeter set to the particular aifield QFE reads zero when an aircraft is on the ground (strictly the height of the altimeter above the ground ) In the circuit, the height idicated is the height above official airfield datum.

QNH: 

The pressure reduced to mean sea level, assuming ISA TEMPERATURE PROFILE from the station /airfield  to MSL. An altimeter set to the airfield  QNH reads the elevation of the airfield when on the ground

QFF: 

Barometric pressure reduced to mean sea level , assuming AN ISOTHERMAL ATMOSPHERE from the airfield/ station to MSL, using current (screen) temperatures.The difference between QFF and QNH can be considerable when atmospheric conditions are significantly different from ISA : i.e at 'hot and high' airfields.

QNE:

When the ISA mean sea level standard pressure of 1013.2 hPa is set on an aircraft altimeter subscale, the height so indicated upon landing at an airfield is known as the 

QNE reading. More widely, this is also the PRESSURE ALTITUDE , which is  alternatively defined as the height of any level in the international standard atmosphere (ISA-see above), above the level corresponding to a pressure of 1013.2 hPa

 

Standard Atmosphere Modeling:

        For purposes of pressure altimeter celibrations, aircraft and rocket performance and their design, and so forth, knowledge of the vertical distribution of such quantities as pressure ,temperature, density and speed of sound is required. Since the real atmosphere never remains constant at any particular time or place, a hypothetical model must be employed as an approximation to what may be expected. This model is known as the standard atmosphere. The air in the model is assumed to be devoid of dust, moisture, and water vapor and to be  at rest with respect to the Earth.

       The first standard atmospheric models were developed in the 1920's in both Europe and the United States. The slight differences between the models were reconciled and an internationally accepted model was introduced in 1952 by the International Civil Aviation Organization (ICAO). The International Standard Atmosphere is defined in ICAO Document 7488/2. The ISA assumes the mean sea level (MSL) conditions

Temperature Modeling:

  

 Pressure Modeling :

         To calculate the standard pressure p at a given altitude, the temperature is assumed standard, and the air is assumed as a perfect gas. The altitude obtained from the measurement of the pressure is called pressure altitude (PA). 

INTERNATIONAL STANDARD ATMOSPHERE